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Lightbulb Guide to Subwoofer Calibration and Bass Preferences


The Guide, which starts here in Post 1, and which continues in Post 2, is intended to be a general guide to Home Theater, HT calibration, and audio quality. Due to its length, I have had to divide it into two posts. Sections
I through III follow the Introduction in this post. Sections IV through VIII are in Post 2. The thread, which starts after the Guide, is a general audio/HT discussion thread.


Parts of the Guide may seem very thorough and detailed, but readers can choose the parts they wish to read at any one time. Over a period of time, however, most of the information contained in the Guide is likely to prove valuable for those who want to understand how things work, and for those who want to get the most from their HT/audio systems.

The Guide began on the Audyssey thread as a way to explain Audyssey's calibration process, and to provide guidance with respect to adding bass boosts after calibrations. But, over time, the Guide has expanded to encompass discussions of a wide-ranging series of topics involving our HT and audio systems. The Table of Contents, and the Introduction to the Guide, provide an overview of the topics which are discussed.

* The general principles of room/system interaction, of system calibration, and of room EQ, which are contained in the Guide, are believed to be applicable to other methods of HT calibration, and to other systems of automated room EQ.

With respect to the thread itself, ideally this will be a thread where people will be comfortable discussing any HT calibration, subwoofer, or audio-related issues.



Cliff Notes:


Several people have suggested having some abbreviated HT calibration tips, so I have added a few prior to the actual Guide. Anyone wanting more detail on anything from room and speaker setup, to setting crossovers, to the differences between sealed and ported subs, to selecting and positioning subwoofers in a room, can find the pertinent information in the Guide itself. There are some similar abbreviated tips, to selecting subwoofers, at the beginning of Section VIII.


1. Try to position speakers and subwoofers strategically, doing a "subwoofer crawl" if necessary. You can Google the procedure, or refer to Section VIII-E for instructions on how to do that.


2. Set the subwoofer phase at 0, and the low-pass filter (sometimes labelled crossover) on the subwoofer at the maximum setting.


3. Allow your AVR to calibrate your audio system for you, using its automated routine. The first thing that will do is to calibrate your HT system to Dolby Reference, and it will provide a common basis for comparing listening levels and subwoofer boosts. That calibration process will insure that all of the channels are playing the same volume levels at the MLP (main listening position) and that all of the sounds will arrive at the same time.


4. The second thing that the calibration will do is to set EQ filters for all of the channels to remove some peaks and dips in the frequency response caused by the interaction of your transducers (speakers and subwoofers) with the room. Those random peaks and dips in volume, at different frequencies, interfere with the quality of the sound we hear. With subwoofers, boomy, one-note bass is often the result of a random peak. After room EQ, the bass may sound smoother, but correspondingly less impactful, until the subwoofer volume is increased.


5. It is important to understand that two different things are occurring during a calibration. The initial calibration process insures that equal volume levels, from all of the channels, will arrive at the MLP at the same time. The room EQ process (that is also part of the calibration) sets filters, for all of the channels, in an effort to improve the overall sound quality in the room.


6. There is a model calibration procedure shown in Section I-B which may help you to achieve an optimum calibration. Starting with good speaker setup, and a good calibration, can take some effort. But, doing those things can make an audible difference in the resulting sound.


7. As noted, when an AVR calibrates your audio system, all of your channels including your subwoofer(s) will typically be set to play the same volume at the main listening position (MLP), and you will be listening to Reference volumes when you are at a listening level of 0.0 MV (master volume). Most people probably listen at an average volume of about -20 MV to -10 MV. Individual listening volumes, however, may vary much more widely than that.


8. Since all of the channels are now playing equal volumes, and since we don't hear low-frequencies as well as other frequencies, after calibration most people will need to add more bass to their audio systems. That is particularly the case when we listen at below Reference volumes, where bass frequencies were designed to be in better equilibrium with frequencies in our normal hearing range. As we drop below Reference, bass frequencies drop-out of our hearing more quickly than other frequencies do.


9. We can compensate for the audible reduction in bass by turning-up the volume of the subwoofers, and how much volume to add is strictly a user preference issue. To state this in a different way, after an Audyssey calibration, very large subwoofers will be playing at exactly the same volume level as very small subwoofers would be, since all of the channels in a calibrated HT system are level-matched to play the same SPL at the main listening position. In order to use the greater headroom of more powerful subwoofers, it is simply necessary to turn-up their volume.


10. It is desirable to add most of your subwoofer volume increase with your subwoofer gain control, while not letting your AVR sub trim go above about -5. Section II explains the best ways to use the subwoofer gain in some detail, and explains why it is generally advisable to keep AVR subwoofer trim levels well in negative numbers.


11. Some Denon AVR's have a feature called Subwoofer Level Adjust. That feature should typically be turned off, and any volume adjustments should be made either with the subwoofer's gain control, with the subwoofer trim control in the Audio menu, or with the trim control in the Speaker: Manual: Test Tone area of the AVR. Again, in making any subwoofer trim adjustments, it is desirable to keep the trim levels at -5 or lower. To add more subwoofer boost in excess of a -5 trim setting, it is always possible to use the gain control on the subwoofer.


12. It is important to understand that Audyssey will measure your speakers, at their specific positions in your room, and your AVR will set preliminary crossovers in accordance with its own programming. Those aren't actually recommendations. They are just observations based on the measured response of your speakers. After calibration, reset crossovers as suggested below.


13. If there is a subwoofer in the system, speakers should typically be set to Small, and crossovers should typically be set at 80Hz or higher. The LPF of LFE, in the AVR, should typically be set to 120Hz (which is usually the default setting in our AVR's). There are exceptions to that, which are explained in Section III, but this is typical best practice advice for starting with an HT system.


14. It is always acceptable to raise crossovers from their initial calibration setting, but it is not generally desirable to lower them from wherever your AVR set them. Among other things, Audyssey will not be EQing speakers below the crossovers set during the calibration process.


15. If you have Audyssey, you can add subwoofer boosts on top of Dynamic EQ, or turn off DEQ and add your own subwoofer boosts. Section V explains DEQ in detail. Experimenting with it on, and with it off, is a good idea. Turning it off will probably require you to add more subwoofer volume, but for some people, it may change the sound in a positive way.


16. You can turn Audyssey off, to hear how things sound without room EQ, and then turn it back on without changing the room correction filters that it set for any of the channels. And, you can experiment with Audyssey Flat. Audyssey and DEQ are always the default settings after a calibration. Audyssey Flat and/or DEQ off are user preference options.


17. You can change any settings in your AVR without affecting the room correction filters that Audyssey sets.


18. If you make a significant change to a room, such as moving a speaker or a subwoofer, or adding room treatments, you should recalibrate.


Specific sections of the Guide deal with how to setup speakers and how to perform good automated calibrations. Other sections deal with setting crossovers, and selecting and positioning subwoofers. Most of the terms used in the Guide are defined in the first two sections. Readers can choose what sections to read for a particular purpose, although some of the information does build as you go.



* The Subwoofer Guide is organized into the following major sections. Each section (and subsection) is hyperlinked so that readers can go directly to that part of the Guide by clicking on it in this Table of Contents. Readers can also link a specific section (or subsection) for others by right clicking on it in the Table of Contents, and copying the link location.


Table of Contents:


Introduction to the Guide:


Section I: Room/System Setup and Sound Quality:

I-A: Distortion, Speaker Placement, and Room Treatments

I-B: Room EQ and Calibration Techniques


Section II: Audio System Calibration and Subwoofer Levels:

II-A: Audyssey Calibration And Dolby Reference

II-B: Why We Add Bass After Calibrations

II-C: Where And How To Add Bass

II-D: Master Volume Levels And Sub Boosts

II-E: Gain Settings And Maximum Sub Output


Section III: Setting Crossovers:

III-A: Crossovers From Speakers to Subwoofers:

III-B: Low Frequency Effects Channel:

III-C: Cascading Crossovers:

III-D: LFE+Main:


Section IV: Audyssey EQ and Multiple Subwoofers:

IV-A: Setup and Calibration:

IV-B: Room EQ:


Section V: Audyssey Dynamic EQ and Dynamic Volume:

V-A: Dynamic EQ

V-B: Dynamic Volume


Section VI: Audyssey Thread History of Recommended Subwoofer Trim Settings:


Section VII: Bass Frequencies, Room Gain, and The Equal Loudness Contours:

VII-A: Bass Frequencies:

VII-B: Room Gain:

VII-C: The Equal Loudness Contours:


Section VIII: Bass Preferences, and Subwoofer Selection and Placement:

VIII-A: Sealed Versus Ported Subwoofers:

VIII-B: Comparing Subwoofer Performance:

VIII-C: Selecting Single Versus Multiple Subwoofers:

VIII-D: Internet Direct Subwoofers:

VIII-E: Subwoofer Placement in a Room:



Introduction to the Guide:

The most commonly asked question on the Audyssey thread, and on a number of subwoofer owners' threads, involves subwoofer settings. People who have new audio or home theater (HT) systems, or who have upgraded and/or added subwoofers, are naturally anxious to be able to get the most from them. In addition, there is a fairly universal perception that bass volumes sound somewhat softer after running Audyssey (or other systems of automated calibration), and people are frequently curious about whether that perception is normal, and if so, about the best way to increase their bass.

The Guide was originally written to explain why it may be perfectly normal to perceive bass levels as lower, after running Audyssey or other forms of automated calibration. And, it was written to explain the best ways to use a combination of subwoofer gain and AVR trim to make bass boosts. In attempting to address issues involving subwoofer boosts, however, I have found that it is also helpful to understand some of the basic principles of HT system calibration, and their relationship to Dolby Reference.

And that, in turn, has led to discussions of how we hear bass frequencies in relation to other frequencies, and of how our preferences influence our subwoofer selections and our subwoofer placements. As the Guide has continued to expand, I have also decided to try to address some fundamental issues of speaker placement and of how rooms influence the sound we hear. And, I have added some general suggestions on techniques to use during the Audyssey calibration process.

* Much of the information in the Guide is applicable to non-Audyssey systems of audio/HT calibration and automated room EQ.

[For people wanting a quick answer to a question about subwoofer levels. It is normal to add approximately +3db to +6db of boost to subs after running Audyssey. That is a typical boost even with Dynamic EQ (DEQ) engaged. I should point out, however, that this note concerning an average boost of about +3 to +6db does not constitute a recommendation. It is just an observation of what most people typically seem to add when DEQ is engaged. Individuals must decide for themselves whether to add any sub boost at all, and if so how much.]

It may be worth pointing out that we all like having some reassurance that we are operating our audio systems correctly, with the "correct" settings, and that we are getting the maximum benefit from them. I believe, though, that the more that we understand some of the basic audio principles involved (which I certainly didn't when I first got into home theater) the more confidence we will be able to have in our own individual setting preferences, and in the resulting sound quality. As with almost everything in audio, sound quality can be very subjective.

Part of the key to developing a satisfactory audio system, in my opinion, is informed experimentation. AVS gives all of us an opportunity to share information with each other, so that we can enjoy our audio/HT systems more, and be more confident in the choices we make. And, that's really what the Guide is about--sharing information and, in some cases, speculation. I have learned a lot from writing it, and continue to do so as I try to add more detailed explanations. I hope that others will find it of benefit to them as well.


Sections I Through VIII:

* There are eight major sections in the Guide, which begins in this post, and continues in Post 2. All but one of the eight sections are divided into multiple subsections, which cover a wide range of related material. The first section involves system setup, and how rooms influence the sound quality we hear. It offers some advice on room treatments and also some calibration technique tips. The second section explains how Audyssey calibrates our audio systems. It is broken down into subsections which are labeled. The section explains the basic principles of how Audyssey works during the set-up process, and how it EQ's our audio systems.

The second section also explains how audio systems are calibrated to a Dolby/THX Reference standard. It offers some best practice advice for getting the most from our subwoofers, and explains the relationships among subwoofer gain, AVR trim levels, and master volume levels. The section emphasizes the general desirability of keeping subwoofer trim levels in the negative range and using subwoofer gain to add sub boosts. And, it explains different ways to do that.

* Since bass management is such an important component of all our audio systems, the third section explains some basic principles to consider in setting crossovers. And, some creative crossover suggestions are discussed. Following the crossover section, there is a fourth section which explains how Audyssey calibrates and EQ's multiple subs. The fifth section examines Audyssey's DynamicEQ (DEQ) and Dynamic Volume in some detail, and the sixth section explains something of the Audyssey Thread history with respect to setting subwoofer trim levels.

* The last two sections have relatively little to do with Audyssey directly, or with room EQ in general, although there are some overlaps with room EQ. But, understanding some fundamental audio concepts can enhance our ability to get the most from our audio systems. The seventh section is a longer one which explores some of the general relationships among bass frequencies, tactile response, how room gain amplifies our bass, and how the way we hear bass frequencies influences the settings we use. In that section, the Equal Loudness Contours, which illustrate normal hearing, are explored. Ideally, Sections VII and VIII will be read in conjunction. I think that it would be especially helpful to read subsections A and B, of Section VII, before reading Section VIII.

* The eighth and final section provides some fairly detailed guidance for people who are in the process of selecting subwoofers, and also provides some basic advice on positioning them within a room. Many people start threads on which subwoofer to buy, without having a good idea of what they are actually looking for, or how to distinguish among the options which people suggest to them. Section VIII will help with that. The section starts with some general rules to follow in selecting subwoofers--sort of like the Cliff Notes shown above.

The five subsections in Section VIII go into considerable detail in describing differences between sealed and ported subwoofers; ways to compare subwoofer performance; the pros and cons of initially buying a single large sub, versus two smaller ones; and some descriptions and comparisons of some of the more popular ID (Internet Direct) subwoofer companies. Since subwoofer placement in the room is so important, a separate subsection is devoted to that.


[It is worth noting that the Audyssey FAQ, which is linked in my signature, and especially the Technical Addendum to the FAQ, have a wealth of additional information and explanation on some aspects of Audyssey which are not covered in this Guide. Interested readers are highly encouraged to read the FAQ for both quick answers and for in-depth detail. Wherever the Guide conflicts with the FAQ, the Guide presents more current information, as explained in the Audyssey Thread History in the sixth section.]


* HT owners who are encountering specific problems with their frequency responses, or who are curious about what is actually happening in their rooms, may wish to implement REW, which is a free download. REW will require a calibrated measurement microphone and a computer (preferably a laptop) which can be connected to their AVR's or AVP's. Anyone interested in learning more about REW, and how to implement it, is encouraged to consult the following step-by-step guide by AVS member @AustinJerry .

https://www.dropbox.com/s/zdhq72a1pu...%20Version.pdf





Guide to Subwoofer Calibration and Bass Preferences



Section I: Room/System Setup and Sound Quality:

There are a number of factors which can affect the sound quality in our rooms. Those factors include our speaker choices and their placement, distortion from the room itself, and the use of room treatments and automated room correction. There are a number of potential reference sources which can help us with our initial system setups, in terms of positioning our speakers, or with room treatments, or with specific room EQ calibration tips. But, I think that it would be worthwhile to try to address some basic concepts in this Guide, so that it can be a more general resource.

With that in mind, I would like to try to explain some basic concepts of system/room interaction and to offer some general advice on the relationships among the room, our system setups, and our sound quality. I would also like to offer some general tips on performing a successful calibration with room EQ.

It would probably be helpful to define some terms that are used in audio, and throughout the Guide. (Additional definitions and abbreviations are presented as they are used in individual sections.) I will start by using a good online definition of sound. Sound in air is made when air molecules vibrate, and move away from the vibrating source, in a pattern we refer to as sound waves. In our context, the vibrating source would be our transducers--our speakers and our subwoofers.

Sound pressure level (SPL) is a measurable quantity of sound volume. It is measured in decibels (db). "Loudness" is not a measured quantity of volume; it is a perceived amount of sound. For instance, "That sounds really loud!" is a very different statement than "The SPL in the room is 100db, as measured at the main listening position (MLP)." Loudness is a perception of how something sounds, while SPL is a measurable quantity of sound volume. The distinction between those two terms becomes very important when we are selecting our preferred listening volumes and our bass volumes.

The Subsections in Section I are as follows:

A: Distortion, Speaker Placement, and Room Treatments

B: Room EQ and Calibration Techniques


I-A: Distortion, Speaker Placement, and Room Treatments

To begin the discussion in this section, it may be helpful to talk a little about distortion, in the most general way. There are much more sophisticated explanations of distortion that readers can investigate, but I want to try to cover some very basic concepts. First, distortion can be defined as "any alteration in the waveform of an audio signal." (All sound is composed of frequencies--sound waves--so any alteration in the waveform can affect what we hear.)

In practice, "distortion" is sort of a catchall term for nearly anything which adversely affects perceived sound quality. Some audio sources we listen to may have compression in the source itself, or distortion in the specific recording. Speaker, subwoofer, and AVR makers go to great lengths to minimize distortion, and particularly Total Harmonic Distortion (THD) in their products. Anyone interested in understanding more about audio distortion, from audio sources and transducers, is encouraged to consult more authoritative articles such as this one:

https://www.axiomaudio.com/blog/distortion

Since frequency response (FR) is mentioned so often in audio discussions, and in the Guide, it is also worthwhile to define what is meant by a "smooth", or "even", or "flat" frequency response. A smooth, even, or flat frequency response would be one in which no frequencies are playing significantly louder or softer than other frequencies. (We will often refer to peaks and dips in the FR.) In music, and in movies, some sounds will be deliberately emphasized over other sounds--such as a trumpet solo in a music recording, for instance, or low-bass in movies. That would be inherent to the recording and not dictated by the room. However, the room will inevitably influence the evenness of the FR in unpredictable ways, causing some frequencies to sound louder or softer than others. We can remediate that potentially uneven FR with good placement of our transducers, with room treatments, and with room EQ.

Unless we are creating a house curve to intentionally change the interaction of our recordings and our room in some specific direction (such as having less treble, or having more bass) most of us would not typically want the room to arbitrarily dictate which frequencies are louder or softer than others, in a way that has nothing to do with the original intent of a recording. Where a frequency response is reasonably "flat" or "smooth", all frequencies are playing at approximately the same volume, at a particular point in space, unless we, or the recording, dictate otherwise. The selected point in space where everything is more-or-less in equilibrium is the main listening position (MLP). As we move away from the MLP, the relative smoothness of the FR may change, as some frequencies become slightly louder or softer, compared to others.


1. Room Distortion:

If we drive a particular speaker, or subwoofer, or AVR too hard, we may introduce distortion into our signal chain. But, even if we are playing content at moderate volumes, once we play an audio recording, from any kind of transducer (a speaker or subwoofer which converts electrical energy into sound) located inside a listening room, the interaction with the room itself influences the sound that we hear. Not all influences are negative, as the room may influence the sound we hear in a number of positive ways. But, not all room influences are positive, either. It is the negative influences caused by interaction with a room, which we may characterize as distortion, that I am going to try to address in this brief discussion.

The later section on Bass Frequencies and Room Gain explains in more detail the difference between frequencies below the Schroeder, or transition frequency in a room, and frequencies above that point. Briefly, below the transition point in a room, which is typically about 200Hz or lower (depending on the size of the room), bass sound waves collect as standing waves. And, some frequencies are amplified and some are cancelled as a result. Overall, however, room boundaries reinforce the bass SPL. Higher frequencies, especially above about 500Hz, behave differently, bouncing around the room until they run out of energy.

Higher frequencies may not really reinforce the measurable SPL in a room, but they may sound as if they do, especially in very lively-sounding rooms. In a room with a lot of hard surfaces, some of what we are perceiving as loudness is probably distortion. We typically perceive high-frequency distortion as being louder than undistorted sound. In fact, we may perceive it as being much louder. And, most of us don't seem to tolerate higher frequency distortion as well as we do lower frequency distortion.

For instance, we may describe high-frequency distortion as screechy or piercing (which suggests a somewhat painful sound), where we might describe low-frequency distortion as muddy or boomy (suggesting a lack of clarity). The words we use to describe the different types of distortion are significant, and the difference in the way we hear distortion at different frequencies can be important. Understanding differences in the way we hear some frequencies, and the distortion that may accompany those frequencies, can be helpful as we try to improve the sound quality in our rooms.

[As an aside, home theater (HT) rooms of about 20,000^3 or less sound perceptually louder than commercial cinemas, which are always much larger than even very big HT's. A number of audio experts estimate that our HT's may sound anywhere from about 5 to 8db louder than the same SPL would seem when played in a commercial cinema. The 5db to 8db range of difference varies with the size of the room, with very small rooms being as much as 8db louder sounding. As noted above, some of that louder-seeming sound might be due to distortion. But, even in a heavily treated room, the smaller size of the room would appear to somewhat amplify the loudness level we would perceive. I have heard room treatment consultants estimate that a well-treated room might sound up to ~2 or 3db less loud than it did prior to room treatment. But, it would still sound significantly louder than the same SPL played in a commercial theater.]

In order to understand how rooms can create distortion, it is important to understand something about how sound waves behave in a room. For the purpose of the discussions in the Guide, I am defining bass frequencies as those frequencies below 500Hz. I would probably define mid-range frequencies as the range from 500Hz to about 2500Hz. Frequencies from about 2500Hz and up would be high-frequencies (the treble range, typically played by tweeters). Sound waves are vibrations, with different lengths and different vibration speeds. (They vibrate--moving back-and-forth, sort of like a slinky toy.) They all travel at the speed of sound (which is approximately 1 millisecond per foot at sea level), but how they behave in a room, and how we perceive them when they reach us, makes all the difference in what we hear.

As frequencies get higher, the vibrations become much shorter in length, and they oscillate (move or travel back and forth between two points) much faster. They also tend to travel in straight lines unless absorbed, or redirected by contact with some surface. Longer frequencies oscillate more slowly, and they tend to bend rather than always simply ricocheting in a straight line. They also go right through solid objects in a way that higher frequencies cannot. Those differences can have a direct bearing on what we hear in a room. And, the differences in sound wave lengths are significant.

For example, a 10Hz frequency is only 10 cycles per second (meaning that it vibrates 10 times per second), and the wavelength is about 112' long. By contrast, a 100Hz frequency is 100 cycles per second, and the wavelength is now only about 11' long. As we go up in frequency, the wavelengths get shorter and shorter. A 1,000Hz frequency is 1,000 cycles per second, and the wavelength is just a fraction over a foot long. By the time we are up to 10KHz, and 10,000 cycles per second, the wavelength is only a little more than an inch long. The table linked below illustrates frequency length:

http://www.soundoctor.com/freq.htm

Where low-frequencies oscillate relatively slowly, bending and pooling in corners, the frequencies above the transition point (which is usually above about 200Hz or so in most rooms) oscillate increasingly faster, and bounce around in a room, reflecting off of hard surfaces like a billiard ball bouncing off the cushions on a billiard table, until they run out of energy. The billiard ball analogy actually falls a little short, as all frequencies, but especially mid and high-frequency sound waves, ricochet off of all six surfaces in a room (the four walls, the floor, and the ceiling) until they run out of energy, or are absorbed by something.

They may also bounce off of other hard surfaces, such as table tops or other furniture. Those reflected sound waves which don't arrive very close in time to the first sounds to reach our ears, may be somewhat ignored by our brains. We typically hear the first-arriving sounds as being louder, and we concentrate more on those sounds. Our brains can typically filter-out (or separate, if sounds are very delayed) most of the late-arriving sounds, in order to concentrate on those louder, first-arriving sounds. But, higher frequency sounds arriving within about 6 milliseconds (ms) or so of each other may be associated with distortion, although I suspect that can vary somewhat, depending on the individual, and on both the frequency and the SPL of the sounds. (As noted above, sound travels at approximately 1ms per foot at sea level.)

Sounds arriving close behind the first-arriving sounds from our speakers, within approximately that 6ms window or so, may distort the sound we hear. As a best case, early reflections may make sounds seem louder or more three-dimensional. But, they may also be perceived as contributing to a harsher or more strident sound.

For instance, a reflected upper-mid or high-frequency sound from a wall or other hard surface, which arrives at our ears at almost the same time as the direct sound from a speaker, may create a type of distortion, as our brains will have difficulty combining the separate close-arriving sounds into a single coherent sound. In that case, the resulting distortion simply represents a loss of clarity in the sound we are hearing. It may, or may not, be particularly noticeable depending on whether we are actually accustomed to hearing undistorted sound from our audio systems.

Our initial system setup can contribute to the relative clarity of the sound that we hear in a room. Some suggestions regarding subwoofer placement in a room are addressed in the last section of the Guide. In this first section, though, I would like to concentrate on the speakers, and on their relationship to the room and to the listening positions. I would particularly like to concentrate on the speakers located in the front soundstage, as they carry the majority of the content in a movie or music recording, and will have a much greater influence on our overall sound quality.

(There are numerous on-line guides illustrating placement objectives for surround and overhead speakers, but I believe that insufficient attention may often be paid to the front soundstage which is so critical for both music listening and movie/TV viewing.)

In the context of the discussion of how sound behaves in a room, anyone who is interested in how specific frequencies sound, and how the room may influence our perception of sound, may find the following video helpful. In the video, individual frequencies are played, starting at 20Hz and continuing to 20,000Hz. The test should be conducted only in stereo, and the sound should always be coming from the center. Where the sound seems to come more strongly from one ear than from the other, the room your speakers are located in is influencing the sound, as room reflections from one speaker or the other momentarily seem to dominate the sound. It's a great illustration of how the room influences what we hear.

Something else that the video demonstrates is how low some low-bass frequencies may actually sound to us. Most of us probably believe that some frequencies we hear are lower than they really are. Getting a feel for how low in frequency an 80Hz, or 60Hz, or 40Hz tone actually is can be quite revealing. That is why a consistent theme in the later sections on bass is that we feel as much as hear frequencies below about 30Hz. And, it can take a fair bit of SPL to hear and feel those very low-frequencies, in conjunction with the sounds in our more normal hearing range, which may be accompanying them.

Pure tones, like those in the video, and the complex sounds which we hear in music and in movies, are very different. Complex sounds may consist of multiple frequencies, each containing both fundamental frequencies and harmonics (overtones) of those frequencies. Most of the time, the very low-frequencies are simply adding some bass weight to sounds, rather than standing-out as distinct sounds themselves, although there can be some exceptions to this with some low-bass content.




2. Front Speakers:

To continue the discussion of room distortion, let's start with the front speakers. Many speakers require some distance from the wall behind them in order to achieve good sound quality. Electrostatic speakers may require even more separation from a wall as they don't radiate sound in the same fashion as direct-firing speakers. Bipole speakers are designed to radiate sound both forwards and backwards toward walls. And dipole speakers are designed to radiate most of the sound backwards toward walls, in order to create a more diffuse sound field.

Most speakers are direct firing--meaning that the sound primarily travels in the direction they point toward. With direct-firing speakers, the sound leaves the drivers, in the front of the speaker, in a cone-shaped cluster of sound waves. But, even with direct-firing speakers, other sound waves travel backwards, through the speaker cabinet, to reflect off the wall behind the speakers. (Rear-ported speakers may also require some wall clearance to operate properly.) Bass frequencies, of course, will radiate omnidirectionally from all sides of the cabinet.

Several things are, therefore, happening when a speaker plays inside an enclosed space, such as an audio/HT room. First, the direct sound is arriving at the listening position. Second, sound waves which bounced off the wall behind the speaker, are arriving just a little later at the listening position. Third, sound waves which bounce off the side-walls are arriving at the listening position just a little behind the direct sound. And fourth, sound waves which bounce off the floor and ceiling (and to some extent off the wall behind the listener) are arriving at the listening position just a little behind the direct sound. And, all of those sounds will arrive very close together. Our brains are remarkably adept at sorting-out all of those very early and later-arriving sound waves into a single coherent sound. But, when the sounds arrive too close together, they can't, and we hear distortion.

Opinions vary as to how to address distortion caused by the speakers and the room. For instance, speaker makers typically try to walk a fine line with respect to having drivers with wide horizontal dispersion, in order to provide a wider-sounding soundstage and in order to maintain the same SPL off-axis as they do when they are pointed straight ahead. But, if they get too wide, early reflections from side-walls may become more of a problem. Again, it is helpful to remember that sound waves above about 500Hz leave a typical direct-firing speaker in a cone shape, with the narrow portion of the cone closest to the speaker, and the mouth of the cone getting wider as the sound waves move further away from the speaker.

I think that most speaker makers try to achieve about 30 degrees of horizontal off-axis dispersion, at a distance of about 3 to 4 meters. That would represent the typical listening distance for most tower, or larger bookshelf speakers. Vertical dispersion is usually more restricted. I believe that about 15 degrees is typical in order to avoid too much reflection from floors. It is important to note that some very large speakers are intended for relatively large rooms, and for longer listening distances--up to 5m or more. Knowing this can be important, because large tower speakers, which are too close to a listening position, may not be able to achieve as good inherent sound quality, without the intervention of room EQ, as speakers which are designed for that typical listening distance referred to above.

[This issue of having large speakers, designed for greater listening distances, may be worth pursuing a little further. If we think of a typical three-way tower speaker, the speakers (woofer, mid-range, and tweeter) are arranged vertically, with each driver spaced a prescribed distance from the other drivers, so that they will voice appropriately at a particular distance. (There is more to it than just the driver spacing, but this conveys the general idea.) That means that the drivers are designed to all play at the same volume at a designated distance. The distance is usually a range. I believe that 3 to 4 meters is typical for a tower speaker, or for a large bookshelf speaker.

Smaller bookshelf speakers, however, may be designed for a listening distance of only about 2m. For instance, My two-way Audioengine A5+ speakers only have a tweeter and a 5" mid-range/woofer driver. They are designed to be used primarily as desktop computer speakers, without an external amplifier or AVR. And, they are self-powered for that purpose. They are intended to be separated by anywhere from about 3' to 6' from each other, and they are designed to voice most effectively at about those same distances. To facilitate that, the drivers in each speaker are in a relatively short vertical cabinet, and the drivers are spaced closely together. Spaced appropriately, they image extremely well in my desktop system, with good sound separation and a good phantom center in stereo.

A very large tower speaker which is designed to be employed in a large room, on the other hand, may have speakers which will play at equal volumes at a distance of 5-6 meters or more. I actually have a pair of very large (18^3) speakers like that. They were specifically designed to be separated from each other by about 20' and to each be about that same distance from the main listening position. They were always intended for larger listening venues, and they don't really sound as good in a smaller room. Automated room correction can help the various drivers, within a vertical cabinet, to play the same volumes at the calibrated listening distance. But, starting with speakers which are actually sized reasonably appropriately for the listening distance is still a good idea. And, this is a concept which is sometimes overlooked when people select speakers.

As noted above, automated room EQ can help to alleviate issues with our listening distance from our speakers, because EQ filters (control points) can be set at various frequencies to insure that all of the frequencies reach the listener at the same volume level. That is slightly different from the distance setting which governs the arrival time of the overall sound from a speaker. But, when speakers are situated at approximately the appropriate distance from a listener, and at the appropriate height with respect to the listener, all of the drivers in a well-designed speaker should start-off playing at about the same inherent volume level, at least prior to room influences. That makes it much easier to achieve a smooth frequency response, and good sound quality, even before automated room EQ sets filters to improve the speakers' specific interaction with the room. It should be noted that not everyone will choose to use room correction for higher frequencies. More on that later.]

Finding the sweet spot for a particular pair of speakers, in a particular room, usually requires some experimentation. For instance, if a speaker is pointed too far away from the listening position, there may be an adverse impact on the high-frequencies (which are typically very directional), and it may result in an increase in early reflections from the side-walls. So, determining how far to keep speakers away from the wall behind them, and from the wall beside them, and how much to toe-in the speakers toward the listening position, is something that may require some trial-and-error. Ideally, listeners will try to perfect the sound quality of their speakers, in their rooms, before running room EQ. That is because the better the native response is to start with, the less that room correction is likely to interfere with the quality of the sound. (There is more on this subject in the following subsection on calibration tips.)

The old audio rule-of-thumb about trying to keep a roughly equilateral triangle with respect to the two front speakers and the MLP (main listening position) is still a pretty good starting point, in order to determine the best separation for the front speakers. That doesn't relate directly to distortion, but it does relate to the ability to easily generate good stereo and HT imaging, and it may also relate to the ability to create a realistic soundstage width, which extends beyond the screen for HT, or beyond the speakers for stereo. So, soundstage width, and the overall benign or malign influence of wall reflections, may also be factors when trying to decide how close front speakers should be to side-walls.

Another factor involving the placement of the front speakers concerns the early reflections from side-walls referred to above. It was suggested earlier that most sound waves leave a direct-firing driver in a cone shape, and that most speaker makers try to get about 30 degrees of off-axis horizontal dispersion. Depending on how close a speaker is to a side-wall, and how much (or how little) the speaker is toed-in toward the MLP, the more that early reflections from side-walls may be an issue. That might be especially true for speakers where speaker manufacturers specifically suggest pointing speakers straight ahead, or even toeing them away slightly, as that could result in more side-wall reflection.

Speaker positioning, with respect to early reflections from adjacent walls, is entirely a matter of personal preference. Some manufacturers of acoustic panels, and some AVS members, believe quite strongly in always treating side-walls for early reflections. On the other hand, listening tests conducted by Dr. Todd Welti, Dr. Floyd Toole, and others, indicate that most people prefer the additional ambient sound provided by untreated side-walls. According to those listening tests, most people hear a wider and more realistic sounding soundstage when those side-wall first reflection points are either untreated, or at least, less heavily absorbed. For some, that wider soundstage may make the sound seem to come from beyond the sides of the speakers.

Listeners need to make their own determinations of what they prefer in their specific rooms. People wanting to experiment with their preference for more ambient sound from side-walls, versus treating those early reflections, can try putting some sort of fabric on a wall temporarily, to test whether or not they like the result. Determining where to put a piece of test fabric, such as a blanket or folded towel, is fairly easy. One person sits at the MLP, while another person holds a mirror on the side-wall. Where the seated person can see the speaker in the mirror is the first reflection point for that wall. If a mirror test results in subjectively better overall sound quality, for a particular listener, he now knows where to install nicer and more permanent acoustical treatments.


3. Center Channel:

The center channel (CC) is an extremely important component in an HT system, as it carries so much of the content in movies and TV programs. And, the placement of the center channel can be very important for dialogue clarity in both movies and TV. The front speakers are typically designed so that the tweeters are more-or-less at the ear level of a seated person. That is typically the case with both tower speakers, and with bookshelf speakers on stands. But, most center channels (except for those placed behind an acoustically transparent screen) are necessarily either below or above the display (or screen). And, they are typically horizontal speakers in order to facilitate those placement options.

When CC's are placed above or below a screen, higher frequencies (which are highly directional) are not beaming straight at most listeners. As noted earlier, vertical dispersion, from both mid-range speakers and from tweeters, is usually restricted in order to avoid too many early reflections from floors. Consequently, when the speakers don't point more directly at listeners, that can interfere with overall sound clarity, and particularly with dialogue intelligibility.

(This is just speculation on my part, but I believe that where a center channel is not pointing directly toward us, we are still likely to be able to hear most of the lower frequencies in the human vocal range, and that would include most of the vowels. But, if the higher frequencies are attenuated, due to being so directional and pointing away from us, or if they are distorted, some of the consonants such as "B", "C", "D", "G", "P", and "T", especially at the beginning or end of words, may lose some clarity. And, that may make it harder to understand what is really being said. That might especially be the case with softer, or more rapid speech, or where foreign accents are involved.)

It is advisable to make sure that CC's are pointed as much toward ear level as possible. Where a CC is above the screen, a shim placed under the back edge of the speaker will point it down more toward ear level. Where a CC is located below a screen, a shim placed under the front edge of the speaker will point it up toward ear level. Getting the center channel pointed more directly toward ear level can sometimes make a substantial difference with respect to audio clarity. And, when a center channel points directly toward our ears, our brains will usually adapt even more readily to the illusion that the speech we hear is coming from the mouth of the speaker on the screen.

Another placement issue that can affect both overall audio clarity and dialogue intelligibility, is the extent to which a CC is recessed on a shelf or within a cabinet. As sound waves emerge in a cone shape, high-frequencies reflect off the top and bottom (or sides within a cabinet) of a shelf, causing what's called comb-filtering effects. The result of those very early reflections from the shelf, arriving so close in time to the direct sound waves, can create a distorted sound and interfere with our ability to hear dialogue clearly. Pulling the center channel forward so that the front of the speaker protrudes about an inch clear of the shelf or cabinet will prevent comb-filtering.

[Where it is possible to do so, it is always better acoustically to not enclose speakers inside a cabinet. It may be counterintuitive to think that enclosing a wood cabinet speaker, inside another wood cabinet, would negatively impact the sound. We may think that the cabinet would act to reinforce the sound and add more bass. It should reinforce the sound, from the standpoint of having sound waves bouncing around inside the display cabinet or whatever the enclosure is. But, those sound waves which bounced off the sides, top, bottom, and back of that cabinet will reach our ears just enough later to interfere with the clarity of the direct sound from the speakers. If possible, speakers should be at the forward edge of a shelf, and if they can be taken completely out of cabinets, that's even better.]

Irrespective of good center channel positioning, it can sometimes be a little difficult to hear dialogue in movies clearly. There are a number of factors that could contribute to a loss of clarity in general, or to dialogue intelligibility in particular. Sometimes, center channels are simply a little too weak to keep up with the other speakers in a system, or with the subwoofers. The CC may be the most important speaker in an HT system, as it is involved in playing at least about 80% of the content in a movie, and nearly all of the dialogue. Heavy bass boosts can sometimes contribute to difficulty in hearing dialogue, especially where crossovers of 100Hz or higher are used for the CC. When a higher crossover is used, strong bass boosts may make voices sound deeper, thicker, and harder to understand. In that case, a better CC, which can utilize an 80Hz or 90Hz crossover, may be helpful.

The use of DEQ (an Audyssey program) may also affect dialogue intelligibility, in some cases, as it slightly boosts the bass in all of the channels, including the center channel. And, it boosts the surround channels by about 1db for every -5db of master volume. The louder sounding surround channels could make it harder to hear the CC. (DEQ is discussed in detail in a later section.) Even without the action of DEQ, ambient noises, music, and special effects in a movie or TV soundtrack, may make sounds from the center channel harder to hear, and voices harder to understand. Many people prefer to boost the volume of their center channel a little, depending on the specific program, or on their specific circumstances.


4. Early and Late Reflections, and Room Treatments:

I have mentioned early reflections from side-walls that may involve the front speakers, and from cabinets and shelves that involve the center channel. There are other areas of the room that involve both the front speakers and the CC. If the floor between the front soundstage and the listening position is a hard surface, such as concrete, tile, or wood, there are almost certainly going to be some early reflections which can interfere somewhat with sound quality. In that case, an area rug in front of the listening position can be very helpful. Typically, people will use a foam rubber pad under the rug to prevent slipping, and to partially attenuate reflections from frequencies above about 1000Hz.

Coffee tables directly in front of a sofa can also be a source of early reflections, as high-frequencies bounce off the hard surface and reach our ears just behind the direct sound from the speakers. Any softening influences that can be added to the table can help. Even scattering some magazines on a table can help to disperse high-frequencies, so that they don't reflect directly toward a listening position. Remember that higher frequencies leave the speaker in a cluster, and they tend to travel in a straight line, so scattering (diffusing) them can also help to reduce the distortion we may hear.

Two other areas of a room can also be particularly problematical. First, as noted earlier, speakers radiate some sound waves backwards. If they are fairly close to a wall (perhaps within a foot or two), it can be advisable to put something such as an acoustic panel, or an oil painting, or decorative fabric behind the speaker. That will prevent early reflections from the wall from interfering with the clarity of the sound reaching our ears. If a dense acoustic (rockwool or fiberglass) panel is used, in a 1" or 2" thickness, it will typically only affect frequencies above about 300Hz or 350Hz. Foam rubber treatments, such as the egg crate versions shown in a video below, may not absorb frequencies below about 1000Hz, although they can also act as diffusers. But, enough of them in a room can have a significant effect, especially with high and mid-range frequencies.

If a listening position, such as a sofa, is within about 2' or 3' of a wall, it may be very helpful to put something on the wall behind the sofa. Ideally, something like an oil painting, or a tapestry, or an acoustic panel, would be used in order to disperse or absorb sound waves. Otherwise, those mostly high-frequency sound waves would bounce off the wall behind us, and into our ears, just a few milliseconds behind the direct-arriving sound from our speakers. Reflections from that back wall could also interfere with our audio clarity. Aside from hard surfaces, such as a floor or table, directly in front of a listening position, this might be the most important early reflection point when a listening position is near a wall.

This idea of reflections from front and back walls, and from floors and table tops, is probably worth expanding on a little bit. If there are reflections from a cabinet top holding a center channel, or from the floor, or from a table located within a few feet of a listening position, the result may be the same. That is because, in each case, the sound would arrive at a listener's ears only a few milliseconds behind the first arriving sound. (Again, sound travels at ~1ms per foot at sea level.)

The same thing would apply to reflections from a wall behind a speaker, or from a wall just behind the listener. In both of those cases, the reflected sounds would arrive just a few milliseconds behind the first arriving sounds. For instance, let's assume that the main speakers are pulled out 2' from the front wall. Some mid and high-frequency sound waves would travel backwards through the cabinet to reflect off the wall behind them. Their round trip of 4' to strike the wall and ricochet toward the listening position, would put their sound arriving ~4ms behind the sound waves that came from the drivers firing directly toward the listening position. That 4ms second difference could be just enough to create some audible distortion.

Distortion could also occur from sound waves reflecting from a wall behind a chair or a couch. If the couch were within a couple of feet of a wall, then sound waves would go past the couch, hitting the wall and ricocheting back toward the listening position. That could also create distortion, as the sound waves would still arrive at our ears well within that approximately 6ms window referred to earlier. (Again, when our brains can easily separate sounds, the first arriving sounds will seem louder, and we will tend to ignore the later arriving ones.)

I have observed that when someone is able to move the couch out about 3' or so from a wall, the problem is much less noticeable. In that case, the reflected sounds would be arriving a full 6ms or more behind the first arriving sounds. And, in some cases, that is just enough for the first arriving sounds to mask the later arriving ones. When in doubt, putting some acoustical material on the wall behind a couch is a good idea.

Examples of decorative products which can absorb higher frequencies, and which can be strategically placed in appropriate locations in a room, are all over the Internet. Here is an example of one such product:

https://www.acoustimac.com/acoustica...icart-custom-1


* Reducing Reverberation (Ringing):

Most of what I have addressed so far would be classified as early reflections. But, as noted in the first part of this section, frequencies continue to bounce around a room until they simply run out of gas. As noted earlier, in the discussion about distortion, we may particularly notice when higher frequencies do too much of that. And, the presence of many bare or hard surfaces in a room allows higher frequencies to continue to ricochet around the room for a much longer period of time. When a relatively bare room has a lot of excess sound energy in it, the result can sometimes be perceived as shrill or harsh sounding. That shrillness can often make it difficult to tolerate very high volume levels.

Listeners sometimes note becoming fatigued by their high-frequencies. Some speakers may be more likely to create that sensation than others, and some listeners may be more susceptible to that sensation than others. But, in either case, a very bare room will usually exacerbate the sensation of brightness or shrillness in an audio system. I have been in a listening room in a "high-end" audio store that sounded like fingernails on a chalkboard to me, at anything above a low volume level, simply due to all of the bare surfaces in the room. Some listeners, such as the employees at that audio store, can become so acclimated to a particular sound that they may not even notice the overall distortion they are hearing.

Some audio magazines may contribute to an impression that bare room surfaces are appropriate for "audiophiles" by publishing photographs of audio systems in very bare rooms. I have seen photographs of very high-end speakers, with equally expensive amplifiers, in rooms with all bare surfaces, including polished hardwood floors. The rooms were attractive and the speakers looked very nice in those photographs, but I would personally not have enjoyed listening to music in those rooms.

Whether or not to add any rugs, or drapes, or other softening influences to a room, is certainly a personal decision, and sometimes there may be important WAF (wife approval factor) considerations, as well. But, in general, bare rooms will promote distortion, which may be especially noticeable at mid and high-frequencies.

Where a listener lacks measuring equipment, but suspects that the bare surfaces in his room may be contributing to excessive high-frequency energy, a simple handclap test will help to determine how much of a factor the room itself is playing in the sound he hears. Standing at the listening position and simply clapping your hands sharply will tell you a lot about the room. If the sound of the handclap is prolonged, the reverberation we are hearing is called slap echo, or ringing. Ringing interferes with our ability to hear individual sounds distinctly, because louder or first-arriving sounds linger for too long, drowning out harmonics of the initial sound, or the more subtle sounds which follow.

A very good example of this is an instrument such as a chime or a cymbal. There are subtle harmonics from the fundamental sounds produced by those instruments (and many others) which go way up in frequency. (A fundamental sound or frequency is a single note, but as stated earlier, all musical notes have harmonics which go up in frequency, at a somewhat reduced volume.) When the room itself exhibits ringing, we won't hear those harmonics as clearly, or at all. We will just hear the first loud fundamental sound, and the ringing in the room will drown-out the more subtle harmonics, which would otherwise cause the sound of the chime to linger in the air for just a moment, with the correct timbre, as the recording intended for it to.

This idea may seem a little counterintuitive at first, but what I am saying is that if the room itself has ringing (prolonged reverberation which would be almost like an echo in a larger space), then the natural timbre of an instrument, which is captured in a good audio recording, may be eclipsed by the inherent ringing of the room itself. And, the musical, or other sound, will be cut-off somewhat abruptly, and harmonics of that sound may be supplanted by room reverberations of the fundamental sound, which continue to reverberate in the room.

Other good examples of instruments to which this might somewhat subtly apply are the piano or other string instruments, or wind instruments such as the flute or clarinet. I suspect that most of us won't even be aware of what we might be missing unless we experiment--comparing very lively-sounding rooms, to less lively-sounding rooms, and listening for the differences.

The YouTube video I am linking below has a terrific illustration of the ringing phenomenon I have been describing. In this instance, the instrument is a snare drum, playing in a room with all hard surfaces, oblique angles, and no furniture. This is a deliberately extreme example, as most rooms will have a little more favorable geometry, some furniture, and some other softening influences.

So, in most cases, the ringing effect would be much more subtle than what we hear in the video. And, in some cases we might not really notice it without a before-and-after comparison. It would also typically take much less effort to reduce the ringing in a normal room, and we might not want to deaden the room quite as much as is illustrated in the video. (This is an issue that will be explored a little more below.) But, the video dramatically makes the point of what excessive reverberation in a room can do to distort mid and high-frequencies.




Individuals who want to enhance the clarity of frequencies above the transition point may wish to investigate whether early and late reflections are a problem in their rooms. If so, simply adding some softening influences to the room may greatly enhance the quality and clarity of the sound in the room. Room treatments can be subtle and don't necessarily have to involve the use of acoustic panels. Area rugs, thick drapes, bookshelves filled with books, and other softening, absorbing, or diffusing influences can also be very effective with higher frequencies.

When ringing in a room has been addressed, a handclap test reveals a single fairly sharp sound, with relatively little lingering quality. And that, in turn, allows the full frequency range of individual instruments, and of individual sounds to be heard, without the room itself getting in the way. To what extent this is an issue in a particular room, for a particular listener, is another YMMV decision. As a general rule, it is probably better to add acoustical treatments and/or softening influences to a room gradually, listening as we go. Concentrating on the specific areas of the room, mentioned earlier in this section, would also be a good idea.

How much overall reverberation, or ringing, we prefer will undoubtedly vary from individual to individual. Most of us would probably want to shoot for a reverberation time (RT60) of somewhere between about 0.5 and 1.0 seconds. (RT60 is an industry standard for the time it takes for a sound to decay inside a room by 60db.) I have seen 0.7 seconds (700ms) described as an optimum HT target, although individual preferences will surely vary. The article which follows explains reverberation time in both simple and more technical terms, as we follow the associated links.

https://www.nti-audio.com/en/applica...rberation-time

To get a sense of how we might determine the approximate reverberation time in our rooms, with a handclap test, the following series of short videos may be very helpful. In the first brief example, the RT is probably about 0.2 seconds (200ms). That room would feel very dead or dry acoustically--similar to a recording studio. The next one demonstrates an RT of 1.0 second. I would personally consider that room to be a little too lively for music listening and for HT. Somewhere between the first two examples is where most people will probably want to be. Short of measuring our rooms with appropriately implemented acoustical analyzers, performing a handclap test should give us a good general sense of how we are doing when we add acoustical treatment to our rooms.

https://www.nti-audio.com/en/support...for-rt60-value

If we go slowly with our acoustical treatments, adding softening influences gradually, and paying attention to our own perceptions of sound quality, there should be no risk at all of over-treating a room in a way that compromises desirable ambient sounds or helpful reverberations. It is probable that different individuals will prefer slightly more, or less, room treatment, so two different people could probably address the same room in somewhat different ways. The main thing is to become aware of the ways that our rooms may be negatively affecting our sound quality, so that we may better please ourselves with the aesthetic/acoustic choices that we make.

A rule-of thumb which always made sense to me, for audio in general, was that a room which sounds comfortable for normal conversation is likely to sound comfortable for listening to recorded music, and perhaps also for watching movies. That is because, just as with normal conversation, the room itself wouldn't be adding or subtracting too much to the sound for music listening, or for movie watching. A room which is so lacking in reverberation, that it isn't very comfortable for hearing normal conversation, at normal speaking levels, may also be adversely affecting the normal sounding timbre of voices, and of other sounds as well. That same dullness may be expected to carry over to what we would hear with recorded music.

Alternatively, a room which is so reverberant that voices sound abnormally loud or distorted for conversation, probably won't sound very good for music, or for movies, either. Musical instruments, playing higher frequencies, and vocals, may be particularly affected, as we will miss out on subtle harmonics. And, some sounds may seem relatively shrill. Movie dialogue can also be especially affected by excessive room reverberation, as the center channel producing the dialogue is already competing with several other sound sources, even without excessive room reverberation to exacerbate the problem. So, in HT systems, excessive room reverberation may mask dialogue clarity in movies. In all cases, though, this is a YMMV issue, which individuals will need to resolve to their own satisfaction. Sometimes aesthetics, or even simple inertia, will trump acoustics, and that's fine too.


** Bass Traps:

For frequencies below about 250Hz or 300Hz, bass traps can be very helpful. But, it takes thick and dense acoustic panels to affect frequencies below about 120Hz. Bass traps are acoustic panels which are specifically designed to absorb some longer, bass wavelengths. As explained in more detail in the section on room gain, low-frequencies behave more sluggishly than higher frequencies do, pooling in areas where any two (or more) room surfaces meet. When they collide, the low-frequency wavelengths either reinforce each other, causing peaks at certain frequencies, or they cancel each other, making certain frequencies harder to hear.

As some of those excess sound waves are collected by the acoustic traps, random peaks and valleys in the frequency response are reduced, allowing other low-frequency sound waves to be heard more clearly. Boomy sounding bass is often the result of a peak at a particular frequency that obscures other low-frequency sounds. On the other hand, bass traps can often allow bass to sound both clearer and louder than it did before, when significant cancellation was occurring.

Bass traps are typically at least 4" thick and are typically made of compressed fiberglass or rockwool. The ones with which I am familiar have a plywood backing with a large hole cutout in the plywood. The hole works something like a Helmholtz resonator to attenuate low-frequencies. Then, the entire panel is covered with fabric. The best results are obtained when the thicker panels can stand-out away from the wall, with a gap between the panel and the wall behind them. I believe that using about a 4" gap behind a bass trap is usually recommended for very low-frequency absorption. The combination of the thicker acoustic panel, and the air space behind them can make bass traps somewhat effective down to about 60Hz or 70Hz, where the panel sitting flush against a wall, might only be effective down to about 120Hz.

Thick bass traps (4" to 6" thick) can be placed anywhere, but are frequently recommended for placement in corners, as that is nearly always a location where low-frequencies collect. The large corner traps are typically wedge-shaped to fit in corners, and may be about 12" deep at the point of the wedge. That 12" depth consists of a 4" or 5" thick panel which faces out into the room, with two plywood side pieces, which form the triangular shape that fits into the corner. The entire corner bass trap is covered in fabric.

The side pieces create a built-in air pocket behind the thick front-facing panel, so that the wedge-shaped bass trap can fit flush into the corner. As noted above, the thick bass traps, with air gaps behind them, can be at least somewhat effective down to about 60Hz or 70Hz. Bass traps are typically designed to also absorb mid and high-frequencies, in addition to the low-frequencies for which they were designed, unless otherwise specified. Those "broadband" bass traps can, therefore, also reduce ringing in mid-range and treble frequencies.

Sometimes, where bass traps can be partly effective with mid-bass frequencies (defined as about 50Hz to 120Hz), they can allow room correction to complete the job of smoothing-out peaks and valleys in the frequency response. (Room correction alone, is not always effective in doing that.) There are a number of good sources for these more specialized acoustic panels, and they can also be DIYed. Most of the acoustic panel makers will offer free room treatment advice. I am showing a link to one well-known maker, but Ethan Winer's RealTraps, and ATS Acoustics are also good sources.

http://www.gikacoustics.com/product-...ry/bass-traps/


I-B: Room EQ and Calibration Techniques

Systems of automated room EQ, such as Audyssey, measure the frequency response within a listening area, and then set filters (control points) at specific frequencies to equalize sound pressure level across the entire frequency range. Automated room EQ is generally believed to be helpful with bass frequencies (<500Hz) and especially helpful below the transition frequency in a room. The extent to which systems of room correction can correct higher frequency distortion in a room, caused by setup issues or by fundamental room issues, is a more controversial question.

In my opinion, this question can only be answered by individual listeners on a case-by-case basis. If the overall audio quality sounds better with Audyssey on, then the room correction is successful in that specific instance. And, if the overall sound quality sounds better without Audyssey, or some other form of room correction, then I see that as strictly a user preference issue. (Before writing-off a system of room EQ, such as Audyssey though, it is a good idea to experiment extensively with settings, such as DEQ. A number of user-controlled settings can influence the potential sound quality for a particular listener in a particular room. They are discussed throughout the Guide.)

* Note: Some systems of automated room EQ allow users to limit the use of room correction above a certain frequency, such as 500Hz. That is strictly a user preference issue. The Audyssey app, which is available with newer model Denon/Marantz AVR's/AVP's, has that feature, along with other user adjustability options. There is a thread devoted to that app, so I am not going to address it in the Guide. Instead, I am going to explain ways to maximize the sound quality of Audyssey calibrations in general. Readers who want to learn more about the use of the Audyssey app are encouraged to consult this thread:

https://www.avsforum.com/forum/90-re...-pre-pros.html


There are some things that we can do initially, to enhance our chances of improving our audio quality, when we do a room correction calibration with a system such as Audyssey. First, we have to understand that there may be a limit to what Audyssey can do to correct pre-existing problems involving improper speaker setup, or too many early and late reflections. Better speaker setup, and even a modest use of room treatments, may augment what room correction can do to improve the sound quality.

Second, we need to understand that, in some cases, room correction can actually create distortion (or exacerbate it, if it already exists) if we don't observe good calibration technique when we EQ our rooms. So, this section of the Guide will offer some general tips which may assist users in getting better calibrations. Although the general focus will be on Audyssey, some of the tips may apply to other systems of automated EQ as well.

We need to realize that measurement microphones, such as the one employed by Audyssey, do not "hear" sounds in the same way that we do. To start with, the Audyssey measurement microphone is far more sensitive than our own hearing is. Small variations in volume at certain frequencies, which might completely escape our notice, will be picked up by the Audyssey microphone. And, Audyssey will try to "fix" them, even if they don't really need fixing, and, even if we would already have been able to hear those frequencies with subjectively good sound quality.

For instance, if speakers are toed-away from the MLP a little too much, Audyssey may detect that the high-frequencies are a little too low, in comparison to the frequencies which are not quite so directional, and Audyssey may boost those high-frequencies accordingly, causing some shrillness. The reverse can also be true, when some types of tweeters are pointed too directly at the MLP. For example, some manufacturers (or experienced product users) may recommend that specific front speakers not point directly at the MLP. That can especially be the case with some horn speakers, which have high sensitivity, and which may sound bright compared to other speakers.

In either case, Audyssey may exacerbate a barely noticeable (or completely unnoticeable) but pre-existing situation, by trying to do too much correction in a frequency range where a lot of correction may not be required for an individual to hear subjectively good sound quality. I think that Audyssey can actually be used as a kind of test instrument to help users discover how to point their speakers in just the right direction to maximize sound quality, although the resulting differences in SQ may be subtle.

I found that to be the case in my own room. A little less toe-in gave me more harshness in the upper frequencies, post-Audyssey. A little more toe-in, and Audyssey did less to affect those higher frequencies, resulting in better, more natural, sound quality. In my particular room, and with the specific setup and adjustments I use, Audyssey simply sounds better on that off for the full frequency range, although the most noticeable difference is still below 500Hz.

What I am saying here is that if we wish to fully benefit from automated room correction, and if we are really serious about the sound quality in our rooms, then it may be necessary to experiment a bit with speaker positioning, with appropriate softening influences, and then with subsequent Audyssey calibrations, in order to achieve the best overall sound quality that we can. It is a little bit of extra work initially, but the long-term results can be worthwhile.

We probably shouldn't just assume that the Audyssey microphone will hear exactly what we hear. And, we shouldn't assume that it will EQ our systems in accordance with our personal listening preferences, if we don't experiment a bit with our speaker placements and our EQ technique when we are running our Audyssey calibrations. It's really just trial-and-error to find out what works best for a particular listener in a particular room.

[I should note here that my intent in writing this is not to make people obsessive about their speaker placements, or about any other factor being discussed. The real intent is just to inform people that factors such as speaker toe-in can potentially impact an Audyssey calibration. If the initial Audyssey calibration sounds good, then don't worry about it. If, over time, someone wonders whether any further improvements can be made, typically to the higher frequencies, then some additional experimentation might be helpful.]

A second example of the difference between what we hear, and what Audyssey hears, concerns the nature of the omnidirectional Audyssey microphone. The Audyssey microphone hears sounds equally in all directions, but we don't. The pinnae (flaps) in our ears funnel sounds into our ears from the front and from the sides. But, they partly block and deflect sounds coming from behind us. So, early reflections from a wall behind us are going to be noticed far more by the Audyssey microphone, than they are by our ears. And, in trying to over-correct those early reflections, Audyssey may actually contribute to the distortion we hear.

[As a general rule, we don't want Audyssey to become extremely busy in the high-frequency range. Reflections from hard surfaces, bouncing into the Audyssey microphone at close range, can cause Audyssey to set too many control points at high-frequencies, causing additional distortion which may sometimes be characterized as comb filtering. The more that we understand why Audyssey might be doing things to "correct" mid-range or high-frequencies, the more that we can enjoy the overall benefits of room correction for low-frequencies, without adversely affecting our higher-frequency sound quality.]

The nature of the omnidirectional microphone is why Audyssey advises keeping measurement microphones at least 18" away from a wall or other hard surface. Perhaps an even better example of the difference between the way we hear, and the way the microphone hears, involves chair or sofa backs. Most chairs or sofas in our HT's or mixed-use rooms have relatively smooth surfaces. Some of them have fairly firm leather surfaces.

Those smooth or firm surfaces reflect high-frequency sound waves directly into the Audyssey microphone, in a way that they never could if we were actually sitting there. And, the sounds from the back of a sofa would be sufficiently attenuated by our pinnae (ear flaps), and would reach our ears so simultaneously with the direct sound, that we would never hear them. But, the omnidirectional Audyssey microphone would hear them, and in trying to correct something that didn't need correcting, it could introduce comb-filtering (high-frequency distortion) into the sound.

One way to avoid that problem would be to keep the Audyssey microphone at least 18" away from a chair back. But then, we wouldn't be measuring where our ears are, and that could negatively impact our calibration. A better solution is simply to drape a fluffy blanket over our chair backs during calibration. That will enable us to get our microphone within about 4" or 5" of the chair back, and where our ears actually are as we listen. At the same time, that will prevent high-frequency sound waves from bouncing into the mic from very close range. And, Audyssey will leave those spurious high-frequency reflections alone, concentrating its EQ resources on broader areas of the frequency range. (Chris Kyriakakis, the creator of Audyssey, has endorsed this solution.)

Audyssey employs a system of fuzzy-logic weighting to average the results from either six or eight microphone positions (depending on the Audyssey version). In general, I believe that the more we can give Audyssey more consistent measurement results to work with, the more that we can achieve a smoother frequency response, and consequently improved sound quality. This is something that Chris Kyriakakis commented on recently in response to a question. He suggested that the more uniform the sound is within a measurement area, the more uniform the Audyssey EQ is likely to be. And, the more uniform the Audyssey EQ is, the more likely it will be to provide good sound quality to a larger listening area. (We should recognize that no system of automated room EQ is likely to be able to EQ an entire room. The smaller the listening area we are trying to EQ, the better our resulting sound quality is likely to be.)


** The examples above illustrate one aspect of microphone placement, within a more consistent measurement area, but general microphone placement is also a factor. We typically want to have our microphones at ear level, even if not all of our speakers are at that same height. Keeping the mic at roughly ear level seems to be consistently important. Some users, including myself, have achieved good results by taking just a couple of measurements 2" or 3" above ear level. We typically do not want to go behind a chair back with any of our measurement positions, unless we are deliberately trying to EQ for a second row of listening chairs. And, even then, it would be a good idea to experiment both ways--going behind the MLP, and not going behind it. It helps to recognize that Audyssey is simply trying to EQ a fairly uniform listening area, and not individual seats. Other than mic position 1, which is typically centered on the primary listening chair, the mic positions don't need to correspond at all to any actual seats.

It is also a good idea to use a boom microphone stand with an extendable arm for Audyssey calibrations. That allows the base of the stand to remain on the floor. If a tripod is placed in a chair or on a sofa, vibrations from the furniture could be passed up through the Audyssey microphone. Whether that would always make a significant difference in the calibration is somewhat debatable, although there have been some good before-and-after examples where it definitely did. But, for the small cost involved, I think that it makes sense to use a better and more stable stand than the one that Audyssey provides.

The type of stand I am recommending will also provide more exact mic placement and better repeatability for calibrations. That repeatability is important, once someone has found a mic pattern that works well for his room and equipment. There are a number of different stands that can work, although I have heard that some of them which come with adapters included, are flimsy and don't stand-up well. The one that I am linking below is sturdy and durable, but it does require a separate adapter to hold the Audyssey microphone. I am also linking two different adapters. Either adapter can work.

https://www.amazon.com/gp/product/B0...KIKX0DER&psc=1

https://www.amazon.com/gp/product/B0...KIKX0DER&psc=1

https://www.amazon.com/gp/product/B0...KIKX0DER&psc=1


*** As a general rule, it is a good idea to measure smaller areas, as opposed to larger areas, for the reasons cited above. We want our measurement area to be large enough to accurately represent the binaural (hearing with both ears simultaneously) nature of our hearing. But we may not want to measure such a large area that we present Audyssey's fuzzy-logic weighting system with too much anomalous information. Patterns that vary in size from as small as about 6" to 12" out from the MLP (mic position 1), to as large as about 24" to the side and forward are typically used. I would not generally recommend going further to the sides, or forward more than about 24" from mic position 1.

It is interesting to note that, in the last couple of years, Audyssey has revised it's owner instruction manuals to recommend a smaller microphone pattern than they used to recommend. They used to recommend 3' to 4' out from the MLP. I believe that they now recommend about 2' or less. Their revised recommendations seem to parallel the experience of many Audyssey users, who discovered that smaller microphone patterns often resulted in better sound quality, over a wider area, than large mic patterns did. That is consistent with my own experience, and with that of a number of others on the Audyssey thread.

But, I suspect that finding an optimum microphone pattern is at least somewhat room and system dependent, so I suggest that interested users experiment in an effort to discover the specific microphone pattern which produces the best sound quality in their rooms. Once they find a mic pattern that they really like, I recommend that they write it down, or draw it, so that they can return to it for future calibrations. Sometimes, fairly subtle differences in microphone placement can yield significant differences in the resulting sound quality.


**** For people who are looking for some preliminary guidance in selecting microphone positions, the following visual aid is offered. This roughly 2' by 2' pattern is one that a number of people have successfully used. But, it is only shown as a starting point and not as a specific recommendation. People still need to experiment to discover what pattern works best in their particular circumstances.

In this pattern, mic position 1 is about 4" or 5" in front of a blanket covered chair (or couch), the center of which is the MLP. Remember that MLP stands for main listening position, and mic position 1 is always, by definition, the MLP. The MLP can be the center of a couch, or the center of a chair, depending on the specific room. For purposes of the illustrated diagram, mic position 1 is right between your eyes (and ears) and about 4" or 5" way from the blanket covered surface of a chair or couch. The mic is at a height which approximately corresponds to the center of your ear canal, as you would sit when listening to music or watching movie. That is what we mean by ear height.

Positions 2 and 3 are out to each side of 1 by about 10" to 12". Positions 4 and 5 are straight out in front of 2 and 3 by about 20" or 24". Number 6 is in a straight line out from 1, but this time only about 14" to 18" away. All six of those mic positions are right at ear height. Positions 7 and 8 are in fairly close to the chair back--perhaps only about 6" away from the blanket and about 6" out to the side of mic 1. (That clusters some mic positions very near the head, and where the ears on each side of our heads are located.) Both of the last two positions can be raised up by 2" or 3" above ear level. None of the mic positions go behind the chair.


2--------1---------3
-----7--------8-----

---------6-----------

4-------------------5


The specific order of the mic positions is not important, so after mic position 1 (which is always the MLP) the numbers assigned are arbitrary. Users can follow the diagrammed positions in whatever numbering sequence works best for them. It is only important to keep the mic level (so that it points upward) and close to ear height for at least about the first six positions. People who have a version of Audyssey which only uses six mic positions might wish to eliminate 7 and 8 from the diagram shown, or they could experiment with an even more compact configuration for their six. Experimentation is the key to finding a result which pleases the individual user.



Section II: Audio System Calibration and Subwoofer Levels:

This section explains how Audyssey calibrates and EQ's our audio systems, and offers some advice for the best methods to boost our subwoofers. It also explains what Dolby/THX Reference is and how that standard specifically relates to the subwoofer boosts we may prefer.

The subsections in Section II are as follows:

II-A: Audyssey Calibration and Dolby Reference

II-B: Why We Add Bass After Calibrations

II-C: Where and How to Add Bass

II-D: Master Volume Levels and Sub Boosts

II-E: Gain Settings and Maximum Output


II-A: Audyssey Calibration And Dolby Reference


Audyssey Calibration:

Audyssey is a room correction software program designed to reduce room/speaker interactions which may adversely affect audio quality. Once we put a speaker or a subwoofer inside a given room, it's native frequency response changes, depending on a number of factors, including its specific placement in the room. Audyssey attempts to remove undesirable room influences by setting control points to even-out the frequency response, so that we don't have large dips or peaks in sound pressure level (SPL) at certain frequencies. Audyssey's goal is to make all frequencies approximately +/- 3db from a standardized calibration SPL of 75db.

It does this by sending test tones at 75db to each of the channels in an audio system, and by then measuring the frequency response for each channel, from 10Hz to 22KHz. Once the measurements have been completed, Audyssey calibrates the results, using a system of fuzzy-weighted logic. It then sets control points at individual frequencies, or groups of frequencies, in order to correct for peaks and valleys in the sound. Once the control points have been implemented, the room EQ is complete.

* When Audyssey, and other systems of automated calibration, perform a system calibration they ignore all prior settings, such as the master volume level, trim levels, distances, and crossovers. Once the calibration is complete, the master volume level will typically return automatically to the volume that the AVR was on prior to the calibration.

It is important to note that all measurement microphones, including the Audyssey microphones, have an inherent error factor of anywhere from +/- 1.5db to +/ -3db. That includes very expensive calibrated microphones used to measure SPL or frequency response. If you measure the Audyssey test tones, or your SPL post-Audyssey, you may find that your SPL measures two to three decibels above or below 75db. I think it would be fairly rare for two different microphones to measure exactly the same. The important thing is that all of the channels in your system are playing the same SPL, as measured at the MLP.

It should be understood, that two separate actions are performed during calibration. First, the system will be calibrated to Dolby Reference using 75db test tones. All channels (including subwoofers) are set to have the same sound pressure levels, as measured at the main listening position (MLP), and all sounds are set to arrive at the same time, via the distance settings. Preliminary crossovers will also be set during this process. (As noted in the Cliff Notes, the preliminary crossover set by an AVR is not a recommendation. It is actually just an observation, as the AVR uses it's own default programming to make the preliminary crossover settings.)

The second thing that will occur, during the calibration, is the actual room EQ process. In that process, Audyssey will measure the frequency response from various microphone positions, and will use a system of fuzzy logic weighting to set filters for all of the channels, in order to remove some of the peaks and valleys in the frequency response that inevitably occur whenever transducers (speakers or subwoofers) are played inside a typical home theater or mixed-use room.

It is important to distinguish between the two processes. The initial calibration process insures that equal volume levels, from all of the channels, will arrive at the MLP at the same time, and it calibrates each of the channel volumes to a Dolby/THX Reference standard. The second part of the calibration process is the room EQ process, which sets filters for all of the channels, in an effort to improve the overall sound quality in the room.

The room EQ software program, and the filters (control points) it applies to an audio system are independent of the various AVR settings. So, once the calibration is complete, changing any settings will not change the room EQ that Audyssey applied to an audio system. This is a recurring question, so I have emphasized it here. AVR setting changes do not affect the room EQ that Audyssey applies. And, Audyssey can be turned off, and then turned back on again, as often as we like. Once Audyssey is turned back on, the same room EQ will be applied. (Using the Pure Audio mode will disable Audyssey. When Audyssey is disabled, features such as DEQ and Dynamic Volume are not available.) If significant changes are made to the room, however, or to any of the speakers or subwoofers (new locations, for instance) a new calibration should be performed.

One of Audyssey's goals, in any Audyssey version, is to set the volume levels of all channels in a system, including subs, to 75db, as measured at the MLP, by the calibrated Audyssey microphone. The MLP is microphone position 1, by definition, wherever the user chooses to place the microphone. And, that point in space is where Audyssey will set timing (distance) and trim levels (volume levels), for all of the channels, to coincide.

The .1 in a 2.1, or 5.1 (or larger) audio system is the LFE (low frequency effects) channel, and that .1 designation has nothing to do with the number of subwoofers in a system. The .1 designation was originally selected because the LFE channel only plays a fraction of the total frequency range of an audio system. (The LFE channel is explained in a little more detail in Section III.) Where there are subwoofers configured in an audio system, Audyssey will measure and calibrate all of the subs in a multi-sub system together, so that their combined SPL is 75db. Whether there is one subwoofer, or there are many subwoofers in an audio system, the combined sound of the subwoofers as a whole will be set to 75db.


[The difference between subwoofers and the .1 LFE channel may be a little confusing at times to all of us, so I decided to add some clarification to this section. Most people reading the Guide will have subwoofers connected to sub outs in their AVR's or AVP's. Those AVR's and AVP's are designed to play Dolby 5.1 programming, which contains the .1 LFE channel, briefly described above. But, it is possible to connect subwoofers to some stereo amplifiers. We can also listen to 2-channel content, or watch older movies (made prior to the development of Dolby 5.1, which occurred in 1992) on our HT systems. However, the audio system is only playing LFE (.1) content, which is recorded 10db louder than the regular channels, when 5.1 or higher material is being played on an AVR or AVP.

For any other program material, the subwoofers only play content contained in the regular channels (such as 2-channel content) even if the content is upmixed using a surround mode such as Dolby Pro-Logic (PLII). When subwoofers are employed for non-native 5.1 material, they simply enhance the bass in the regular channels by playing frequencies below the crossovers assigned to those speakers. It is important to understand the distinction I am making, because all of us tend to equate subwoofers with the .1 LFE channel, and they are two different things. The .1 channel is for low-frequency effects content, recorded 10db louder than other content in a 5.1 soundtrack. Subwoofers are transducers designed to play bass frequencies, below crossover points, in the regular channels. And, in addition to that function, they also play .1 LFE content, whenever a 5.1 program is played through an AVR or AVP.]


The trim levels and distances (timing) for all of the channels (including the subwoofers) will be set at mic position 1, from the initial test tones at that first mic position. Audyssey will disregard any previous settings and set levels, distances, and crossovers from scratch, whenever a new calibration is run. As noted, trim levels and distances for all of the channels are set based on microphone position 1. Crossovers are set after all of the test tones are completed, based on a fuzzy-weighted average of all of the microphone positions in a calibration.

It should be noted that, although the test tones are full band, from 10Hz to 22KHz, only a portion of that frequency range is used in setting trim levels. For the regular channels, the range that Audyssey analyzes to set trim levels is 500Hz to 2000Hz. For the subwoofers, the range is from 30Hz to 80Hz. All trim levels are set before Audyssey adds control points to the channels. And, all internal test tones which govern those trim levels remain independent of the EQ which Audyssey performs.

[The sweeps that Audyssey uses, during the calibration process, provide an interesting illustration of the way that our hearing works. The sweeps (test tones) used for the regular channels sound much louder than the sweeps used for the subwoofers. But, all of the sweeps are playing at the same 75db volume.

All of the sweeps, for all of the channels, are playing the same frequencies, but we aren't hearing them the same way. At a volume level of 75db, the subwoofers can't go very high with audible sounds, and the speakers can't go extremely low with audible sounds. We may hear some difference in tone between larger speakers, which may sound lower than smaller ones. And, we may hear some difference in loudness between more distant speakers and closer speakers.

But, the biggest difference in both tone and loudness will be between the sweeps for the regular channels, and the sweeps for the subwoofers. The difference in the relative loudness between the sweeps for the regular channels, and the sweeps for the subwoofers, is the difference between the way we hear frequencies in our normal hearing range, and the way that we hear low-frequencies. Frequencies in our normal hearing range of about 500Hz to 5,000Hz sound much louder than the frequencies played by our subwoofers.

The sweeps are a good example of why most of us need to add more bass, once all of our channels are level-matched. That is especially true as our overall listening levels drop, because low-frequencies drop-away faster, relative to those in our normal hearing range.]

As noted above, the first microphone position is also used to set distances for all of the channels. Audyssey sets distances for the channels based on the time it takes for the sound to arrive at mic position 1. Subwoofers have their own internal amplifiers, and their own internal processing, which typically delays the timing of the sound arriving at the MLP. Audyssey compensates for that delay by setting subwoofer distances as longer than the physical distance from mic position 1. Setting a greater distance for a channel causes the AVR's internal programming to speed-up the arrival time of the sound (and vice-versa). In the subwoofer's case, speeding up the signal, with respect to the sound from the other channels, allows the sounds from all of the channels to arrive simultaneously at the the MLP.


Dolby/THX Reference:

When Audyssey finishes, all channels in the system (including all of the subwoofers' combined SPL) will play at the same volume, at the MLP, as determined by the calibrated Audyssey microphone. And, when all channels in a system are playing at the same volume, the sound at the MLP will be approximately in balance with what the film mixer intended whenever a movie is played at "Reference" volume, which is 0.0 master volume. However, when the listening level is lower than about -5 MV, most people will not hear bass frequencies quite as well as other frequencies, or quite the way that a film mixer intended for them to be heard in equilibrium with the other frequencies in the film.

The Dolby, or THX Reference standard is intended to provide some degree of uniformity in the maximum volume levels of movies, and is intended to provide a way for commercial cinemas and home theaters to make sure that their audio systems correspond to what film mixers intended for them to hear. The Reference level is capped at a peak volume of 105db for the regular channels, and 115db for the LFE (low frequency effects) channel. The LFE channel enables additional bass effects, below about 120Hz, to be mixed into a 5.1 movie (or music) track. A system that is calibrated to Reference will, if it has the capability to do so, play those 105db and 115db peaks when the master volume control is set at 0.0.

As stated, Dolby/THX Reference is a guideline that provides a degree of standardization for the way that movies are recorded and for the way that commercial cinemas and home theaters are calibrated. But, all the Reference standard really does is to establish maximum volume levels for the regular channels (105db) and for the LFE channel (115db). There is no specific uniformity with respect to the average volume level of movies, and there is no specific uniformity with respect to how frequent, or how sustained, crescendos of up to 105/115db will be in a particular movie.

Some movies may be much louder than other movies, both in terms of the average volume level of the film, and in terms of whether the film actually hits crescendos at the upper limits of the Reference standard. Some movies may never hit the upper limits of the standards, and some movies may hit those upper limits again and again. That is entirely at the discretion of the director and the film mixer. That is easy to understand if we compare blockbusters to light romantic comedies. But, even among different blockbusters, or among action films in general, there may be volume differences.

Once an audio system has been calibrated to Reference, how much or how little of the total SPL capability of an audio system is actually employed, is entirely a matter of personal preference. Some people prefer to listen at much louder levels than others do. It is important to understand that Audyssey will set all of the channels in a system to play at equal volumes at the MLP. And, when the master volume is at 0.0, the audio system will be playing Reference volumes, if it is capable of reaching those sound pressure levels. But, it is up to the individual user to decide how loudly he wants his audio system to play, via the master volume control.

That same principle also applies to our subwoofers. Sometimes, people will add a second subwoofer (or a larger subwoofer) to a system, and then question why the bass doesn't sound any louder after the system is recalibrated with the greater amount of subwoofer headroom included. Having multiple subwoofers increases our potential bass headroom, so that we can hit higher bass SPL's. But as noted earlier, the combined SPL of our subs will always still be calibrated to 75db, so that the combined volume of the subwoofers will be equal to the volume levels of the other channels in our system. In order to actually use any additional bass headroom we may have available, we either have to play our audio system at higher listening levels than we were before, or we have to increase the volume level of our subwoofer(s). Or, we can do both.

At one time, test tones were employed at the "nominal" average Reference volume of 85db. (I use the term nominal average volume, since there really isn't an average volume level for movies.) The 85db number was selected when the Dolby/THX Reference standards were developed, to represent a hypothetical average with 20db of headroom for the regular channels, and 30db for the LFE channel. That same 85db number was chosen as a uniform calibration number. And audio systems were calibrated to Reference at 0.0 master volume with 85db test tones.

However, the original 85db test tones which were used were uncomfortably loud for most people, so most systems of automated or manual calibration, including Audyssey, converted to a less uncomfortable test tone of 75db. And, all channels are calibrated equally to that 75db level. Our AVR's then do an internal recalculation to add 10db to the regular channels, so that a master volume of 0.0 will equal approximately 85db. And, once that internal recalculation takes place, the audio system is now calibrated to Reference (allowing for 105db max for the regular channels, and 115db max for the LFE channel) at a master volume of 0.0.

Once an audio system has been calibrated to Reference, whether that particular audio system will subsequently be able to play 105db peaks in the regular channels and 115db peaks in the LFE channel is entirely a function of the capabilities of the individual audio system in that particular room. And, as mentioned earlier, whether an individual user will ever decide to play his audio system at Reference levels, even if the system is capable of doing it, is entirely an individual choice. In fact, most people don't ever play their audio systems that loud.

[Based on anecdotal information, the average HT listening level is probably in a range from about -20 to -10 MV, with a master volume of -10 sounding twice as loud as a master volume of -20. (Each additional 10db of SPL equals a doubling in perceived loudness.) Many people will be either below or above that average range, but most of us probably fall within it. It is worth noting that a number of audio experts have stated that rooms under ~20,000^3, which would include almost all home theaters, sound anywhere from 5 to 8db louder at the same SPL as would be the case in a commercial cinema. Commercial theaters are much larger than home theaters, and higher sound levels don't sound quite as intense in those large spaces. That probably helps to account for the fact that relatively few people, even in treated rooms, listen at 0.0 MV.]

Regardless of preferred listening levels, however, it is important to have all of the channels in an HT system playing at the same volume at the MLP, so that sounds from all of the speakers (and subwoofers) will be in proper balance. But, as a practical matter, starting with all of the channels (including the combined sound of all the subwoofers) playing at the same volume is probably also the only way to set the audio system to Audyssey Flat. The intent of the Flat response curve is to have every frequency from down to as low as 10Hz, and as high as about 22KHz, play +/- 3db. Audyssey accomplishes this by setting control points which add boosts to some frequencies, and cuts to other frequencies, within every channel in an audio system.

Speakers and subwoofers are typically designed to play a reasonably flat frequency response, so that some frequencies don't stand out in comparison to others. But, once those speakers, and especially subwoofers are placed in a room, the room itself will affect the frequency response, causing peaks at some frequencies and dips at others. Proper speaker and subwoofer placement within a room will help, but bass frequencies in particular often need help from some form of room EQ to even-out the frequency response.

Audyssey attempts to provide that help by setting control points within every channel. When an audio system has been EQed to a relatively flat frequency response, the room is at least partly taken out of the equation, allowing the speakers and subwoofers to play a naturally flatter frequency response. This is generally believed to be particularly helpful for bass frequencies (from about 500Hz down), and may also be helpful for higher frequencies, depending on the particular room and listener preferences.

The Audyssey Reference curve changes Flat by creating a slightly downward curve to the high-frequency response. The Reference curve slightly rolls-off the treble frequencies above 4,000Hz (by about -2db) and it adds more roll-off (about -6db total) above 10KHz. It also adds mid-range compensation (a dip between 2,000Hz and 3,000Hz). Many people prefer those high-frequency roll-offs, and it is strictly a YMMV issue as to which of the two Audyssey settings we use. But, to create either the Flat curve, or the Reference curve, Audyssey needs to start with all channels and frequencies playing at the same volume at the MLP.

[Again, it should be noted that a similar methodology, of setting all channels, including the combined sound of the subwoofers, to the same volume, is used by other systems of automated or manual calibration, whether any room EQ is being attempted or not.]


** Occasionally, someone may say that he prefers to hear the "natural" sound of his speakers, or subwoofers, without any room correction at all. As far as I am concerned that is an entirely personal decision. There isn't any right or wrong way to listen to audio, in my opinion. But, I have never really believed that Audyssey significantly changes the "natural" sound of speakers. Horn speakers still sound like horn speakers, and electrostatic panels still have their own characteristic sound. What Audyssey may do, however, is to change the way that speakers and subwoofers interact with a room. And, whether that change is positive or negative is up to the individual to determine.

What any system of room correction attempts to do is to change the interaction of the speakers with the room, in order to allow the speakers (and subwoofers) to play with less distortion caused by room-induced peaks and dips in the frequency response. As noted below, those peaks and dips are especially prevalent in bass frequencies. Whether Audyssey or any other system of automated room EQ is successful in creating a smoother frequency response, and whether attempting to do that results in improved sound quality, is strictly up to the individual user to determine. And, that may depend on variables which include the specific room, the speakers/subwoofers, the AVR settings employed, and the personal preferences of the particular individual.

But, in order to fully experience Audyssey, I would encourage users to take pains in system setup and in their Audyssey calibrations. As noted in Section I, differences in system setup can yield different results, as can differences in microphone placement. Some calibrations may sound and/or measure better than others, and once a good calibration routine is developed, it is a good idea to keep a record of the mic placements that produced the most positive results. I would also encourage users to experiment thoroughly with various settings, such as the Audyssey Reference curve versus Audyssey Flat; with DEQ on and off; and of course with bass boosts. Individual setting changes, such as those, can sometimes significantly change the nature of the sound.


II-B: Why We Add Bass After Calibrations


The room strongly influences bass response, due to the action of room modes, causing peaks and dips at various frequencies. That is why Audyssey can be so helpful in EQing subwoofers. Audyssey can implement boosts up to 9db and cuts up to 20db, at selected frequencies in all of the channels in order to achieve a flatter frequency response. When Audyssey is successful at flattening out most of those bass dips and peaks (at least to some extent) the result is a smoother, clearer, and more uniform sound.

That less distorted and less boomy sound, without some frequencies peaking at much louder SPL's than other frequencies, may give the impression that there is less overall bass playing. And, there may actually be less bass playing, at some frequencies, if a particularly noticeable frequency (say at 50 or 60Hz) were peaking quite loudly prior to EQ. Just hearing all of the bass frequencies in better equilibrium with each other may contribute to the initial impression that there is less bass.

But, setting aside whatever impressions of lower bass volumes we may have, when we hear less distorted bass in our rooms, there is more to it than just hearing a smoother frequency response. Most people don't listen at Reference Volumes (0.0 MV) which is where the low-frequency content in 5.1 movies was mixed to be in correct balance with other frequencies. Once the volume level of a movie is reduced below Reference, in a typical home theater, those low-bass frequencies will typically be harder to hear, in relation to the frequencies where our hearing is stronger.

That is because, as the volume level drops, it will appear to drop faster for frequencies outside our normal hearing range. And, that particularly includes low-bass frequencies, which carry much of the special audio effects in movies. So, in a properly calibrated audio system, listening at anything less than very loud volume levels, it is pretty normal to perceive the bass as sounding too soft.

[It is worth noting that people sometimes attempt to double-check their subwoofer volumes, after an Audyssey calibration, with an SPL meter. Even if the SPL meter is correctly set to C-weighted Slow, however, it may not be able to accurately record sound pressure levels in the 30Hz to 80Hz range, where Audyssey is measuring SPL. Unless the SPL meter is a slightly more expensive one, which is calibrated, it may not yield correct readings for subwoofer frequencies. That is particularly true for smartphone apps, and for the inexpensive meters sold at places like Radio Shack.]

After the level-matching process from mic position 1 is complete, the low frequencies (which, as noted, are harder for us to hear) are playing at the same volume as all of the other frequencies. This phenomenon of lower frequencies being harder to hear than higher ones (except for very high-frequencies) is well known. Our hearing is strongest from about 500Hz to about 5,000Hz. So, frequencies played by our subwoofers may require more volume than the frequencies played by our regular channels. Some additional explanation of this is included in the section on DEQ, and in the Addendum on the thread history. A more complete discussion of the Equal Loudness Contours has also been included in Section VII of the Guide.

[For the sake of this discussion of bass boosts, it should be noted that the Equal Loudness Contours, which are a slight modification of the original Fletcher Munson Curves, are based on averages in normal, healthy human hearing. It may be assumed that, as with other human attributes, hearing generally follows the shape of a bell curve, with some individuals hearing bass frequencies somewhat better and others hearing those same frequencies somewhat worse than the average. And, our hearing may (will) change somewhat as we age. Therefore, a given individual may be able to hear lower frequencies relatively better, or relatively worse than would be predicted by reference to the Equal Loudness Contours. Consequently, the final bass levels that we pick may be partly a function of our individual hearing capabilities, and also partly a function of our specific psycho-acoustic preferences.]

Although a lot of the discussion so far has focused on movies and on Reference levels, our desire for stronger bass may not be limited to 5.1 movies or TV shows at below Reference levels. Even if we are watching a 5.1 movie at Reference levels, some of us might still prefer to have more bass than what Audyssey provides with a flat bass frequency response. DEQ won't add any bass at all at a listening level of 0.0. The decision of how much bass we want to hear (at any listening level) is an entirely personal one, which may depend on a number of factors.

We may also prefer to apply bass boosts to music. And, we may wish to add sub boosts for some of the same reasons that we would add them for movies. Even in the absence of special effects in movies, we may not hear low-frequencies in music as well as we hear those in our optimal hearing range, or we might just prefer more bass, period. That would be especially true at lower listening levels. And, some music and some TV shows may have less low-bass in the soundtrack to begin with. For instance, some older music may have very little content below 60 or 80Hz, due to the nature of the recording process, and we might be used to hearing more low-bass in our HT systems than that. So, some people might wish to boost the subs just to hear what sounds like a more appropriate amount of bass.

[I think it is important to emphasize that the degree of sub boost selected for any program content is entirely a matter of personal preference, as is the overall listening level. And, individual preferences may change, as we go from one source to another, from one song or movie to another, or depending on our moods, from one day to another.]

Audyssey's DynamicEQ (commonly abbreviated as DEQ) is a separate software program which boosts the low-frequencies in all of the channels, including the .1 subwoofer channel. It also slightly boosts the high-frequencies in the regular channels. It is engaged by default whenever an Audyssey calibration is run. The boosts that DEQ adds are intended to, at least partly, compensate for the inherent difficulty in hearing lower frequencies (and to a lesser extent, high-frequencies) at below Reference volume levels. DEQ is explained in detail in Section V.

How much boost DEQ adds varies depending on the MV selected, with more boost added as listening levels reduced below Reference (0.0 MV) at a rate of about +2.2db per 5db below Reference. So, at -15 MV, for instance, DEQ would add a little over 6db (6.6db) of bass boost to all of the channels, including the sub channel.

Whether DEQ fully restores bass equilibrium to movie soundtracks is an interesting question. Most people seem to prefer more bass boost than DEQ provides, and typically add an independent sub boost, even with DEQ on. With DEQ engaged, the typical sub boost appears to average about +3db to +6db. With DEQ off, sub boosts are typically much larger. Additional information, regarding DEQ, may be found in a later section. But, with or without DEQ, the question of how and where to add a sub boost is important for most people.


II-C: Where And How To Add Bass

Most modern commercial subwoofers have a gain (sometimes labeled "volume") control. That gain control determines how much voltage will go from the sub amp to the driver (woofer). During the Audyssey calibration, the initial setting of that gain control will determine where Audyssey sets the trim level for the sub(s). So, if the initial gain control is high, Audyssey will set a low trim setting in the AVR (such as -9) in order to insure that the sub is playing 75db at the MLP, just as all the other channels are. If the gain control setting is low, Audyssey will set a high trim level in the AVR (such as -3.0, or 0.0, or even +3.0) to insure that all of the subwoofers in a system are playing at that same 75db level. A simple way to think of what happens during the initial calibration is: high gain level = lower trim level; low gain level = higher trim level.

[To emphasize this point more clearly, during the automated calibration process the subwoofer gain level and the AVR trim level are inversely proportional. For every decibel that we increase the gain level on our subs, the AVR will subtract one decibel from our AVR trim level. So, if we start with our subwoofer gain level at X, where X = 75db, the AVR will set our subwoofer trim level at Y. If we set our subwoofer gain level at X + 5db (80db) our AVR will set our trim level at Y - 5.

That is because it is the AVR's job to make all of the channels play that same 75db volume. Otherwise, our HT system can't actually be calibrated to the Dolby/THX Reference level that was our original target. The same thing would be true, in reverse, if we set our subwoofer gain level below 75db, to 70db, for instance. Our AVR would raise the AVR trim level by +5db to compensate for the low volume of the subwoofer amplifier. Doing that would insure that all of the channels were calibrated to play that same 75db at the MLP.]

So, that explains the relationship between the gain control and the AVR trim level during the initial calibration. But, what about after the calibration? If we want to add a subwoofer boost after Audyssey has set all of our channels to play at exactly the same volume level, how should we do it? should we use the gain control, or should we use the AVR trim control? We can actually use either one, but the decision of which one to use in a particular situation is just a little complicated.

The first thing to understand is that it is desirable to make the subwoofer amplifier send voltage to the driver, rather than having that voltage come from the AVR amp, because the subwoofer amplifier is much more robust and powerful than the amps in the AVR. This is an extremely important point, because using the subwoofer amp will help to prevent clipping. Clipping is a form of distortion, due to an alteration in the waveform. It can be audible in some cases, and if prolonged, can lead to overheating the voice coil in the woofer. When a waveform is clipped, the round top of the wave is squared-off---clipped-off.

That can happen when a subwoofer attempts to play low-frequencies with inadequate voltage. That's why you want to have the real voltage coming from the subwoofer amp, and just let the AVR amp supply enough voltage to turn-on the subwoofer from its Auto-On mode. -5 is sort of an arbitrary number, but several subwoofer makers suggest using that as about the max AVR trim setting for subwoofers. Holding our AVR trim at about -5, or lower, forces us to use the gain controls on the subs to add any really substantial subwoofer boosts.

After running Audyssey, simply making any adjustments in sub boost using the gain control on the sub(s) would insure that the sub amp is being used. So, that would be a perfectly good way to add sub boost. And, as noted a little further down, using a higher gain control may enable some subwoofers to achieve higher SPL's than they can with low gain levels and high trim levels.

But, most people find it more convenient to make adjustments using the AVR trim controls, with a remote control. And, in that case, it is desirable to start with a high sub gain level, and a low AVR trim level. Remember that a high gain level = a low AVR trim level. So, we would need to take certain steps during the calibration process if we wanted to have a low AVR trim level--let's say in about the -10 range. And then, we could adjust the trim level upward after the calibration. Using the trim settings in the AVR to make sub volume adjustments, after running Audyssey, allows the user to make convenient and fairly exact (.5db increments) adjustments to subwoofer volume, by using the AVR remote.

Typically, in order to achieve a low AVR trim level, though, it will be necessary to start with a measured sub SPL of higher than 75db. An SPL level of about 78db to 80db may be required. That would be in the red zone for Denon/Marantz units during the subwoofer level-matching process. Audyssey is specifically trying to set the sub(s) SPL to 75db. That is in the green zone on Denon/Marantz. However, to get a strongly negative trim level, a higher than 75db level will be required, and that will be in the red zone. The specific SPL used is not as important as the resulting low AVR trim level.

It should be noted that there is no harm in telling Audyssey to proceed with the calibration, even though the subwoofer is not in the green zone. That notification of red zone just gives owners the opportunity to adjust the gain on the externally-powered subwoofer to the same 75db volume level which Audyssey is trying to achieve for all of the channels. But, if the owner chooses to proceed with the calibration anyway, Audyssey will simply calibrate the subwoofer(s) with a low trim level, and that is typically exactly what we want it to do. That way, we can start with a low AVR trim setting, and add some subwoofer boost in a very convenient way, while remaining at about -5 or lower with our AVR trim.

It should be emphasized that there is no particular reason not to just use the gain control on a sub to add volume post-calibration. For people wanting to add really substantial bass boosts--up to, or in excess of 10db or 12db, some gain increase, in excess of the original gain setting, is generally necessary anyway, in order to achieve the bass boost desired by the user.


* [It should be noted that some Denon AVR's have a feature called "Subwoofer Level Adjust". When this feature is used, the subwoofer trim level is reset to 0.0, and the adjustment is made on top of that. So, starting at -11.5, post-calibration, and adding 5db of boost with that feature, will actually result in a net trim level of +5.0 in trim, instead of -6.5 in trim. That is a net increase of 16.5db instead of 5db. It is highly recommended to turn that feature off, and instead to make any necessary subwoofer volume adjustments either with the Channel Level Adjust in the Audio menu, or with the trim controls in the Speaker: Manual: Test Tone area of the Denon AVR.]


The usual recommendation to employ the AVR trim is more a matter of convenience and accuracy than one of necessity. Some subs don't have digital gain controls, for instance, so fine tuning the gain can be more difficult, as can on-the-fly adjustments during a particular movie, or music listening session. And, it gets even less convenient when multiple subs are connected together, or when gain controls are difficult to access easily. Using the trim controls in an AVR allows for very convenient and precise adjustments in sub volume. But, the most important thing is to make sure that the real boost comes from the subwoofer amp, and not just from the AVR, whichever adjustment method is ultimately employed.

Assuming that some initial sub boosts are to be accomplished using AVR trim, then starting with a low trim level post-calibration would be helpful. A low trim level might be defined as -9 to -11, but not exceeding -11.5 in Denon/Marantz units. (With other manufacturers, just determine what the minimum trim level settings are in order to ascertain what your optimum low trim setting should be.) As stated earlier, it may take an SPL of 78db, or higher, to achieve that optimum low trim level. However, it is important not to go lower than -11.5 in trim, in Denon/Marantz units. (For example, I believe that the lowest trim levels we should calibrate our audio systems to are -14.5 with Onkyo, and -9.5 with Yamaha, based on their respective trim level limits of -15 and -10.)

If a trim level of -12 is set, with Denon/Marantz units, there is no knowing what the actual volume of the subwoofer is. The AVR simply ran out of negative trim at -12. The actual sub volume might be 80db, or even 85db. If so, you might not like the way it sounds to have your sub so much louder than the rest of your system. And, you would not have an easy way to turn down the subwoofer volume, if your trim level were already at the lowest setting. You also could never be sure what your actual sub volume is, and as a result, you could find yourself running out of headroom sooner than expected. So, for instance, you want a negative trim setting not exceeding -11.5 in Denon/Marantz units.

Think of the process of adding a sub boost this way. When you raise the gain level in the sub, so that the sub produces more than 75db at the MLP, you are making a deposit in the bank, of amplifier power from the sub. So, for instance, let's say you start with a trim level in the AVR of about -9 to -11. Now, you can withdraw amp power from the bank, using your AVR trim control. You would, for instance, do that by increasing your trim setting to -6 or -5. As noted earlier, a +3 to +6db boost would be pretty typical, even with DEQ engaged. But, there is no free lunch. As you begin to approach 0.0, the bank deposit of amp power that you made with the higher gain setting is used up, and now you are using AVR amp power, which as noted, is not as powerful. Using AVR amp power can, in some instances, result in clipping (distorting) your subwoofer(s) or it can, in some cases, result in undesirable mechanical noises.


II-D: Master Volume Levels And Sub Boosts

There is a relationship between subwoofer volume and master volume (MV). As your MV increases, the subwoofer volume goes up correspondingly, and more demands are placed on the sub. It is important to remember that the subwoofer is not only playing the LFE channel, but also providing bass support for all of the other channels in a typical HT system. So, as the MV increases, the demands on the sub go up much faster than for the other channels, particularly in a movie with a lot of low-frequency content. It is worth noting that 5.1 movies (and some bass-enhanced music) can have very low frequency content in all of the channels, and not just in the LFE (low frequency effects) channel. The subwoofer has to (and should) play all of that low frequency content.

It is recommended by a number of subwoofer experts, two of whom are quoted in the FAQ, that it is advisable to keep sub trims well in the negative range (below 0.0). That is particularly important as MV's approach, or exceed, -10. In Denon/Marantz units, that is 10db below Reference (or 70 on the absolute scale) in your AVR master volume. Both of those experts quoted in the FAQ, Ed Mullen of SVS, and Mark Seaton have, subsequent to the entries in the FAQ, recommended staying well in negative trim levels, period. To follow their advice, and to avoid the possibility of distortion, we would want to keep our trim levels in about the -5 range, or lower, at even moderate listening levels. Again, that is easy to do by simply raising the gain on the subwoofer(s).

* [In addition to the possibility of clipping the subwoofer signal, with higher AVR trim levels, there is another potential reason for keeping gain levels relatively high. It is addressed in the following section titled Gain Settings and Maximum Sub Output.]

High sub gain levels, which still result in high trim levels, are indicative of a sub which is under-powered for the space, and/or the distance from the MLP. It could also be indicative of a specific placement problem, where either the sub or the MLP is located in a null. In the first instance, the only remedy would be a more powerful sub, or multiple subs, or a different (probably closer) sub placement. In the situation where the sub or the MLP were located in a null, a subwoofer crawl should be done to determine proper sub placement. Although subwoofer placement is not a direct part of this particular discussion, it is a very important factor in sub performance.

If you never intend to approach about -15 MV or higher, then the advice to set your sub gain high enough to obtain a strongly negative trim level might be less important. (Even then, however, that could be somewhat dependent on the use of DEQ and/or the use of independent sub boosts.) And, if you don't believe that you will ever want to listen at high volumes, or to boost your subs, then starting with a trim level of about -5 or -6, should be perfectly fine.

But, most people on this and other threads seem to average at least a +3 to +6db bass boost after calibration, and some people add much more than that. When DEQ (with its own bass boost) is not employed, boosts of 12db, or even more, are not uncommon. So, the advice you will most commonly see on this thread is to start with a negative trim setting of about -9 to -11 post-calibration, in order to maximize your ability to add sub boost, with your AVR trim control, while still using the sub gain you deposited in the bank.

Although this advice is not consistent with the explanations and recommendations in the FAQ, the more current advice supersedes the older advice in the FAQ. I would personally recommend following the advice to maintain a negative sub trim, preferably of -5 or lower, as a matter of best practice, even if you believe that you will never approach -10 or -15 MV. (As noted in the section just below, keeping a relatively higher gain level may work to your advantage, in any event.)

There is no telling who might, inadvertently or otherwise, run the volume control up on your system, or when unexpected peaks in very low bass (in electronically-enhanced music, or in movies) might cause some distortion to occur. And, if your sub happens to be approaching its max output limits, even at lower master volumes, the lower trim level would provide an additional measure of confidence that you weren't clipping the subwoofer signal.

While it is unlikely that most good modern subs would be damaged by a bit of distortion, or by an inappropriate use of AVR amp power, I know of two well-documented instances of a JTR Orbit Shifter, which is an extremely powerful and well-made sub, frying a voice coil (due to overheating) just from playing electronic music, downloaded from YouTube, at a very high volume, with a high AVR trim level. (Both instances involved the same user, who didn't quite remember the lesson from the first instance.)

And, even if no damage could ever be done as a result of clipping the sub signal, listening to distorted bass is sort of antithetical to the whole idea of good sound quality, and of using automated room EQ to achieve it. When proper gain/trim protocols are followed, it is also less likely that inappropriate noises, such as port chuffing, or of drivers hitting limiters, could occur prematurely. So, an ounce of prevention is worth more than a pound of cure, in this case.

Again, you can use a combination of increased subwoofer gain, and some increase in AVR trim, to raise the volume level on your sub to any level you choose, while still maintaining an AVR trim of about -5, or less. (Since originally writing this guide, I have seen even more recommendations from subwoofer makers to be at -5 or less in the AVR trim.) That will help you to avoid the possibility of clipping the subwoofer signal. And, raising the gain control on the sub(s) post calibration, will have no effect at all on the way that Audyssey EQed your system.


II-E: Gain Settings And Maximum Sub Output

There is another aspect to the gain/trim issue that is worth mentioning. Depending on how the DSP in a given subwoofer is implemented, the subwoofer may only be able to achieve max output levels with the gain control set very high. Some subwoofers are only able to achieve maximum output levels when the gain control is set to, or very near, the highest setting. So setting a lower gain control, and a correspondingly higher AVR trim control, might not result in the same amount of peak bass SPL, irrespective of issues of clipping.

Apparently, this issue may be more common in subwoofers with digital (rather than analogue) controls. But, according to several examples I have observed from various threads, the issue is not at all limited to subwoofers with digital controls. Some subwoofers with analogue controls may have the same issue of not being able to achieve higher max output levels with low gain settings.

How important this max output issue actually is probably depends on the situation. For instance, I believe that a relatively lower gain setting might cause a ported subwoofer to chuff prematurely. Again, depending on the situation, even someone who is listening at a fairly moderate listening level, let's say -15 or -20 MV, might experience issues if he were using a significant subwoofer boost, either independently or on top of DEQ.

Putting a sudden peak demand on the subwoofer, with the right low-frequency content, might not enable the subwoofer to access the full output that it is designed to produce, if the gain level isn't fairly high. In that case, the subwoofer just wouldn't play the low-frequency content at the volume it was supposed to. In other words, it would simply stop getting any louder during that peak content. Whether we would even notice that, or whether we would hear the subwoofer make any audible sounds of distress, are separate questions.

But, unless we are sure that our subwoofers can achieve max volume levels with low gain settings, it is probably a good precaution to keep gain levels fairly high. Typically, that will mean using corresponding lower AVR trim levels for our subwoofers. This is not an issue that I have often heard addressed by subwoofer makers, but I suspect that many would intuitively know that some subs produce max volumes only with high gain levels.

This is just speculation on my part, but I think one reason that this issue isn't discussed more is because sub makers are not typically testing their subs as part of a calibrated HT system. So, they aren't dealing with gain/trim relationships at all in their design and testing process. When they want to push one of their subwoofers to its limits, or they want to measure its maximum output, they just max out the gain control on the subwoofer itself. It is only when subwoofers are calibrated as part of an HT or audio system, with an inverse relationship between gain and trim, that this becomes an issue. But, I believe that it can be an issue, and I believe that is another potential reason for attempting to keep gain settings fairly high.

CEA 2010 testing, performed by Data-Bass, always measures max output with gain controls at the maximum setting. And, as noted, some subwoofers may not be able to produce those same max SPL numbers, that we see on Data-Bass or from other professional sources, with lower gain settings. This won't be true for all subwoofers, but as a matter of best practice, I believe that it may be generally advisable to keep gain settings fairly high, and AVR trim settings fairly low, in order to maximize available headroom. An exception to this general policy could be a situation where a lower trim setting didn't successfully power a subwoofer on, when it was set to Auto On mode. But, that would be extremely unusual with Denon/Marantz, or Onkyo, receivers and processors.


[People with Yamaha AVR's are apparently much more likely to experience issues with subwoofers not turning on automatically unless AVR sub trim levels are relatively high--perhaps even fairly close to 0.0. That is due to the lower voltage signal sent from some Yamaha AVR's to the subwoofer. Newer high-end Yamaha AVR's are reported to have addressed the problem.

If subwoofers will not turn on automatically in Auto mode, without higher AVR trim levels, then the higher trim levels may be slightly less likely to lead to clipping issues, since the voltage from the AVR was lower to start with. Some Yamaha owners use a Y-connector into both subwoofer inputs in order to double the voltage going to the sub. And, that typically resolves the Auto On issue. Of course, Yamaha owners can also choose to just leave their subs on all the time, if the Auto On issue proves to be a real problem. That will consume slightly more energy, but will not affect the operation or longevity of the subwoofer.

AVS member @Basshead recently mentioned a clever solution for achieving more headroom, with higher gains and lower trim levels, which seems to circumvent the Auto On problem with Yamaha AVR's. He went from a -1.5 subwoofer trim level to a -4.5 trim level, with a comparable gain boost, and obtained 3db more headroom, prior to clipping. But, his subwoofer didn't power-on reliably when watching TV at very low master volume levels. So, he lowered the trim levels on all of his other channels by -3db, and raised his MV level by +3db, and is now able to have his sub power-on reliably for low-volume TV content, while still having more headroom available for louder movie viewing. This is an additional technique that Yamaha owners might wish to try.]



Section III: Setting Crossovers:

This general discussion of bass-management, and of setting crossovers, applies to other systems of automated calibration and not just to Audyssey. The same questions come up so many times that I think it is worth emphasizing some of the basic crossover concepts. We use crossovers between our speakers and our subwoofer(s) in order to bass-manage our audio systems. In audio systems where there is no subwoofer, there will be no bass-management required, and speakers will always be set to Large or Full-Range.

The Subsections in Section III are as follows:

III-A: Crossovers from Speakers to Subwoofers

III-B: Low Frequency Effects Channel

III-C: Cascading Crossovers

III-D: LFE+Main


III-A: Crossovers From Speakers to Subwoofers:

Subwoofers can be used with two speakers in a stereo system, or they can be used with a 5.1, or larger, audio system. Whenever they are used, it is necessary to have a way to cross over from the speakers to the subwoofers, so that the subwoofers can play bass content below a designated frequency. Good subwoofers are designed for the sole purpose of playing bass frequencies below about 150Hz.

(Subwoofers will typically have a setting labelled "LPF" (low-pass filter) or "Crossover". It may be an analogue knob on the amplifier plate. That setting should generally be at the highest setting, which will usually be 150Hz. That will allow the subwoofer to play frequencies up to 150Hz, before it starts to roll-off. An exception to that general rule is explained in subsection III-C: Cascading Crossovers.)

As frequencies go below about 120Hz, and especially below 80Hz, subwoofers typically perform their specialized function much better than even large tower speakers can. We set crossovers to allow our subwoofers to take over duties below a selected frequency. That selected frequency will depend to some extent on the native capability of our speakers, and it will depend somewhat on the speakers' placement in the room, since room placement will strongly affect low-frequency performance of any transducers in both positive and negative ways, as explained in Section I.

Section I also noted the importance of placing tower or bookshelf speakers where they can point toward the listener, so that mid and high-frequencies will be heard clearly. The advantages of spreading speakers apart to achieve a wider soundstage, and potential issues with early reflections from side-walls were also discussed. But, those are all issues affecting mid and high-frequencies. Bass frequencies have different issues with respect to placement, and good subwoofer placement can be completely independent of the placement requirements for our tower and bookshelf speakers. This is why, even those of us with very capable speakers may wish to have subwoofers which we try to strategically locate for optimal bass performance, and which we bass-manage for optimal integration with our speakers. That is especially helpful for movies where the low-bass demands can be very significant.

When we look at a 5.1, or larger, system, we see even more importance attached to the subwoofers. First, the subwoofers must provide bass support for all of the regular channels, just as they would in a two-channel system. In 5.1 content, those regular channels can have bass peaks up to 105db, depending on master volume and bass levels, and the frequencies can go very low at times. Second, the subwoofers must play all of the content in the LFE (low frequency effects) channel, at peaks of up to 115db, also with potentially very low-frequencies. Subwoofers which are powerful enough for the room, and for the individual listener's preferences, perform this double duty just as they are designed to.

Those two different subwoofer functions are controlled by two different mechanisms. As explained below, the bass content redirected from the speakers is controlled by crossovers. The LFE content is controlled by a low pass filter, called the LPF of LFE. That LPF setting is explained in more detail in the next subsection. The default LPF setting in most AVR's is 120Hz.

The low-bass content in the regular channels is controlled by crossovers set for each pair of speakers (or for the center channel). In order for bass to be redirected from the regular channels to the subwoofer(s) speakers must be set to Small, with a crossover. The assignment of crossovers for each channel is accomplished during the initial calibration process, and then may be modified by the user. Crossovers may be assigned globally (such as 80Hz for all channels) or they may be assignable individually, for speakers pairs and for the center channel, depending on the specific AVR.

As stated earlier, where we have subwoofers in our audio systems, and wish to employ them as they were designed to be used, some system of bass-management is necessary to split the signal between the speakers and the subwoofer, so that the subwoofer can handle low-frequency content in the regular channels, while the speakers continue to play all of the other content. And, that split can only be accomplished through a setting of Small, with a crossover. Determining where that split between the speaker and the subwoofer should occur starts with the calibration process, where initial crossovers are assigned by the AVR. And, it continues after the calibration process, as listeners adjust their crossovers to achieve their specific listening objectives.

The frequency at which a signal split should occur may be different for different speakers in our audio system, depending on their low-bass capabilities and on their placement in the room. The section on room gain explains something of the unpredictable ways that a room, and placement within a room, can affect the bass response of a subwoofer. The same explanation applies to speakers. Room placement can affect a speaker's bass response, which in turn determines where a crossover will be set. (Where crossovers must be set globally for all of the speakers in an audio system, some compromise may be necessary.)

It was noted in an earlier section that distances (timing) and channel trim levels are determined by microphone position number 1. That is not the case with crossovers. Crossovers are set based on the fuzzy-weighted average of the frequency response from all six or eight microphone positions. As a general rule, crossovers in a full calibration will not vary much from where they would be set at mic position 1. But, they may vary slightly once Audyssey has calculated the FR at all available mic positions.

Audyssey (and other systems of automated calibration) accomplish bass-management during the initial calibration. When Audyssey measures all of the speakers in an audio system, it reports the measured F3 point of each channel to the AVR or AVP. (The F3 point is the frequency where a speaker is reaching the bottom of its frequency response, and rolling-off in SPL by 3db.) Once Audyssey has completed its measurements of frequency response for each speaker, the AVR then sets that speaker, or speaker pair, to either Small or Large (also called Full-Range), based on its own internal programming. If a speaker begins to roll-off in the mid to high 30's (or lower) the speaker will be set to Large. At any frequency above the high 30's, the speaker will be set to Small, and a crossover will be assigned. The weaker of two speakers in a pair will control the crossover, as Audyssey is specifically designed not to EQ below the F3 point of any speaker, or subwoofer.

As stated, if a speaker's F3 point is somewhere in the upper 30's, the AVR will round-up, and set that speaker's crossover to Small with a 40Hz crossover. Crossovers will always round upward, so an F3 point of about 42 or 44Hz would round-up to a 60Hz crossover, and an F3 point a little above 60Hz would round-up to an 80Hz crossover. The exact number used to round upward would probably vary somewhat among different AVR makers, but the basic principle involved is applied in both Audyssey and non-Audyssey systems. From the user's standpoint it is important to note that, without independent measurement, there is no way of knowing exactly where a speaker's roll-off actually occurred. So, it may be advisable to be conservative with crossover settings post-calibration.

[Sometimes people observe changes in crossover settings that seem to coincide with a change in the type or number of subwoofers. Crossovers may vary slightly from one calibration to the next, and certainly can change due to relatively small shifts in speaker positioning. As explained in the later section on room gain, boundary gain due to proximity to walls can affect the low-frequency response that Audyssey is measuring, as can specific room modes. Small changes in microphone positioning between calibrations can also affect crossovers, as Audyssey is averaging the results of all of the mic positions performed in a calibration. But, Audyssey is only measuring each speaker in isolation, without reference to subwoofers. Any change that appears to coincide with some change to a subwoofer is coincidental.]

The initial setting of Large, or of Small with a 40Hz, or 80Hz, or higher crossover, does not constitute a recommendation, either by Audyssey or by the AVR. It is simply a modest initial setting designed to somewhat protect the speaker, while providing information about that speaker (or speaker pair) to the user. The information the initial crossover setting provides tells the user something about where a particular speaker is actually rolling-off by 3db in that particular position in the room, and informs the user that no EQ is being performed below that approximate point defined by the crossover. It is then the user's responsibility to interpret that information, and to decide whether to leave the crossover at that initial setting, or to change it.


* It has already been noted that setting speakers to Small with a crossover is the appropriate way to direct lower bass frequencies from the regular channels to the subwoofer(s). As a general rule, I would suggest that where a calibration sets crossovers to 40Hz, an increase to about 80Hz might be advantageous. I believe that the 40Hz crossover setting covers a very narrow range of frequencies from about 36-38Hz to about 42-44Hz. Anything higher than about 42Hz or 44Hz will probably automatically round-up to a 60Hz crossover. With speakers already rolling-off at about 40Hz or so, a good subwoofer should handle that frequency, and those at least a half-octave higher much more effectively. I believe that 80Hz would typically be a much more conservative setting.

[FWIW, I think that an initial crossover setting of 60Hz may be especially problematical. Assuming that our AVR is rounding-up from anywhere in the low-40's to about 60Hz or so, then it can be very difficult to know where the F3 point of our speaker actually is. A speaker might be starting to roll-off at about 42 or 44Hz, or it might already be down 3db in SPL somewhere in the mid to upper-50's. In the absence of measurements to tell me where my actual F3 point is with a 60Hz crossover setting, or a strong preference for the sound with it set that low, I would personally be more comfortable raising the crossover to at least 80Hz.]

With crossovers already set to 80Hz during the calibration, users might also wish to experiment with slightly higher crossovers, or they might just leave them set at 80Hz. The lower bass frequencies put more demand on a speaker, or even a subwoofer, than the mid-bass frequencies do. And, that demand can create distortion in our speakers, particularly at higher volume levels. So, it typically makes sense to be more conservative with our speakers below about 60Hz or 80Hz. Different speakers, different rooms, and different speaker/subwoofer interactions, however, could all influence the selection of a crossover. Actual experimentation, with careful listening and/or measurement, may be required to make final setting decisions, and the selections may depend heavily on individual user preference.

As an aside, the reason that the Large or Full-Range setting is still necessary in modern audio systems is because not every system has a subwoofer, and not every user employs subwoofers for all listening material. For instance, some listeners may choose to listen to a music genre (which may have relatively little low-bass content) with their speakers set to Large, and without any subwoofers engaged. But, in order to employ a subwoofer for anything except the LFE channel, it is first necessary to set speakers to Small with a crossover.

[Some AVR's also have a feature which allows a Large setting with subs employed. The setting is called LFE+Main, or Double-Bass, and is explained in some detail at the bottom of this section. That setting may increase the apparent quantity of bass, but may also introduce considerable distortion in the process. It is not generally recommended by Audyssey and others, from an audio quality standpoint, although that is strictly a user-preference issue.]

It should be noted that owners are often surprised by the crossovers set by their AVR's. Sometimes, they are surprised that the crossovers are set so high, and sometimes, they are surprised that the crossovers are set so low, because in either case the crossovers don't align with their expectations. Two major factors contribute to that surprise. First, speaker makers frequently inflate the low-frequency specifications of their speakers. Second, room placement plays just as important a role in the low-frequency performance of our speakers as it does for our subs. The optimum location for a particular speaker (or speaker pair) may give it extra low-frequency response, due to boundary gain or due to favorable room modes at particular frequencies, or it may rob it of some low-frequency performance. Audyssey will simply measure what it detects, and report that to the AVR, which will set the crossovers in accordance with its own algorithm.

As a general rule, crossovers can always be set higher than where they are set automatically by the Audyssey calibration. This is a user-preference issue, and may depend on what sounds (or measures) best to a particular individual. It is not a good idea to set crossovers lower than where they were set automatically during calibration, because those speakers will not be receiving any benefit from room EQ, somewhere a little below the original crossover point. They will also be down 3db in measured SPL at the point where the EQ stops. And, they will continue to play softer and softer as the frequencies go lower, so, they will not be providing much audible benefit at that point, anyway.

in addition, running speakers with crossovers below the original calibration setting, may consume valuable amplifier power, and may result in some distortion, while the more powerful subwoofers are being correspondingly under-utilized. So again, it is typically better not to reduce crossovers from wherever our AVR's set them. Crossovers of at least 80Hz are typically recommended in THX standards, and for general best-practice purposes, as subs will nearly always do a better job of reproducing the mid-bass frequencies up to 80Hz or so. 80Hz is also used as a standard frequency for where most people will not be able to localize a subwoofer.


** Sometimes, HT owners may be a little reluctant to set crossovers of 80Hz, or higher, due to concern that they won't be using the full capabilities of their tower, or large bookshelf, speakers. But, in considering that, it is important to understand how the crossovers in our AVR's actually work. Crossovers are not like brick walls, where the speaker suddenly stops playing everything below 80Hz, and the subwoofer suddenly starts. We might be able to hear that kind of abrupt transition from speaker(s) to subwoofer(s).

Instead, when we set a crossover, the AVR implements a high pass filter (HPF) for the speaker(s) and a low pass filter (LPF) for the subwoofer(s). The high pass filter is designed to pass all of the frequencies above that point, while the low pass filter passes all of the frequencies below that point. But, both filters have a slope which gradually reduces the volume of the speaker or subwoofer to insure a smooth transition at the crossover point. (The filters also gradually reduce the volume to insure that neither speakers nor subwoofers are damaged by trying to play frequencies that they shouldn't be playing, at anything approaching full volume.)

The HPF for the speaker(s) is typically a 2nd order filter with a slope of 12db per octave. The LPF for the sub(s) is typically a 4th order filter with a slope of 24db per octave. Putting it into practical terms, that means with an 80Hz crossover, a speaker would still be playing 70Hz content, but -3db softer than it plays 80Hz content. It would play 60Hz -6db, 50Hz content -9db, and 40Hz content -12db softer. The frequencies between 40Hz and 80Hz would constitute one octave (40Hz times 2).

[Note: an octave is a series of 8 notes, between any two notes, where the frequency of one is twice that of the other. So, from 40Hz to 80Hz would be an octave. From 80Hz to 160Hz would be one octave.]

Looking at that same 80Hz crossover from the standpoint of the subwoofer, the sub would still play 100Hz, but at -6db compared to 80Hz. At 120Hz, the sub would play -12db; at 140Hz, it would play -18db; and at 160Hz, it would play -24db. The subwoofer's 24db octave would consist of the frequencies between 80Hz and 160Hz (80Hz times 2). Those crossover filters, set by the AVR, allow the speaker to gradually give way to the subwoofer, while allowing both subwoofer and speaker to play a little above/below the crossover point. (To put this into perspective from the standpoint of protecting the subwoofer, a sub playing with an 80Hz crossover would be playing 24db softer by the time it reached 160Hz.)

The crossovers within our speakers are similarly designed to create gradual transitions from woofer to mid-range, and from mid-range to tweeter (in a three-way speaker), although the precise amount of attenuation employed in the filters may vary among different speaker designs. When employed properly, crossovers enable us to listen to our audio content with no audible transitions at all from driver-to-driver within a speaker, or from speaker-to-subwoofer within an audio system.

As far as setting crossovers within our audio systems is concerned, there may be circumstances in which a crossover even lower than 80Hz is desirable, where measurements, or our own perceptions of sound quality, guide us. A situation where our speakers were set to Large during the calibration (because the measured F3 point was in the mid-thirties or lower) might lend itself to setting a crossover lower than 80Hz. For instance, in some cases, it is possible that a 60Hz crossover might provide an apparently smoother transition, or some other audible advantage, compared to the standard 80Hz crossover.

But, concern that we are wasting the capabilities of our speakers should not be a strong factor in our decision regarding what crossovers to use, as the speakers will always be playing content somewhat below the crossovers we select. It is also helpful to remember that our speakers have to play a very large frequency range, and that relieving them of some low-frequency burden (which requires a disproportionate amount of the amplifier power assigned to that channel) may help them to play their entire frequency range more effectively, and with less potential distortion.

Our subwoofers, on the other hand, have just that one specialized job to perform--playing low frequencies. And, as noted earlier, they typically do that single specialized job much better than even very large tower speakers. Particularly where bass-heavy movies or bass-enhanced music are concerned, crossovers of about 80Hz or higher are usually likely to improve our sound quality. In considering the use of crossovers for 5.1 movies, and for bass-enhanced music, it is important to realize how much additional low-frequency demand may be placed on our speakers. That is especially the case at higher master volume levels.

It is also worth noting that we may be better able to mitigate negative bass influences with our subwoofers than we can with our speakers. We will usually position our front speakers in specific locations on the front wall where they form an equilateral triangle with our listening position, or where they look good aesthetically. As we drop below 120Hz, and especially below 80Hz, room modes may be affecting more and more of our bass response. In some cases, we may be able to position our subwoofer(s) more strategically to sound good just for bass frequencies, where our front speakers had to be positioned to sound good for all frequencies.

If the subwoofers are better positioned with respect to room modes than the front speakers, then transferring more of the bass load (below about 80Hz) to our subwoofers may help to promote a superior frequency response with better sounding bass. This is always an issue that has to be resolved on a room-by-room basis, but it helps to understand some of the reasons behind the typical advice to use 80Hz or higher crossovers.

I think that the more that we understand how much more capable our real subwoofers are compared to the woofers in our speakers, the easier it is to use slightly higher crossovers. It isn't just a matter of woofer diameter, it is also a function of the subwoofer driver's motor strength and excursion capabilities (literally moving in and out to displace air and create SPL), the cabinet volume, the amplifier power, the DSP employed, and even the tuning point of a ported speaker, compared to both sealed and ported subwoofers. The subwoofer is typically far better at playing frequencies below about 80Hz or so, with higher SPL and less distortion.

A final crossover issue which is worth exploring involves bass localization. A subwoofer can be localized when the bass sounds that we hear are obviously coming from the subwoofer itself. Early listening tests determined that most people can't locate the specific direction of bass sounds below about 80Hz (or a little higher in most cases). At very low frequencies, bass is just not very directional. It was for that reason that 80Hz was originally chosen as a recommended crossover point from speakers to subwoofers. This is a fairly complex subject, in its own right, as explained in a little bit more detail in Section VIII-E: Subwoofer Placement in a Room. In general, where someone cannot identify bass sounds as coming directly from a subwoofer, an 80Hz, or slightly higher, crossover may be a very good choice.



III-B: Low Frequency Effects Channel:

There is another setting, associated with our subwoofers, which it is important to mention in the context of this discussion. It is not a crossover, but it does control the content we hear in the LFE channel. The .1 LFE (low-frequency effects) channel is a separate bass channel, played only by subwoofers, as long as there are any subwoofers configured in an audio system. (If there are no subwoofers connected to the system, and turned on in the configuration menu of an AVR, then the front speakers will automatically be set to Large, and LFE content will be routed to those speakers.)

The LFE channel is intended to give audio mixers an opportunity to add more bass SPL to 5.1 audio tracks. As explained in several sections of the Guide, we don't hear bass frequencies as well as we do other frequencies in our normal hearing range, so adding even more low-bass SPL, via a separate channel, can really enhance bass sound effects in movie soundtracks.

[It may be worthwhile to distinguish between two-channel music--stereo-- that has been bass-enhanced electronically, and 5.1 channel music. Some bass-enhanced music can contain very low-bass frequencies, and even bass sine waves, and it can be recorded at higher than normal volumes. The subwoofers will handle the low-frequency content in that music via the normal crossovers. But, only 5.1 music (or of course movies) will have an LFE channel which is dedicated to the subwoofers, and which is programmed to be 10db louder than the regular channels. Two-channel music which has been up-converted to surround sound, via Dolby Pro Logic, or by some other surround mode, is still two-channel music and doesn't have an LFE channel.]

As noted in other sections, the LFE channel has a max SPL of 115db for peaks, compared to 105db for the regular channels. That provides an additional 10db of bass SPL for audio mixers to be able to add to their 5.1 tracks. And, that allows selected low-bass sounds to stand-out more. In addition to the obvious difference of 10db between the regular channels and the LFE channel, however, there are also some differences in bass content.

The bass content in the regular channels ranges from the upper-end of the bass spectrum (500Hz) to the lowest frequencies contained in a recording. So, the range could be from 500Hz down to as low as single digits (~2Hz to 5Hz). The great majority of that bass content would be played by the woofers in the regular channels, with the subwoofers taking over at the crossover from the speakers to the subwoofer(s).

The LFE channel has a much more restricted and concentrated frequency range, and is exclusive to the subwoofers, if there are any in an audio system. Bass content in the LFE channel may still go down to the low single digits, depending on the film, but there is a filter which attenuates the bass content above about 120Hz. But, even beyond the difference established by the restricted frequency range, most real content in the LFE channel appears to be concentrated below about 80Hz. And, I believe that is generally intentional.

It is important to understand that the LFE channel is specifically designed to give more weight to the low-bass (and not quite as much to the mid and higher bass) in scenes where low-frequency effects are deemed appropriate by the sound mixer. So, the LFE channel may not be in very obvious or continuous operation during many movies. In some cases, the LFE channel may be used sparingly, and it may just kick-in with more really low and loud bass, whenever a sound mixer wants to emphasize the sound effect at a particular point in a film. In other movies, with sustained and intense low-bass, the LFE channel may be extremely involved throughout the movie. I believe that is the case in Batman Versus Superman, for instance.


* The LFE channel, which exists only in 5.1 (or higher) movies and music, has it's own setting in our AVR's, called the LPF of LFE. Since the LFE channel is intended to contain bass content up to 120Hz, the typical setting for the LPF is 120Hz. And, that is the customary setting which most AVR's engage by default. However, a number of audio experts, including Mark Seaton and Roger Dressler (formerly with Dolby Labs and one of the creators of Pro logic II) believe that, it can make sense to experiment with lower LPF settings.

Some people have suggested that relatively little meaningful bass content is mixed into the LFE channel above about 80Hz, as the LFE channel is primarily intended to emphasize lower bass sounds and special effects. That may or may not be correct in general, although some film mixers have indicated that most of their meaningful LFE content is in the lower bass range. It would make sense for that to be the case, since the original Dolby/THX standard was for speakers to crossover to subwoofers at 80Hz, and the <80Hz frequencies were always the ones most commonly associated with subwoofers.

In some cases, setting a lower LPF might emphasize low-bass frequencies a little more, and might also result in slightly clearer bass. Since the LPF is simply a filter, which gradually attenuates volume levels, setting a lower LPF will not completely eliminate bass above the filter, but it will roll-off the higher bass content a little earlier. For instance, an LPF setting of 80Hz would roll-off the 100Hz frequencies by 6db, and the 120Hz frequencies by 12db. Doing that would provide relatively more emphasis to the low-bass frequencies, compared to the mid-bass frequencies. That is similar to, but probably more subtle than, approaching the bass from the bottom by lifting the lowest frequencies with a rising house curve.

Mark Seaton has made the point that the more someone is boosting his subwoofer(s), the more that an 80Hz LPF may be helpful in making the bass blend well with the speakers in an audio system. That would particularly be the case where someone was using 80Hz crossovers for the regular channels. Remember that a subwoofer boost lifts all of the bass frequencies symmetrically, in both the regular channels and in the LFE channel. Where significant subwoofer boosts are employed, the bass frequencies above 80Hz in the LFE channel (which are already 10db louder than the regular channels) might seem to stand out too much in comparison to the lower bass frequencies. Again, that might be more likely to be noticeable where 80Hz crossovers are employed for the regular channels.

Some people may notice a little more bass clarity, and a little greater concentration on the low-bass, with an 80Hz setting. Others may prefer the fuller mid-bass sound with the default 120Hz setting, or may perhaps prefer a compromise setting of 90Hz or 100Hz. The differences among the various settings are probably fairly subtle, depending on the listener, and which setting sounds better is strictly a user preference issue. Although AVR makers typically employ a default LPF setting of 120Hz, there is no absolute right or wrong way to use the LPF of LFE. (FWIW, I do think it is possible that a higher LPF setting might contribute to subwoofer localization, when bass-heavy 5.1 content is playing.)

As a general rule, there may be no particular reason to experiment with the LPF unless a fairly significant independent sub boost is employed--perhaps at least 3 or 4db on top of DEQ, or even more than that without DEQ, and unless crossovers of 80Hz or 90Hz are employed for the regular channels. Or unless someone is specifically looking for greater clarity in the mid-bass range. There is some additional discussion of methods to achieve mid-bass clarity in the next section on Cascading Crossovers.

[Some additional discussion of the LPF of LFE, including comments from Mark Seaton and Roger Dressler, can be found in the Audyssey FAQ, linked below. It should be noted, however, that one comment at the very end of the FAQ discussion is not correct. LFE material is not "brick wall" filtered at 120Hz. As noted above, the LPF (at any setting) simply rolls-off content gradually, just as any other low pass filter does.]

"Official" Audyssey thread (FAQ in post #51779)

[It should also be noted, that just as some AVR's only offer crossovers which are already fixed at 80Hz, some AVR's do not allow LFE adjustments. Yamaha AVR's, for instance, do not have variable LPF of LFE settings. In that case, the default setting will be 120Hz.]



III-C: Cascading Crossovers:


The concept of using cascading crossovers, to increase mid-bass clarity, is one that has been around for a while. It may be especially helpful where someone is using significant subwoofer boosts in order to emphasize low-bass frequencies, or mid-bass chest punch. The process is typically defined as setting two crossovers in different places, such as on your subwoofer, and in your AVR, to combine at the same frequency. (In this case, we won't actually be setting a "crossover" in the subwoofer. We will just be setting a low-pass filter in the subwoofer which corresponds to the crossovers in our AVR.)

Combining two crossovers may potentially cause some cancellation at that specific frequency, such as at 80Hz, in some cases, although that may or may not be audible if it does happen. (As noted in an example on the Guide thread, by @bscool, if some measurable cancellation does occur, it can typically be corrected by either adjusting phase or subwoofer distance, as illustrated at the end of this subsection.) In any event, cascading crossovers will typically increase the strength and clarity of the mid-bass frequencies as a whole. And, many other listeners who have tried the process have reported an overall improvement in sound quality and in mid-bass impact.

I recently decided to experiment with the concept of cascading crossovers in my system, and I liked the results very much. I will explain what I did and what I liked about the way it influenced my sound quality. (I will note at the outset, that for anyone using Audyssey or some other form of room EQ, nothing about implementing cascading crossovers interferes with the filters set by automated room EQ. This is strictly a post-calibration tweak.)

First, I should explain that I have very capable speakers in my 7.1 system, and I never use crossovers higher than 80Hz. (I think that having reasonably capable speakers, which can handle crossovers below about 100Hz may be a prerequisite for successfully implementing cascading crossovers.) Second, I also prefer to use an LPF of LFE setting of 80Hz. I get better bass clarity when I set my LPF to 80Hz, rather than to the default 120Hz. Potential advantages to using the lower LPF are briefly described at the end of the previous subsection, and in greater detail in the Audyssey FAQ. The fact that I was already using a lower than typical LPF of LFE made me think that I might be a good candidate to try cascading crossovers.

Since I was already getting a very smooth transition at 80Hz, from my speakers to my subwoofers, I decided not to set my subwoofers' internal low-pass filters to that same 80Hz. Instead, I chose 100Hz, for my initial experiments, and then later tried 90Hz and 80Hz. Before attempting to explain what I experienced when I tried this, I should explain the physical mechanism involved. As discussed earlier, when we set a crossover for our speakers, in our AVR's, the speakers typically roll-off below the crossover at 12db per octave, and the subwoofers roll-off above the crossover at 24db per octave.

That means that at 100Hz, the subs are playing content at that frequency 6db more softly than they were at 80Hz, and at 120Hz, the volume in the subs is down by 12db. But, they are still playing the content, and their SPL still contributes to the overall sound and consumes some headroom from the subs. When, I set a 24db per octave, 100Hz LPF in my subwoofers themselves, I didn't affect the frequencies below 100Hz. But, I increased the magnitude of the roll-off occurring above 100Hz.

What I found when I tried this was that my mid-bass frequencies (up to 100Hz) seemed relatively louder than they had been, and my overall bass clarity improved. I especially noticed that I didn't have to boost my center channel as much as I had been doing, in order to hear clear dialogue. I think this is due to two factors. First, the higher bass content that had been played by my subwoofers was making the front speakers and surrounds a little heavy-sounding in proportion to the somewhat smaller center channel. And, second, since I was already using a heavy subwoofer boost, cutting-off the subs a little quicker imparted less bass coloration to the voices coming from the CC.

This is one of the reasons that I personally prefer not to use DEQ. I don't like boosting the bass in the center channel, with the voice coloration that I notice when I do that. Deep male voices typically only go down to a fundamental frequency of about 90Hz, so bass boosts above that frequency may make men's voices sound unnaturally thick and chesty to some people. As noted in other sections, however, whether we notice that sort of thing, or care about it, is strictly a YMMV issue. (I make up for not using DEQ by implementing a much more substantial subwoofer boost for movies.)


* I decided to add a little more detail to the explanation of why we may hear more mid-bass and overall clarity when bass boosts don't go above about 80 or 90Hz. Using voices is an excellent way to describe what I think is happening, and that is where I personally notice the additional clarity the most. The human voice is an instrument with a large frequency range. I said that bass boosts above 80 or 90Hz may potentially make male voices sound "chesty". In vocal music, a chest sound is deeper and more resonant than a head tone, which is produced higher in the voice box. The chest tone requires more air, and it resonates lower in the voice box than the head tone does, but it can also sound "throatier", and it has less clarity or "brilliance".

Some consonants, such as "B", "C", "D", "G", "T", "V", and "Z" which all share the same long "ee" sound, may be more difficult to distinguish if they are pronounced with too much chest tone. Some vowels can also be harder to distinguish if more bass sound is added to them, because the voice will sound slightly thicker. I believe that is especially the case if the person speaking has a strong accent, or if he fails to articulate clearly, or if ambient noises in the soundtrack make voices harder to hear clearly to start with.

(When someone articulates, he says each syllable of a word clearly and distinctly. James Earl Jones is a great example of a person with a very deep and resonant voice who is nevertheless very easy to understand. But, he had a speech impediment as a child and worked very hard to learn to speak slowly and with excellent articulation. Most actors do not have that style of speech and that kind of articulate diction.)

Remember also that if subwoofers are strongly boosted, with the normal 80Hz crossover in the AVR, they are only rolling-off at 24db per octave above 80Hz. So, at 100Hz, the subwoofer has only rolled-off by 6db and can still provide quite a lot of bass coloration to male voices. To me, that can make the voices sound a little unnatural as well as more difficult to understand. So, where I may not mind a little additional bass resonance in some music (the cello or the kettle drum, for instance), I may not like it quite as much for some other instruments. And, where I absolutely want it for the low-bass special effects in movies (well below an 80Hz crossover), I may not want that extra resonance at all where the human voice is concerned.

I found that as I implemented cascading crossovers at 100Hz, and then at 90Hz, and finally at 80Hz, I was able to concentrate a little more bass below 100Hz, and then below 90Hz, and then below 80Hz. And, not only did the mid-bass clarity improve with each attempt, but my mid-bass tactile response also increased as a result. That chest punch sensation is explained in detail in Section VII, but briefly, most people seem to feel the sensation most strongly between about 50Hz and 100Hz.

There is some evidence that the sensation may peak for most of us at around 63Hz. That specific number was the conclusion of one study I read years ago, and some subwoofer makers, such as SVS, provide the capability to add a pre-programmed peak at that frequency into their higher-end subwoofer models which have advanced PEQ. If we make our subwoofers roll-off more quickly above 80Hz, by implementing a 48db per octave filter, we are doubling the roll-off.

So, although there is still some transition between speakers and subwoofers, the subwoofers have rolled-off by 12db at 100Hz, and 24db at 120Hz. It is easy to understand how larger subwoofer boosts would allow us to benefit from a quicker roll-off above our selected crossover. And, it is easy to understand how we might be increasing the punchiness of the bass in the range where most people feel those chest punch sensations most strongly.


I offer this method of cascading crossovers as a means of potentially obtaining additional mid-bass SPL and chest punch, combined with potential improvements in overall bass clarity. (The clarity was the real key for me, but again, I use a lot of subwoofer boost for movies.) Determining where to set the LPF in the subwoofers themselves, and what slope to select if that is an option, is something which may require some individual experimentation. But, in my personal opinion, it may turn-out to be an excellent solution for someone wanting to maximize mid-bass SPL and clarity. The setting procedure is summarized as follows:


Setting Procedure:

To recap the procedure to follow in setting cascading crossovers, the following three steps would be performed:

First, all of the speaker crossovers would need to be set not higher than about 100Hz, in the AVR, and 80Hz or 90Hz would be better. So, we would need to have fairly capable speakers for at least our three speakers on the front soundstage. (It might not matter quite as much for surrounds, rear surrounds, height speakers, and so on, as it would be for the channels which carry so much of the fundamental content of both movies and music.)

Second, we would ideally need to be able to implement an LPF of LFE which approximately matches our speaker crossovers. Let's just say we are using 80Hz to make things simple. (If we couldn't adjust the LPF--as is the case in some Yamaha AVR's--we could still try cascading crossovers, anyway.)

Third, we would need to be able to implement a low-pass filter in the subwoofers themselves, or with a miniDSP, which would approximately match the crossovers to our speakers. (Most subwoofers will have either digital DSP, or an analogue knob--sometimes labelled Crossover or LFE--which will enable us to set a low-pass filter for the sub. Some subwoofers may also enable us to manage the slope of that filter. In my case, I rolled-off the bass above 80Hz at 24db per octave.)

When we implement all three of those settings to coincide: the speaker crossovers, and the LPF of LFE in the AVR; and the LPF in our subwoofer(s), the subwoofers will roll-off much faster above our target frequency of let's say 80Hz, and the mid-bass SPL and tactile sensations will be more concentrated below that frequency. I think it would be generally preferable to make the three low-pass filters correspond with each other, but they don't have to correspond exactly (especially with an analogue knob). And, if the LPF of LFE in the AVR can't be adjusted, there may still be some benefit to making the speakers' crossovers, and the low-pass filter in the subwoofer(s), correspond.


* After trying the 100Hz LPF in my subs for a few days, I experimented with dropping the LPF in the subwoofers from 100Hz to 90Hz, and the results were even better. In my particular case, the bass frequencies were even more distinct, and the center channel was even clearer. In fact, I was able to reduce the volume on my CC by about 2db, depending on the movie, and still understand dialogue perfectly well. I like using a large subwoofer boost for the very low-frequencies. I am in a large room on concrete, and it can take a significant subwoofer boost to generate the low-frequency sounds and tactile sensations I like.

But, using a large subwoofer boost also tends to make voices slightly thicker, and a little harder to understand, as explained above. We may get so accustomed to a slight bass coloration in voices, and in other mid-bass sounds, that we may not even notice that we are hearing it any more. At least I didn't. But, when that bass coloration is removed, the resulting sound from all of the speakers is much clearer. And, I can especially hear the difference in the center channel.

After getting used to the 90Hz LPF for a few days, which is how I typically like to test setting changes, I decided to drop the LPF in my subwoofers to 80Hz. At that point, all of my settings aligned at 80Hz. With each change, going from full-range settings in the subs to 100Hz, and then successively down to 90Hz, and then to 80Hz, I liked the results better. The acid test for me was when I watched Battle Los Angeles with the all 80Hz settings. It had been a couple of years since I had seen it, but I remembered how difficult it was to hear some of the dialogue during battle sequences. The mid-bass and low-bass were more impactful than ever, and the dialogue was easily understandable at lower volume levels than it had been before, even when I had boosted the CC.

The cascading crossovers make a noticeable difference to me, and to at least two dozen others who have reported trying them since I first wrote this. I think that the more subwoofer boost we use, the more that this approach may be helpful for us. This is probably not going to be a good solution for everyone. Our audio systems, and individual preferences, are just too diverse for any single method, or setting, to be successful for everyone. But, I definitely recommend trying it, if someone believes that he may be a good candidate, and if his subwoofers allow him to set a lower LPF, which he believes might potentially correspond well with the crossovers he is using in his AVR.

(Just to reiterate, none of the changes I am suggesting will interfere with the automated room EQ that you may be employing, and any of these changes are implemented after a calibration. You would always want room EQ to measure the full capabilities of your speakers and subwoofers. It is only after room EQ has set filters for the various channels that you would implement any limitations to the upper range of the subwoofers.)


Dealing with Cancellation:

I decided to put a brief description of the cancellation that people may sometimes encounter when they incorporate subwoofers into an audio system. Due to a variety of factors, some frequencies may cancel each other, eliminating bass at those frequencies. Cancellation can occur with a single subwoofer interacting with a room, or with dual subwoofers interacting with each other, or with subwoofers and speakers interacting at a crossover. That cancellation may occur either with or without cascading crossovers.

Cancellation always looks bad on a graphed frequency response. (It looks like a a deep V shape in an FR graph.) And, in general, we would prefer not to have any cancellation at any frequency. But, some cancellation may be pretty inevitable in most HT rooms, even where measuring capability and methods of independent EQ are available. Typically, where we have those capabilities, we are trying to move cancellation to another part of the frequency range, where it is less audible, or to another part of the room, where no one is sitting. Perfect bass in every part of a room is extremely difficult to achieve, even where we have four optimally-situated subwoofers.

I think it is worth pointing-out, for people who may be reading this and who do not have measuring capability, that not all cancellation is even audible. That is particularly the case with relatively narrow areas where frequencies are cancelling. For example, let's say that we have some cancellation centered at about 80Hz. That would be a fairly common area to expect some cancellation from speakers interacting (playing the same frequencies) with subwoofers.

We might not be able to hear that cancellation at all unless it covered a wide area. For instance, the octave between 60Hz and 120Hz consists of 8 notes, so each note in that octave covers slightly more than 10Hz. Most sounds we listen to are very complex, consisting of multiple notes (or frequencies, if the sounds aren't musical in nature), and harmonics (overtones) of those sounds. And, our brains are very adept at filling-in missing information. So, a narrow area of cancellation, centered on 80Hz, might not be audible at all.

Wide areas of cancellation, spanning more than 10Hz, might be more audible, as reduced bass in that particular area of the frequency response. But, for someone who doesn't have measuring capabilities, this is not necessarily something to be concerned about with our audio systems (either with, or without cascading crossovers) unless we suspect (either by listening or measuring) that we are losing some significant bass somewhere.

If we do have some reason to believe that we are experiencing cancellation, either via measurements, or from hearing a specific area of the FR where we think there is less bass SPL than there should be, there are a couple of different methods to reduce or eliminate the cancellation occurring at that frequency. One way would be to adjust the phase control on one or more of our subwoofers. Another way would be to adjust the subwoofer distance control in our AVR's. In either case, we could try playing an 80Hz test tone, or using some steady bass content at about that frequency.

Whether to use phase or distance settings probably depends on what kind of subs someone has (and how they are configured); and on what kind of AVR he has. If a single sub has a phase control, adjusting the phase on that subwoofer may be the easiest way to remove cancellation at a crossover. If a subwoofer doesn't have a phase control, adjusting the subwoofer's distance setting in the AVR will work. If someone has two equidistant subs, Y-connected into a single sub out in an AVR, then the use of the AVR's distance control might be easier to use.

If we are using the phase control on a subwoofer, to make it integrate better with speakers, or with another subwoofer for instance, the following information may be helpful. As I understand it, a distance change of 1/2 wavelength corresponds to a 180 degree change in polarity. So for instance, for cancellation occurring at 80Hz (which is a wavelength 14' long), a distance change of about 7' should reverse the phase completely. If a different crossover is used, there are online calculators which make it easy to correlate frequencies with wavelengths.

The real key to remediating cancellation, though, is to adjust the phase or the distance control gradually, measuring as you go, to determine what setting makes the cancellation either disappear (perhaps by moving it to a more remote area of the room), or move to a higher (or lower) frequency where it will have less audible effect. In an extreme case, and where the ability to measure the FR is not available, it might be possible to hear the bass getting stronger with different phase or distance adjustments, while playing some steady bass content.

The second method, the sub distance tweak, is offered compliments of @Alan P. Both the phase change, and the distance tweak, ideally require the use of measuring equipment such as REW. It would be fairly difficult to do the distance tweak, or the phase adjustment by ear, unless cancellation at the crossover were quite audible, and that would be rare. (Of course, if it's not very audible to start with, then many people may not want to adjust the phase, or to perform the sub distance tweak, at all.)

Sub Distance Tweak:

1. Measure the center channel and the subs with REW or comparable software (REW HDMI CH3) using a test tone of 80Hz, or whatever corresponds to the crossover.
2. Add to the sub distance setting of one subwoofer (or of both subs equally, if using an AVR which has a single subwoofer distance setting) in 1' increments. (With some AVR's, you must make sure to back-out of the distance setting menu before the new setting will take effect.)
3. Remeasure.
4. Repeat until you get the smoothest transition around the crossover.
5. If using an 80Hz crossover, it would not typically be beneficial to add or subtract more than about 7' of distance. That corresponds to one-half of an 80Hz wavelength, and a change of one-half wavelength would change the phase of a subwoofer by 180 degrees.
6. We normally have to choose between a good transition for the center channel and the subs, or for the front speakers and the subs. If someone is primarily interested in movies, balance the compromise in favor of CC+sub; and for music, measure with the L/R+sub.

It should be noted again that the procedure outlined above may not be necessary unless someone is either sufficiently curious to measure his results, and discovers something specific in the frequency response, or unless someone hears something that leads him to suspect that significant cancellation could be occurring.


III-D: LFE+Main:

There is a final setting, found in some AVR's (including Denon/Marantz) which some listeners may be tempted to try. It is called LFE+Main, or double-bass. It does literally double the bass, since it allows exactly the same bass content from the front two channels to be played by both the front speakers and by the subwoofers. With this setting, the subwoofers will continue to exclusively play all of the content in the LFE channel, but both they and the front speakers will duplicate the bass content in the regular channels. (Again, as long as a subwoofer is configured in an AVR system, it will always be the only transducer playing LFE content. That doesn't change with the LFE+Main setting.) It is called LFE+Main because the subwoofers still play LFE content, but now they also duplicate the bass content of the main speakers, instead of just handling the bass content that the main speakers don't play.

Some AVR makers added the feature for those HT owners who really didn't want to set their big front speakers to Small, with a crossover, but who still wanted to be able to utilize their subwoofers. But, as explained earlier in this section, the front speakers may not be able to play all of the bass frequencies in movies, or in some kinds of music, nearly as well as the subwoofers can. So, the front speakers may struggle with some low-bass content, causing audible distortion. And, when both speakers and subwoofers try to play the same frequencies, at the same time, the resulting sound quality can suffer in other ways, as well.

When we use LFE+Main, we are not redirecting the bass from our regular channels to our subwoofers as we are with typical bass-management. Instead, we are allowing the front channel content to be played by both the front speakers and the subwoofers. We can still set a crossover from the main speakers to the subwoofers if we wish, and the subwoofers will only duplicate content below that crossover. But, the front speakers and the subwoofers have very different output capabilities, very different frequency responses, and are EQed differently, so what typically results is considerable cancellation at some frequencies, and random peaking at other frequencies. But, the mid-bass frequencies might sound relatively louder, even if there were some resulting loss of clarity in the bass.

[To clarify what may sometimes be a little bit confusing, LFE+Main only operates when three conditions are met. First, the subwoofers must be shown (set) in the Speaker Configuration menu. Second, the front speakers have to be set to Large. Third, a setting of LFE+Main has to be used. That setting is found in the Bass menu, as an alternative to LFE. When LFE+Main is employed, any crossover still works for the front speakers. That crossover determines the frequency at which the subwoofers begin to duplicate the bass frequencies being played by the front speakers. So, for instance, if an 80Hz crossover is set, the subwoofers will softly start to begin operation an octave above 80Hz (as described in the discussion of crossovers) and will be at full-effect from 80Hz down. Meanwhile, the front speakers will play all content, as full-range speakers, to the lowest limit of their individual capabilities. And, the subwoofers will continue to exclusively play all LFE content.]

It may be important to explain a little more about this idea of having transducers with different capabilities, and which are EQed differently, playing the same content. Audyssey and other systems of automated room EQ set filters for each channel independently. So for instance, the specific stereo content from the left front channel is subject to the EQ for that channel, and the specific content from the right front channel is treated the same way. Content below a crossover point for those speakers is EQed for all of the subs as a whole. And, HPF's for the speakers and LPF's for the subwoofers add gradual slopes which help to prevent excessive peaks in the response at bass frequencies. So, as one transducer (the LF, for instance) drops away, the sub picks up the volume.

But, when both speakers and subwoofers play exactly the same content, with front channels and .1 channels that have been EQed differently, the EQ that has been done can no longer be relied upon to prevent bass peaks and cancellation at random frequencies. The distortion that usually results from the LFE+Main setting may produce what is often referred to as one-note bass. That same muddy or boomy-sounding bass can often be heard when room EQ is not operating. Bass clarity occurs when every bass frequency can be heard more-or-less distinctly, without some frequencies peaking and other frequencies cancelling. LFE+Main rarely allows that clarity for individual bass frequencies to be heard.

To summarize the decision of whether or not to use LFE+Main, it may be helpful to compare it to the use of room correction, in general. People who are looking for bass clarity are more likely to find it when they use some system of room EQ. Turning-off room EQ will generally introduce some muddiness to the sound, as bass frequencies randomly cancel, or boominess as frequencies randomly peak. That may very well create the impression that there is more bass, as described in the first section, because we will typically hear some frequencies much more loudly than we will hear others.

That same thing may happen when we engage LFE+Main. It may increase the apparent quantity of the bass in exchange for some quality, in the form of bass clarity. There is nothing wrong with experimenting to determine which setting we prefer, and proceeding accordingly. But, I believe that it may be important to understand the potential trade-offs we are making, and that it may be important to listen objectively to our overall sound quality. Some AVR makers added the LFE+Main feature over the objections of the creators of Audyssey, and of others, who believed that it was contradictory to the fundamental concept of bass-management, and of room EQ as a means to enhance bass clarity.

One of the distinctions that I might personally make would be between the use of our full-range speakers for some types of music, and their use for other types of listening material. As noted earlier in this section, some people may prefer to use just their full-range speakers for music that doesn't have a lot of low-bass content. In that case, turning off the subwoofers in the Speaker configuration menu, and simply setting the front speakers to Large, would enable the listener to enjoy properly EQed bass, played entirely by his front speakers. And, even if someone prefers listening without the use of EQ, more clarity should be achievable with the front channels either playing full-range content by themselves or with the use of properly bass-managed subwoofers.

As a general rule, I think it's fair to say that, if we need to have our subwoofer(s) playing in order to have sufficient low-bass to begin with, then we are usually better off setting our speakers to Small, and bass-managing them with a crossover. Then, if we ever want even more bass, we can just boost our subwoofer volume to get it. That way, we can still benefit from having the right transducers playing the right content, for their specific capabilities, and we can benefit from having properly EQed transducers and from the improved bass clarity which should result. As with all of our settings, however, this is strictly a YMMV issue.


* The Guide continues in the next post, with Sections III through VIII.


Regards,
Mike

GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.

Last edited by mthomas47; 06-03-2019 at 04:06 PM.
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Guide to Subwoofer Calibration and Bass Preferences


* Due to its length, I have had to divide the Guide into two posts. Sections I through III are in Post 1, and Sections IV through VIII are in this post. I have repeated the Table of Contents in Post 2, to facilitate navigation among the sections. Each section (and subsection) is hyperlinked so that readers can go directly to that part of the Guide by clicking on it in this Table of Contents. Readers can also link a specific section (or subsection) for others by right clicking on it in the Table of Contents, and copying the link location.


Table of Contents:


Introduction to the Guide:


Section I: Room/System Setup and Sound Quality:

I-A: Distortion, Speaker Placement, and Room Treatments

I-B: Room EQ and Calibration Techniques


Section II: Audio System Calibration and Subwoofer Levels:

II-A: Audyssey Calibration And Dolby Reference

II-B: Why We Add Bass After Calibrations

II-C: Where And How To Add Bass

II-D: Master Volume Levels And Sub Boosts

II-E: Gain Settings And Maximum Sub Output


Section III: Setting Crossovers:

III-A: Crossovers From Speakers to Subwoofers:

III-B: Low Frequency Effects Channel:

III-C: Cascading Crossovers:

III-D: LFE+Main:


Section IV: Audyssey EQ and Multiple Subwoofers:

IV-A: Setup and Calibration:

IV-B: Room EQ:


Section V: Audyssey Dynamic EQ and Dynamic Volume:

V-A: Dynamic EQ

V-B: Dynamic Volume


Section VI: Audyssey Thread History of Recommended Subwoofer Trim Settings:


Section VII: Bass Frequencies, Room Gain, and The Equal Loudness Contours:

VII-A: Bass Frequencies:

VII-B: Room Gain:

VII-C: The Equal Loudness Contours:


Section VIII: Bass Preferences, and Subwoofer Selection and Placement:

VIII-A: Sealed Versus Ported Subwoofers:

VIII-B: Comparing Subwoofer Performance:

VIII-C: Selecting Single Versus Multiple Subwoofers:

VIII-D: Internet Direct Subwoofers:

VIII-E: Subwoofer Placement in a Room:


********************************************




Section IV: Audyssey EQ and Multiple Subwoofers:

The Subsections in Section IV are as follows:

IV-A: Setup and Calibration

IV-B: Room EQ


IV-A: Setup and Calibration:

There are many advantages to having multiple subwoofers, and many HT owners do have more than one sub. The process for setting trim levels for multiple subs depends on the version of Audyssey being used. For all versions except XT-32, with SubEQ, there will only be one sub out on the AVR. People with multiple subs will typically Y-connect two or more subs into that single sub out. (In some cases, they may also daisy chain the subs.) With only one sub out, it is desirable, to run mic position 1 for each sub independently, letting Audyssey calculate distance and setting gain/trim levels, as described in the section above.

[Note: Although distances and trim levels are always set based on microphone position 1, some more recent AVR's require three mic positions before a calibration can be performed to check trim levels or distance settings.]

However dual subs are connected into the single sub out, once the subs are level-matched, the normal process can be followed to balance the two gain levels symmetrically, increasing or decreasing the gain levels by the same amount, to achieve a good negative trim level. And, distances from multiple subs can be considered, and if necessary, entered manually to more closely correspond to the timing that Audyssey observed when the subs were measured separately.

XT-32, with SubEQ offers users the advantage of having two sub outs, so a pair of subs can be level-matched, and have distances set, automatically. If three or more subs are used with XT-32, and two sub outs, the same process could be followed, measuring each sub independently before pairing it, using a Y-connector, and then measuring the combined pair as Sub Out 1 versus Sub Out 2. If you have tactile transducers in your system, they should be disabled or disconnected before running Audyssey. They can be reconnected or turned back on after the Audyssey calibration, without affecting the room correction that Audyssey has applied, as long as they are not on a separate sub out by themselves. (Audyssey calibrations are invalidated, whenever Audyssey detects a new speaker or subwoofer added to an audio system. But, it can't detect what is added via a Y-connector.)


1. Y-connectors:

People frequently ask about how to use Y-connectors with their subwoofers. RCA Y-connectors can be used for two different purposes. One purpose would be to connect two subwoofers into a single subwoofer output in the AVR. In that case, a Y-connector with two female ends and one male end would be required. A standard subwoofer cable has two male ends. One end plugs into the subwoofer input on the back of the subwoofer, and the other end plugs into the AVR subwoofer output on the back of the AVR. This is an example of a standard subwoofer cable:

https://www.amazon.com/dp/B00NCSQ5M4..._t3_B005979I6G

Where two subwoofers need to be connected to a single sub out on the back of an AVR, a Y-connector with two female ends and one male end should be used. The subwoofer cables from the two subs would plug into the female ends of the Y-connector, and the male end of the Y-connector would plug into the subwoofer output on the back of the AVR. This is an example of that type of Y-connector:

https://www.amazon.com/AmazonBasics-...r%2Bcable&th=1

A second and different purpose for a Y-connector would be to help a subwoofer power-on more easily from Auto mode, as noted in an earlier section. In this application, a Y-connector with two male ends would plug into dual subwoofer inputs on the subwoofer, and the single female end would plug into a standard subwoofer cable. That standard cable would then plug into the subwoofer output on the back of the AVR. That would double the voltage going to the subwoofer amplifier, which would help it to turn on more easily, but it would not increase the overall output of the subwoofer. This is an example of that type of Y-connector:

https://www.amazon.com/AmazonBasics-...r%2Bcable&th=1


2. Subwoofer Collocation Versus Mutual Coupling:


Two identical subwoofers are collocated whenever they are both placed inside a room. And, two identical subwoofers which are optimally situated in a room should average 6db more SPL than a single subwoofer would, when measured with broadband pink noise. That is considered a doubling in max output. However, they probably will not show a symmetrical increase of 6db at every frequency. As explained in the later section on room gain, interaction with the room will cause peaks (reinforcement) and dips (cancellation) at various frequencies throughout the subwoofers' passband.

Depending on which frequencies are peaking and which frequencies are dipping, there may not always be an apparent increase in loudness of 6db from well-positioned dual subwoofers. That could also vary, depending on the material we are listening to, and which frequencies that material is emphasizing. But, if we were to average the peaks and dips, or to measure the total SPL with broadband pink noise, there should be a net increase of 6db from properly positioned and set-up subwoofers which were identical (or very close to identical) to start with. As noted below, room EQ can help to make dual subwoofers both measure and sound more uniform in their frequency response.

Subwoofers are mutually-coupled when they are placed side-by-side or stacked. (Theoretically, subwoofers need to be within 1/4 wavelength distance in order to mutually-couple at every frequency. For instance, 1/4 distance for a 120Hz wavelength would be about 28". So, subwoofers within about 28" of each other should theoretically mutually-couple at every frequency from 120Hz down.) When they are placed in such close proximity to each other, they act as one larger subwoofer, and the room is no longer able to influence their interaction with each other.

Where mutual-coupling occurs, the second subwoofer adds 6db to the output of the first one at every frequency. That may be a very desirable situation where the frequency response of a single subwoofer is already very good. Unfortunately, however, if there are audible or measurable peaks and valleys occurring at particular frequencies, mutually-coupled subwoofers will not help to improve that. The subwoofers typically need to be spread well apart (typically on different walls) in order to smooth-out peaks, and to pull-up dips, in the frequency response.

When subwoofers are on opposing walls, such as front/back or facing side walls, subwoofers sometimes cancel each other at some frequencies. That cancellation may be especially noticeable around the crossover, and it may occasionally occur even when subwoofers are located on the same wall. Where audible or measurable cancellation is occurring, a sub distance tweak, briefly described in the subsection on Cascading Crossovers may be performed.

As noted in other sections, too-loud peaks in some portion of the FR can be even more noticeable than cancellation. Those random peaks are what makes bass sound boomy--what is sometimes described as one-note bass. And, cancellation can rob our systems of audible bass at random frequencies. Neither large peaks, nor dips, in the frequency response are desirable for good audio. Therefore, subwoofer placement and proper set-up are very important with respect to both improved frequency response, and with respect to achieving the full benefits of an expected doubling in max output. Some general suggestions regarding subwoofer placement are made in the last section of the Guide.


3. Increasing the Volume of Dual subwoofers:

A question that comes up periodically involves how increasing the volume on dual subwoofers affects the total bass SPL. Remember that AVR's sum the SPL of dual (or more) subwoofers, regardless of how the subwoofers are connected. When the initial calibration is performed, it is the combined SPL of the subwoofers that is set to 75db. That same principle of summed SPL continues when trim levels or gain levels are increased (or decreased). For example, if someone with dual sub outs symmetrically increases the trim levels of both subs by 5db apiece, the total increase in subwoofer SPL will be 5db, not 10db. The same thing would happen with a symmetrical gain increase. So, symmetrically increasing two or more subs by 'X' decibels, using either the AVR trim controls, or the individual subwoofer gain controls, will result in a total increase of that same 'X' decibels.

Asymmetrical increases would affect the original level-matching that Audyssey performed, but the result would be the same in terms of SPL increases. For example, if the trim level of Sub 1 were increased by 4db and the trim level of Sub 2 were increased by 6db, the net increase would be 5db (10db divided by 2) since the subs are internally summed in the AVR. For obvious reasons, it is not advisable to make asymmetrical increases or decreases in sub level without some specific objective in mind, as the subwoofers would then be playing at different volumes. That would unbalance the work being performed by the two subwoofers (assuming that the gain levels were fairly even) and might make it easier to localize sounds and tactile sensations from one of the subwoofers.

It is important to note that this doesn't mean that all of the headroom of multiple subwoofers can't be employed in an audio system. The total subwoofer headroom will still be whatever it is. It just means that if a particular user wants a net increase in subwoofer SPL of 10db, as in the example above, then all of the subwoofer levels need to be symmetrically increased by that same 10db.

It is easy to apply symmetrical increases using dual AVR trim controls. But, with large SPL increases, the subwoofer gain controls may also be necessary in order to keep AVR trim levels at about -5. In the absence of digital gain controls, however, it can sometimes be difficult to keep track of exactly how much gain increase is being applied. It is always a good idea to increase analogue gain controls by the same number of clicks, or by the same hash marks on the analogue dial. Many users make note of the original starting point, and some may mark that point on the gain dial with tape or a marker, so that they can keep track of how much post-calibration gain they have added. That also facilitates using the same gain setting for future calibrations.



IV-B: Room EQ:

Beyond the volume, distance, and crossover setting functions performed during calibration, Audyssey is a system of automated EQ, whose purpose is to achieve improved speaker/room interaction for the entire frequency range. Most people realize that the room strongly affects the way our speakers and subs sound, once we move several feet away from them. Audyssey's test tones have a range of 10Hz to about 22,000Hz, and Audyssey EQ's that entire frequency range, depending on the capabilities of the individual audio system.

So, when the 75db test tones are played through each channel, Audyssey is measuring the frequency response at multiple locations, and using a system of fuzzy-logic weighting to set filters (technically, control points) for each channel at various frequencies. The goal is to make each channel play 75db +/- about 3db, at every frequency (or cluster of frequencies), and that includes the subs. Audyssey sets filters for each channel independently, with the number and distribution of the filters dependent on the specific version of Audyssey. But, in all versions of Audyssey, and in all other systems of automated EQ with which I am familiar, the subwoofers are EQed together.

This is an important point! Regardless of the number of subwoofers in a system, Audyssey will create a single set of filters for all of the subs. Even with XT-32, once the level-matching is completed, Audyssey will only play separate test tones once (at mic position 1) through Sub 1, and Sub 2. That is for the purpose of setting trim levels and distances separately, so that the sounds of both subs will arrive at the MLP at the same time, and at the same volume.

But, for the remaining 7 mic positions, the same test tone is simultaneously played through all of the subs in the system together, however they are physically connected. That is because Audyssey is only setting filters for the combined sound of all of the subs in concert, and not for the individual subs. Again, using the example of a 5.1 system, all five speakers constitute a separate channel, and all five speakers get all 6, or 8 test tones, depending on the version of Audyssey. And, each of the five speakers gets its own filters. But, with the subwoofers, all subs are pinged and EQed together, and there is only one set of filters for all of them.

Audyssey users are encouraged to level-match all of the subs in their audio systems, prior to running a full Audyssey calibration. And, after Audyssey has set EQ filters, it is generally advisable to add or subtract to sub levels in a symmetrical fashion, by increasing or decreasing sub levels by the same amount. Some users are naturally interested in insuring that their subs are performing equal work, by attempting to have their subs both gain-matched and level-matched at all times. However, this is not always going to be the case after an Audyssey calibration, due to the inherently different influences that different room positions may have on the performance of the subs, as measured by the Audyssey microphone during the calibration process.

It is important to level-match the SPL of the subs in a system prior to running the full suite of test tones in order to present Audyssey with a level playing field. As noted earlier, that level-matching process is typically performed at microphone position 1--the MLP. Once the subs are level-matched to produce equal SPL at the MLP, the full six or eight point calibration is performed, and control points are assigned to the combined output of the subs. But, only one set of filters is assigned for the combined frequency response of all of the subs in an audio system. Once Audyssey sets those control points, changing the SPL of the subs asymmetrically, by raising the level of one sub and not another, or by adjusting sub levels in opposite directions may be counterproductive. That is because the playing field will no longer be level, as it was when Audyssey set its control points, and that may result in an adverse change to the frequency response.

Whether the change in frequency response will be measurable, or audible, will probably depend on the extent of the difference in sub levels from what Audyssey originally started with. For instance, raising one sub by a decibel while lowering the other one by a decibel, in order to make the perceived output of the two subs match better, may result in an imperceptible difference in the overall frequency response. But, it is generally inadvisable to make asymmetrical changes in sub levels without a very good reason to do so, and without the ability to measure the effects of such a change. Negating the positive effects of room EQ is obviously counterproductive to our intent in running the automated EQ to begin with.


Mixing Subwoofers in an HT System:

In addition to level-matching subwoofers prior to running an Audyssey calibration (regardless of the version of Audyssey employed) it is also highly advisable to match similar subs in the system, if at all possible. If similar subwoofers have somewhat similar frequency responses, it is often easy to make-up for some disparity in output with subwoofer placement. For instance, if one subwoofer has 3 or 4db less output than another similar subwoofer, we can place the weaker subwoofer very near the main listening position. Since SPL decreases by about 3db for each doubling of distance, in a room (6db outdoors or quasi-anechoically), the weaker subwoofer can now produce about the same SPL as the stronger one which is located further away.

But, combining subwoofers with inherently different frequency responses is a more difficult problem. It is important to understand what happens when we put subwoofers in a room. When we put a single subwoofer in a room, the subwoofer reacts in unpredictable ways with the room, causing peaks at some frequencies and dips at others. When we put two identical subwoofers in a room, the subwoofers react with both the room, and with each other, in unpredictable ways, causing peaks and dips at random frequencies. As noted throughout the Guide, some forms of automated room EQ help with bass frequencies, by smoothing-out some of those random peaks and dips to create a more uniform bass sound.

However, when we put two subwoofers which are not identical, and which have very different frequency responses--with more and less SPL at different frequencies--we create an additional level of unpredictability. Automated room correction is less helpful, in this case, because the subwoofers will roll-off at different frequencies, and the room correction is designed to stop EQing as the combined SPL of the subwoofers rolls-off by 3db. So, the ported subwoofer may not receive any EQ at all below about 40Hz or 50Hz, where it may still be playing quite strongly, because the sealed subwoofer may have already lost SPL at that point.

Putting sealed and ported subwoofers together in a room is a bit of a crap shoot. The combined bass may sound fine at some frequencies, and it may even measure reasonably well with REW, in rare cases. But, it is much more likely that there will be peaks at some frequencies and dips at others. In the case of random cancellation, due to differences in frequency response, better subwoofer placement may not help to resolve the issues.The typical recommendation is not to mix sealed and ported subwoofers, unless we have something like REW with which to measure the frequency response, and unless we have some means to EQ the subs individually. One way to do that would be via their individual DSP, which can be fairly sophisticated with some subwoofers. Another way would be with something like a miniDSP.

I often refer to Mark Seaton, of Seaton Sound, as a bit of a subwoofer guru, and I enjoy quoting him on subjects like this one. Here is something he said in response to a question about mixing a powerful ported subwoofer with a sealed sub:

"A sub like this should really be the primary bass maker, and if additional smaller subs are used, they should help with the upper octave of the sub range where position and modal issues are key. In the range around the tuning frequency where the port has the most function, the output lags by ~180 deg. This still produces lots of output because the driver is not moving much nor producing much output at this range. As you move to lower frequencies both the port and driver start to move freely again and this is why you see the rapid cancellation well below the port tuning. The port output works great on its own or with other subs having a similar port tuning frequency, but it will typically cancel with a sealed subwoofer or woofer tuned much lower."

So, according to Mark, if we combine ported and sealed subwoofers, we could expect cancellation to occur starting at around the tuning frequency of the ported subwoofer, or above it, and continuing well below that. That would essentially eliminate the value of the ported sub for low-frequencies. (The same thing could happen if the tuning points of two ported subwoofers were very different--perhaps more than ~4Hz.) There might be issues at higher frequencies as well, but the cancellation at lower frequencies would be something to expect.

Ed Mullen, of SVS, put it even more succinctly in the following quote:

"...mixing sealed and ported will typically result in the most phase cancellation of any possible dual sub combination. The reason why is because sealed and ported have very different phase responses, particularly in the deeper octave (18-36 Hz) where sealed is typically being boosted with internal EQ to achieve the target roll-off slope and ported is starting to approach port tuning and exhibit lots of phase rotation and group delay."

As a general rule, automated room EQ won't be effective in eliminating cancellation, as the resulting nulls will be too deep (and sometimes wide) for control points to have a meaningful effect. And, where cancellation was occurring near the tuning point of a ported subwoofer, most systems of automated room EQ wouldn't even be able to try to correct it, as the sealed sub would already have rolled-off the combined SPL prior to that, and room EQ would have quit setting control points.

Here is an example of how Audyssey works. Let's say that you have a ported sub (with about a 17Hz port tune) which produces high SPL from 50Hz down to 15Hz, and a somewhat equivalent model of a sealed sub which may produce even higher SPL above 50Hz, but which can't keep up with the ported sub below 50Hz. That would be a common scenario. Ported subs are specifically designed to produce louder volumes, within relatively narrow low frequency ranges (usually below about 50 or 60Hz), than their sealed counterparts.

Audyssey, in any version, won't inhibit the stronger sub. In this scenario, the stronger (ported) sub will still have more SPL down to it's F3 point of 15Hz, and slightly below. And, it won't overdrive the weaker sub by making it try to play lower and louder than it can. It will simply stop setting filters at the combined, detected F3 point. So, if the sealed sub begins to lose 3db of volume, compared to the ported sub, at about 50Hz, Audyssey will stop EQing at 50Hz, specifically to protect the weaker sub from being over-driven by Audyssey filters. And that means, that in this hypothetical scenario, you wouldn't have the benefit of any EQ in that critical low bass region (from 50Hz to 15Hz) where Audyssey is normally very helpful. And, Audyssey specifically couldn't address cancellation issues occurring anywhere close to the port tune of the ported subwoofer.

Of course, that doesn't mean that you can't try mixing dissimilar subs, and taking your chances that the bass will still sound pretty good to you. It might. And, it also doesn't mean that experienced HT owners, with REW and independent EQ, can't be successful in mixing dissimilar subwoofers. It simply means that when you mix subs with very dissimilar outputs, or different low-frequency extensions, or different roll-off characteristics, you would not typically expect to have good results without being able to measure your frequency response, and without some kind of external EQ.

And, you can no longer count on automated room EQ to help you improve your frequency response, below the F3 point of the weaker sub. There would also be no way to predict how effective a single set of filters would be, even above the F3 point of two very dissimilar subs. For most HT owners, life is much simpler if it is possible to have very similar subwoofers in an audio system.



Section V: Audyssey Dynamic EQ and Dynamic Volume:

The Subsections in Section V are as follows:

V-A: Dynamic EQ

V-B: Dynamic Volume


V-A: Dynamic EQ:

Audyssey's Dynamic EQ (DEQ) is a two-tiered program of loudness compensation, designed to maintain acoustic equilibrium in 5.1 movies, at below Reference listening levels. As noted in the previous section, DEQ is engaged by default whenever an Audyssey calibration is performed. Like the Audyssey Reference curve, which is also the default setting, DEQ can be turned off, or slightly modified, as explained later. Although DEQ can be used in TV, music, and gaming applications, it was specifically created to maintain perceived bass (and to some extent, treble) levels at below Reference volumes in 5.1 movies. At Reference (0.0 MV), DEQ does not effect the bass or treble levels in a system, in any way.

DEQ's sole purpose is to maintain an acoustic balance across the entire frequency range, as listening levels drop below Reference in 5.1 movies. Since 5.1 movies are recorded based on the Dolby and THX Reference standards, described in Section II, DEQ was designed to react specifically to volume reductions below that Reference standard, with accurately-sourced 5.1 movies, in home theater environments.

[It is important to understand that DEQ is tied entirely to the master volume level, once an HT system has been calibrated by Audyssey. Different source material may (and will) vary in actual volume level, with some source material being louder or softer than other material. But DEQ will only respond to the MV setting on the AVR or AVP. What that means is that, as we adjust our master volume level upward or downward, DEQ may be adding more-or-less bass boost, depending on the source. And, therefore, DEQ can sound different with different program material.

It is also important to remember that measuring the volume level with an SPL meter, or even with a calibrated measurement microphone, such as a UMIK-1, is likely to give a slightly different result than the one produced by the Audyssey microphone. All microphones have a built-in error factor that ranges from +/-1.5db to +/- 3db. But DEQ won't know or care what the exact measured SPL is. It is programmed to respond solely to the master volume setting, and to add SPL at certain frequencies, as MV levels are reduced below 0.0.]

The reason why DEQ was developed to make some adjustment to the acoustic balance in 5.1 movies, and the reason why that might be necessary, is predicated upon the Equal Loudness Contours (explained earlier, and in much greater detail in Section VII-C), which define the way human hearing works. Our hearing is most sensitive between about 2000Hz and 4000Hz, and is roughly equal in sensitivity in a range from about 500Hz to 5000Hz. Frequencies above and below that 500Hz to 5000Hz range require more loudness to be heard at the same level as frequencies within that average range. Considering just the lower frequencies for a moment, the lower in frequency we go, the more volume we need in order to hear those sounds in equilibrium with sounds in our more normal hearing range.

That is even more important with 5.1 movies than it is for music, because so much of the bass content of a modern action movie is below 50Hz, and below 30Hz, and frequently even below 20Hz. (With music involving acoustical instruments, on the other hand, relatively little content is below about 50Hz, and almost none is below 30Hz.) Modern 5.1 movies are mixed with a clear understanding of how human hearing works, and low bass sounds are amplified, with respect to other sounds in the film's soundtrack, in order to make them appropriately audible.

Low-bass special effects, involving earthquakes or explosions, for instance, can sound appropriately shocking at Reference, or near Reference volumes. But, that equilibrium among the easy to hear, and harder to hear frequencies, is only correct at a listening volume of approximately 0.0 MV, or just a little less. In other words, low, mid, and high-frequencies are designed to be in equilibrium at close to Reference volumes, but may not remain in balance as listening levels drop very far below Reference.

So, for instance, if we were to listen at about -10 MV, we would be listening only about half as loud as film mixers intended. And, in that case, we would still hear sounds quite well in our normal 500Hz to 5000Hz hearing range. But, we would need additional amplification of very low, and perhaps of some very high-frequency sounds, to hear those sounds in the same way that the film mixer intended for us to hear them. Otherwise, for example, low-bass sounds such as the rumble of thunder, or a train on the tracks, or even the sound of an explosion, might seem somewhat muted, detracting from the realism of the scene. Since, average listening levels on AVS appear to be in the range of about -20 to -10 MV, and each reduction in SPL, of 10db, is a halving in perceived volume, that attenuation of low-bass sounds can be quite a problem.

[It should be noted, that we hear most high-frequencies slightly better than we do low-frequencies, and, low-frequencies drop-off much faster perceptually as the volume level decreases than is the case with high-frequencies. Although DEQ adds a slight treble boost, most people seem completely unaware that DEQ is even slightly boosting very high-frequencies, and may not notice the difference in high-frequencies, if DEQ is disengaged.

It should also be noted that, in a home theater, a master volume of -5 to -8 might actually sound equally loud as 0.0 (Reference) would sound in a commercial theater. This was explained in an earlier section, where it was stated that "small" rooms (of ~20,000^3 or less) amplify the perceived loudness of sound pressure levels, compared to auditorium-sized rooms, such as commercial cinemas. So, in reality, something like -5 MV, or slightly less in a home theater, may actually sound as loud as a movie was intended to be heard in a commercial cinema.

It actually gets even more complicated, though, depending on whether we are listening to a cinema DVD or Blu-Ray, or whether we are listening to an HT release. Some films are specifically rerecorded for HT distribution, and they would not typically be quite as loud as those made for a commercial cinema. Film mixers have home theaters, too, and are aware of the relative difference in loudness between auditorium-sized spaces and HT's. Ultimately, though, we all simply listen at the volume levels which seem appropriate to us, regardless of whether we are watching theatrical releases or HT releases. And, often they are not specifically identified as being one or the other.]

As stated earlier, DEQ was specifically designed to restore acoustic equilibrium in 5.1 movie soundtracks, at below Reference levels. DEQ does that by providing approximately +2.2db of bass boost, and a little less than +1db of treble boost, for each 5db reduction in master volume, below Reference (0.0 MV). So, for instance, at -5 MV, DEQ boosts the bass by +2.2db; at -15 MV, the boost would be +6.6db; and at -25 MV, the boost would be +11db.

That boost is applied in all of the channels, including the .1 LFE channel, and it is applied incrementally at any listening volume below 0.0 MV. For instance, at -0.5 MV, there would be an almost imperceptible boost (about .2db) which would gradually increase as the master volume was reduced until the bass boost reached a maximum of 2.2db at -5.0 MV. The same incremental boosts would happen at -5.5 MV (about 2.4db) with the boost gradually increasing to a maximum of 4.4db at -10.0 MV, and so on.

DEQ does not, however, apply the same amount of bass boost at every frequency. In DEQ's target curve, the bass boost increases gradually, starting almost unnoticeably at about 200Hz. At about 120Hz, DEQ adds about +1db per -5 MV. That +1db boost continues down to 70Hz. Then, starting at 70Hz, the bass boost increases gradually, reaching a maximum of +2.2db at 30Hz. The maximum boost of +2.2db per -5 MV continues below 30Hz, to the limit of the low-frequency response of the subwoofer(s). The treble boost occurs primarily from 10KHz, and up, and is less than half the boost which is applied to the bass frequencies. Based on anecdotal reports from users, the treble boost in DEQ is so gentle as to be unnoticeable by most users.

DEQ's operation is designed to closely correspond to the way our hearing works, as described in some detail in Section VII-C of the Guide. That section presents an explanation of the Equal Loudness Contours. It can be observed in a graph of normal human hearing, that our hearing is significantly less sensitive at 120Hz, compared to frequencies in our normal hearing range of 500Hz to 5000Hz. Compared to the way we hear frequencies above 500Hz, our hearing continues to degrade down to 70Hz, and then degrades even more sharply down to 30Hz.

[Remember, though, that this loss of sensitivity in our hearing is only in relation to frequencies in our normal hearing range. We actually notice volume increases in very low-frequencies more than we notice increases in higher bass frequencies for reasons explained at the end of Section VII-C. This aspect of our hearing is complicated so I will leave that explanation for that section. But, it's important to understand that DEQ's action takes advantage of the fact that we notice a volume increase at 30Hz and below more than we do a volume increase at 70Hz or 120Hz.]

At 30Hz, our hearing levels-off and doesn't degrade further, down to the lowest limits of our hearing, which is probably about 16Hz or so for most people, under most listening conditions. (In any event, we lose our ability to distinguish tone in sounds below about 16 or 18Hz, and we also find it hard to separate sounds from bass tactile sensations at those frequencies.) DEQ's target curve follows the curves shown in the Equal Loudness Curves. This is an important point, because DEQ emphasizes very low-frequencies more than it emphasizes mid-bass frequencies. It adds 1db between 70Hz and 120Hz, gradually more than 1db below 70Hz, and it finally adds its maximum of 2.2db at 30Hz and below.

[Note: For more information on how the DEQ curve is applied, please consult the Audyssey FAQ. The FAQ section on the Reference Level Offset feature of DEQ, discussed later in this section, has graphs showing DEQ's operation at several different master volume levels, with different RLO settings. Those graphs may be found in Section a)3 of the FAQ, linked below.]

https://www.avsforum.com/forum/90-re...html#addendum3

It should also be noted that, as stated in the first section, many users add a personal sub boost to the one that DEQ provides. This is so typical that it may indicate that, for the majority of users, DEQ does not quite succeed in restoring bass equilibrium to movie soundtracks, watched at below Reference volumes.

* Given the nature of DEQ's target curve, which emphasizes the very low-bass over the mid-bass, it is also possible that some users may prefer more mid-bass boost, compared to the very low-bass boost of DEQ. For those users, adding an independent sub boost would proportionately increase the mid-bass, compared to the use of DEQ, since the independent sub boost would affect all frequencies equally where DEQ is emphasizing the lowest frequencies more than the mid-bass ones. This is an important point to understand!

The graph shown below illustrates DEQ's basic operation. Readers will notice that, at above 0.0 MV (master volume), DEQ actually reduces bass and treble SPL's. At Reference (0.0 MV), DEQ does nothing at all. As the volume level drops lower, DEQ creates a larger and larger house curve, increasing both bass and high treble SPL's. As noted above, DEQ affects the lowest bass frequencies more than it does the mid and upper-bass frequencies for reasons explained in Section VII-C: The Equal Loudness Contours.





1. Two-tiered Operation:

DEQ was described above as having a two-tiered operation. According to Audyssey's creator, Chris Kyriakakis, DEQ monitors both the master volume, and the volume of individual channels, in real time. DEQ then adds boosts, in accordance with the maximum limits, listed above, based on the actual volume levels and content, at that moment. This moment-to-moment action has never been explained in any detail, so I am not able to clarify Audyssey's specific actions in this regard. But, from comments made by its creator, DEQ is adjusting both the overall bass volumes, and some individual channel bass levels, on the fly. Those adjustments are made in accordance with DEQ's target curve, which was described above.

In addition to restoring acoustic equilibrium to 5.1 movie soundtracks, at below Reference levels, the designers of DEQ also decided to add another feature to the DEQ software program. Reasoning that sounds from behind were harder to hear than sounds from in front, or from out to the side (due to our pinnae--ear flaps--which funnel sounds into our ear canals) DEQ was also designed to boost all surround channels. That boost increases, as the master volume goes down, at a fixed rate. I believe that about 1db of surround boost is added for every -5db of MV. So, at a listening level of -15 MV (which is a nominal average) there would be approximately 3db of surround boost. Some users find the surround boost of DEQ helpful, or unnoticeable. Other users, who prefer not to hear a surround boost, often slightly reduce individual surround trims, or use the Reference Level Offset (RLO) feature to attenuate the surround boost.


2. Reference Level Offset:

Recognizing that not everyone likes the full effects of DEQ, for all types of listening material, Audyssey added the Reference Level Offset (RLO) to later versions of DEQ. The Reference Level Offset attenuates the effects of DEQ by controlling the point at which DEQ engages. It does that by literally offsetting the Reference point. When an RLO setting is engaged, DEQ will recognize Reference as being at a lower volume, and will not start its operation until a volume even lower than that new Reference point is achieved.

The RLO settings are applied in 5db increments. (When RLO is set at 0, DEQ is in normal operation.) At the lightest RLO setting of -5, Reference is offset by -5db, and DEQ does nothing at a master volume of -5, and adds a bass boost of +2.2db at -10 MV. At the medium setting of -10, DEQ adds +2.2db at -15 MV. And, at an RLO setting of -15, which is the strongest setting, DEQ would offset the Reference point by 15db, and it would add +2.2db of bass boost at a -20 master volume. Remember that DEQ is progressively adding more bass (and treble) boost as listening levels decrease below Reference (0.0 MV). It is adding about 2.2db of low-bass boost for every 5db below 0.0 MV. So, changing where DEQ starts to begin its boost changes the total amount of boost that it adds.

To put that into context with a specific example, let's say that someone is listening at a master volume of -25. With DEQ in normal operation, bass frequencies would be boosted by +11db. (DEQ started below 0.0 MV, so it added five increments of +2.2db each, which equals 11db). With an RLO setting of -15, though, DEQ wouldn't commence operation until the MV was below -15 MV. And, at -25 MV, it would be adding two increments of +2.2db of bass boost for a total of 4.4db. So, there would be a difference of ~6.6db between DEQ in normal operation, and DEQ with an RLO of -15, at a master volume of -25. (Anyone interested can work out the specific numerical amounts of boost, at different master volumes, with various RLO settings.)

Again, the RLO settings change where DEQ starts its operation, and that in-turn, reduces DEQ's effect at lower listening volumes. The lightest setting is -5, the medium setting is -10, and the heaviest setting is -15. Some people, who listen at lower volume levels, find the RLO settings particularly helpful with DEQ. That is because the RLO settings reduce its effect, based on the specific listening level. The higher the RLO setting, the less effect that DEQ has on bass and treble frequencies. Audyssey sometimes defines specific RLO settings as being more relevant for certain types of listening material, such as movies or music. It is up to individual users to decide, however, whether the RLO settings are actually useful, and if so, for what listening material.

DEQ's operation has always been among the most controversial features of Audyssey's operation, and as noted in Section II, it is completely independent of the filters that Audyssey sets for all of the channels including the subwoofer channel. So, DEQ can be turned on or off without affecting the room EQ in any way. Individuals new to Audyssey, or curious about DEQ, are encouraged to experiment. As noted earlier, some people use DEQ for everything, and some people don't use it for anything. Every combination in between those two extremes has also been noted on the Forum. Different RLO settings may produce positive results with DEQ, as may independent sub boosts either with or without DEQ. Experimentation with the tone controls may also positively affect both bass and treble sound quality. (The tone controls, which just affect the front speakers, can only be used when DEQ is disengaged.)

There is no single right way to operate the ancillary programs and features of Audyssey. Personal decisions regarding the use of DEQ alone, or with an additional sub boost, or with an RLO setting engaged, are just as much a matter of individual preference as is DEQ off, with additional personal tweaks to restore acoustic equilibrium (or not) to a particular audio track. As with the choice to use Audyssey Reference, versus Audyssey Flat, the decision of whether or not to use DEQ (in whatever setting) will depend heavily on individual rooms, systems, and listener preferences.


3. DEQ, Audyssey Reference, and Audyssey Flat:

An earlier section explained the difference between the Audyssey Reference curve and Audyssey Flat. To recap, during a calibration, Audyssey tries to make the frequency response as flat as possible from 10Hz to 22,000Hz. It does that by setting control points at frequencies where the SPL varies. Those variations in SPL, at particular frequencies, cause peaks and dips which interfere with the clarity of the sounds we hear, and which may degrade the overall sound quality of what we hear. Audyssey attempts to bring down peaks (by as much as 20db) and to bring up dips (by as much as 9db) so that all frequencies will play within about +/- 3db of the 75db test tones that Audyssey uses. When Audyssey is successful, the frequency response is relatively flat, and that is the basis of the Flat (or Music) setting in our AVR's.

But, there is another curve, which is engaged by default, whenever we perform an Audyssey calibration. It is called the Reference (or Movie) curve, and it is not entirely flat. The first section referred to the Harmon/Welti listening tests that concluded that most people prefer a rising bass response, and a declining treble response. In other words, according to those tests, most people don't like an entirely flat frequency response. Other sections explain some of the potential reasons for our listening preferences. The bass preference may be the easiest to explain, since we don't hear bass frequencies as well as we do those in the 500hz to 5000Hz range. And, as explained in Section VII, there are some good reasons for why we may like having more bass, for its own sake.

But, why we may like having less treble than a flat frequency response would give us is not quite as straightforward. One thing that is clear is that most of us don't tolerate very loud high-frequencies as well as we do very loud low-frequencies. In fact, the extra 10db added to the LFE channel is based on that difference in the way we hear and tolerate different frequencies. And, as explained in Section I, shrill sounding speakers, or distorted content, or rooms with a lot of higher frequency distortion due to the prevalence of hard surfaces, may even be acutely uncomfortable for some people. In any event, Audyssey introduced a high-frequency roll-off into the default Reference curve, in order to create their own Harman curve. That curve is engaged whenever an Audyssey calibration is performed.

The Reference curve does three things that deviate from the flat frequency response that the automated room EQ tried to create. First, the curve rolls-off frequencies above 4000Hz by -2db. That roll-off remains stable up to 10,000Hz. Second, at 10KHz and above, the roll-off increases to -6db. So, irrespective of the master volume level, the Reference curve will roll-off high frequencies by -2db, starting at 4,000Hz, and by -6db, starting at 10,000Hz.

The third thing that the Reference curve does is not related to high-frequencies. It is related to mid-bass frequencies. Reasoning that most speakers have crossovers from the mid-range driver to the tweeter at about 2500Hz, the Reference curve introduces mid-range compensation (MRC), which is sometimes called the BBC dip. That slight dip of about 3db of SPL between 2000Hz and 3000Hz is designed to help speaker crossovers integrate more successfully. Whether that theoretical improvement in integration between the mid-range driver and the tweeter was ever really necessary, however, is debatable. Section a)18 of the Audyssey FAQ addresses that question in some detail.

As indicated, the Reference curve does some things to mid-range and high-frequencies, but it doesn't do anything at all to the bass. So, it is only a partial Harman curve at that point, because there is no rising bass response, only a declining treble response. But, DEQ is also engaged by default, whenever a calibration is run, and it does do some things to the bass (at below Reference volumes), as explained in detail above. So, taken in conjunction, I think that the Audyssey Reference curve and DEQ are intended to resemble a Harman curve (at least at below reference listening levels), with both a declining treble response and an inclining bass response. (The MRC dip in the middle is completely separate from the idea of a Harman curve.)

But, things get a little more complicated when the Reference curve and DEQ are implemented together. The Reference curve rolls-off some of the high frequencies, and DEQ puts just a little of that back. DEQ adds ~1db (a fraction less than 1db) of boost to frequencies above 10,000Hz, at a rate of ~1db per -5 MV. So, if someone is listening at a fairly average volume level of -15 MV, Audyssey Reference will roll-off the frequencies above 10KHz by -6db, and DEQ will put back about +3db in the form of a treble boost.

It may seem a little confusing at first, but FWIW, I believe that the idea was to create a Harman curve, but then to also try to compensate for the greater difficulty people may have in hearing higher frequencies as the volume level declines. That second idea is consistent with the Equal Loudness Contours that describe how we hear equivalent loudness at different frequencies. We don't hear high-frequencies quite as well as we do those in our normal hearing range of 500Hz to 5000Hz.

I am not completely convinced, however, that those two ideas were ever entirely compatible, as the Harman curve was based on the conclusion that most people preferred some high-frequency roll-off at most listening levels, and not just at very loud ones. But, whether or not the theory behind the use of the Reference curve in combination with DEQ, was ever entirely correct, the way that DEQ is implemented does have some implications for our use of the Reference curve, versus the Flat curve.

In general, it seems to me that people with speakers which have a somewhat brilliant sound, located in rooms that have a lot of reflective surfaces, are likely to benefit more from the Reference curve with its high-frequency roll-off. Brilliant speakers might tend to become piercing or harsh-sounding in rooms with a lot of reflective surfaces, and with the Audyssey Flat setting engaged, since there is no high-frequency roll-off with Flat. On the other hand, speakers which have tweeters (such as soft dome tweeters) which do not have as bright a natural sound, or which are located in rooms which have been somewhat treated to reduce early reflections and ringing, might be better potential candidates for the Flat setting. This is strictly a YMMV issue, which can only be determined by the individual listener.

But, in my opinion, where the Flat setting is combined with DEQ, particularly at listening levels of -15 or lower, the high-frequency boost which DEQ adds on top of Flat is likely to make higher frequencies sound a little more shrill. I believe this is something which I may have experienced some years ago, in my mixed-use room, when I was employing DEQ for movies and some TV shows. The Audyssey Flat setting always seemed to be a better natural fit for my speakers, and for my room, than the treble roll-off baked into the Reference setting did. And, I didn't need the MRC dip between 2000Hz and 3000Hz, as my speakers already handled that crossover nicely. But, at listening levels of -15 and -20 MV, I believe that the high-frequency boost of +3 to +4db, that DEQ added, was something that I heard (and didn't particularly like) without being able to completely identify what I was hearing.

I stated earlier that DEQ has always been somewhat controversial. That is probably due in part to the SPL boost it adds to the surround channels, and in part to the bass boost that it adds to all of the channels, including the center channel. For some people, the surround boost, in combination with the bass boost in the CC, makes dialogue harder to hear. Some people describe DEQ as somewhat boomy-sounding. I think that would especially be true with some movies and TV shows which already have a lot of ambient sound, at loud volumes, programmed into the surround channels. (As noted earlier, the RLO settings can help in that case.)

I think that people should decide what they like for themselves, and for their own reasons. And, experimentation is probably the only way to be sure what we prefer. But, it may be worth pointing out that DEQ could (almost unnoticeably) add brightness to sounds which are already bright enough, especially when the Audyssey Flat setting is employed. It is probably fair to say that DEQ was always designed to work in conjunction with the Audyssey Reference setting, rather than with the Flat setting. And, it is probably also fair to say that DEQ may be a less natural fit with the Flat setting. This is just speculation on my part. But, the analysis of the relationships among the three settings might be something to be aware of, and to experiment with, if someone is interested.



V-B: Dynamic Volume:

Dynamic Volume is Audyssey's program for normalizing volume. It is currently offered in Denon/Marantz units. Other AVR brands have similar software programs using different names. The object of Dynamic Volume is to reduce the dynamic range of music, movie, and TV shows so that there will be less difference between the loudest passages and the softest passages. That can be very helpful late at night, with children or family members sleeping, or with neighbors who would be disturbed by louder sound levels or by sudden loud noises.

But, for people wishing to use this feature, it may be important to understand that it is sudden crescendos in orchestral music which creates excitement. And, it is swelling music and gunshots, explosions, car crashes, and other audio effects which do the same thing with many movies and TV shows. Dynamic Volume literally compresses the sounds to achieve a more uniform volume level. But, when that happens some content may be lost. Previous sections have explained that low-bass sounds are harder to hear than sounds in our more normal hearing range.

Film mixers (and composers) may compensate for that by making bass sounds louder, when they want emphasis in a movie scene or in a musical score. Some higher frequencies may be similarly emphasized with temporary increases in the volume level of those frequencies. When the sounds are compressed into a tighter middle ground, some of the high-frequency content, and especially some of the low-frequency content, may be lost. And, as the dynamic range of the content is compressed, some of the corresponding impact of the music, or of the scene, may be reduced. There could be good reasons for using Dynamic Volume, where it is important not to disturb other people, but users may want to experiment a bit before using it when they don't really have to.

The following description of Dynamic Volume is paraphrased from another AVS member. I believe that his description is essentially correct. The "Light" setting (also called Daytime) on some Denon/Marantz units raises the noise floor (the softest parts), and the average level of all of the material, up closer to the maximum volume, so that everything sounds a little (or in some cases a lot) louder. That setting may especially emphasize dialogue. But, even though the overall volume may be louder at first with this setting, there still won't be much variation in the sound level, once a comfortable master volume is implemented.

The "Medium" setting (also called Evening) leaves the average level alone, and instead shrinks both the noise floor (softer sounds) and the noise ceiling (louder sounds) into that average center. Once the master volume level is adjusted to the preferred volume, there won't be any very quiet or very loud volumes in the program.

The "Heavy" setting (also called Midnight) doesn't change the noise floor (the softest parts) or the overall average level, but it significantly reduces the ceiling--the loudest parts. This setting may initially make dialogue a little harder to hear, but once the master volume is adjusted upward to a comfortable listening level, there won't be any sudden loud noises to disturb anyone.

As the names imply, the amount of compression increases as users go from the Light setting to Medium, and then from Medium to Heavy. Audyssey employs a listen-ahead feature with Dynamic Volume, which "hears" volume changes in a recording a few milliseconds before our ears can perceive them, and which changes the volume levels accordingly, based on the setting which was chosen. Chris Kyriakakis, the creator of Audyssey, recommends using DEQ whenever Dynamic Volume is employed, although I suspect that some user experimentation would be helpful there as well.

As noted previously, Dynamic volume is a feature which can provide some benefits, depending on specific circumstances. However, where optimum sound quality and more accurate performances of music/movie recordings are the goals, Dynamic Volume may not be the best way to achieve them. As with all audio settings, the use of dynamic range compression is a user preference issue.



Section VI: Audyssey Thread History of Recommended Subwoofer Trim Settings:


In the interval since I first wrote the Guide, I have come back to it many times to add new sections or to clarify points that I thought were important. And, I have done that in part because I thought it would be valuable to have a single source for some of the best practice recommendations which have evolved on the Audyssey thread and elsewhere, and which have, in some cases, superseded the advice found in the Audyssey FAQ. The FAQ represents a tremendous body of work, in my opinion. But, nothing stands still, and the FAQ hasn't been updated in years.

So, I thought it might be helpful to explain how some of the advice on the Audyssey thread has evolved in recent years. For those who remember, the original advice regarding sub trim levels was to keep them within a range of about -5 to +5. And, the FAQ reflected that advice. Then, after much discussion on the thread, about how sub amps can clip with higher trim settings, the recommended trim setting range in the FAQ was lowered to -3.5 to +3.5, and the advice from Mark Seaton and Ed Mullen was included in the FAQ. But, as explained above, that FAQ recommendation of -3.5 to +3.5 is still too high, particularly depending on master volume levels. As noted earlier, not all subwoofers can develop their full output with lower gain levels and higher trim levels. And with respect to both max output and clipping, sub placement could also exacerbate a higher trim setting, as Audyssey might already be adding up to 9db of boost, at some frequencies.

I also remember people, including myself, speculating at times that Audyssey sets trim levels conservatively, perhaps in an effort to protect less capable subs. I specifically mentioned that to people inquiring about wanting to boost their subs after Audyssey set the trim levels. And, it seemed like a plausible explanation at the time. But, that explanation was never really correct. Audyssey protects less capable subs by not setting filters (control points) below the F3 points of those subs. Audyssey does the same thing with the other channels, setting no control points below the measured F3 of a speaker, or speaker pair. But, it is still the obligation of the user to follow good procedures to insure that the sub(s), and other speakers in an audio system, are used correctly, and are not pushed beyond their specific capabilities.

Audyssey's actual reasons for setting the sub trim levels where it does was explained above. Audyssey uses a 75db test tone to set all of the channels in a system to the same level, as measured at the MLP, so that Audyssey can apply filters to all of the channels, in an effort to achieve a relatively flat frequency response. It can't do that unless all of the channels, including the .1 sub channel, are set to the same volume. And, setting all channels to the same volume, with a 75db test tone, is now pretty much a standard of the industry even for systems having little to no automated room EQ.

But, human hearing is designed/adapted to hear best from about 500Hz up to about 5000Hz. Our hearing quite naturally corresponds somewhat to the range of the human voice. As frequencies drop below about 500Hz, and particularly below about 200Hz, it takes more volume for us to hear those frequencies, at the same perceived level, than it does for the ones in our optimum hearing range. The more that frequencies drop below 100Hz, the harder it is for us to hear them at the same volume level that we hear higher frequencies, and the more subwoofer boost we may require in order to do so. The phenomenon of sharply declining audibility at lower (and slightly declining audibility at higher) frequencies, is graphically illustrated in various depictions of the Equal Loudness Contours, one of which is shown in the last section of this guide.

If we all listened at extremely high listening levels, up around -5 or 0.0, it is unlikely that we would need much bass boost, except that which is added for personal preference. But, most of us don't listen at nearly that volume level. The most common range I see quoted for the average listening level is from about -20 to -10 MV, and some people listen at much lower volumes than that. So, for the great majority of HT users, it was never really about Audyssey setting subwoofer levels conservatively, although Audyssey never added a Harman curve (a rising bass slope) to its two fundamental curves. Audyssey did provide DEQ, which has its own slightly rising slope. But, as noted earlier, not everyone likes using DEQ, and even for those who do, some additional bass boost is typically applied.

For most of us, though, the issue was always about needing more volume to hear subwoofer frequencies, in equilibrium with the higher frequencies played by the other channels in our audio systems, at below Reference listening levels. Even where Reference is not involved, with two-channel music for instance, some of us may still wish to add a bass boost in order to satisfy our own perceptions of appropriate acoustic equilibrium.

As noted earlier, very high frequencies are also a little outside our optimum hearing range, but high frequencies do not fall-off nearly as fast as low frequencies do. And, in-room reflections can emphasize high frequencies, and high frequency distortion, in a way that can be very noticeable. In fact, some studies indicate that most people prefer some high frequency roll-off, and the Audyssey Reference curve is based on that assumption. But, almost all of us seem to perceive any reductions in bass volumes much more easily. How much additional bass we need, or want, in order to perceive our sound as balanced, or to fully appreciate the low bass in specific pieces of music, or in specific movies, is a very personal decision which probably depends on a lot of factors, including our rooms, sub capabilities, sub placement, individual hearing, and personal preference.

None of this is to suggest that other Audyssey users, and specifically the people involved in the creation of the FAQ, haven't already understood some of the concepts listed above. But, the FAQ was written, and edited, over a period of several years, with some sections being revised, and others not. Someone trying to understand whether it is normal to boost his subs, after an Audyssey calibration, may not get the impression that a sub boost is fairly typical, when watching movies at below Reference volumes, for reasons that may transcend simple issues of personal preference. And, of course, personal preference will still always be an important factor in the individual implementation of our audio systems no matter what we listen to.

The FAQ may seem to suggest that it is somehow inappropriate to use the gain control on subwoofers to add bass after an Audyssey calibration. And, that is not correct! To summarize the current best practice recommendation for adding sub boosts: it is to try to keep AVR sub trim levels no higher than approximately -5, if possible, after adding however much sub boost may be preferred. In order to do that, the appropriate use of subwoofer gain, and not just AVR trim, is often required. And as previously noted, the use of the subwoofer gain control may also enable subwoofers to achieve higher max volume levels than they could achieve simply through the use of increased AVR trim.



Section VII: Bass Frequencies, Room Gain, and The Equal Loudness Contours:


The preceding sections have attempted to explain something of the fundamental operations of HT systems and of room EQ, in general, and of Audyssey in particular. Most of the discussion has involved bass frequencies, as bass seems to be a commonly recurring topic of conversation on a number of different threads. I have also tried to explore some of the reasons that people may prefer to add more bass to their audio systems, after a calibration (by Audyssey, or by other means) and the best methods for accomplishing that.

In this section, I would like to discuss how we experience bass, and some of the potential reasons that we seem to enjoy bass frequencies so much to start with. I would also like to explore what I see as some of the relationships among bass, room gain, and the Equal Loudness Contours, and how I believe those relationships may influence our use of subwoofers.

The Subsections in Section VII are as follows:

VII-A: Bass Frequencies

VII-B: Room Gain

VII-C: The Equal Loudness Contours


VII-A: Bass Frequencies:


Different people define bass frequencies, and subdivisions within "bass," in different ways. For the purpose of this discussion, I am defining bass frequencies as 500Hz and lower, as 500Hz is the frequency where our perception of loudness starts to change. Deciding what are the upper bass frequencies, which we don't talk about very much in HT, and what are the mid-bass frequencies is the first major subdivision of bass. For HT purposes, it makes sense to me to define the mid-bass region as starting at about 120Hz. That still leaves two octaves (120 to 480) for upper bass.

Down to about 120Hz or so, the bass frequencies may be played by the woofers in our speakers. From about 120Hz and lower, the bass frequencies are typically played by our subwoofers. Even if we set crossovers at 80Hz, the subwoofers will usually be playing the LFE channel up to about 120Hz. (And, of course, the subwoofers will continue to play above the crossovers we set, but at a rapidly declining volume as explained in Section III.) So, I will define the frequency range from about 120Hz down to 50Hz as the mid-bass range. This is the range usually associated with the tactile phenomenon known as chest punch, so that makes a useful dividing line.

Frequencies below 50Hz will be defined as low-bass, and those below about 30Hz will be defined as very low-bass. As noted in a previous section, most music involving acoustic instruments rarely goes below 50Hz, and even more rarely below 30Hz, so 30Hz seems like a pretty good dividing point for very low-bass. That is also about the practical limit for the bass which can be achieved in most commercial cinemas, so it seems useful to distinguish 30Hz as a dividing line for the low-bass frequencies in home theaters.

In nature (and in some movies) however, bass sounds can be in the low single digits--down to about 2Hz. Below about 20Hz or 25Hz, we will typically feel as much as hear those ultra-low frequencies (ULF). As frequencies drop below about 15Hz, we will almost entirely be feeling low-frequencies rather than hearing them.

I think that the first question we need to ask in this discussion is why do we seem to like bass frequencies so much that many of us are so frequently adjusting our HT systems (or buying more or larger subwoofers) to obtain stronger bass? Granted that we may not hear bass frequencies, as well as we do other frequencies, particularly as volume levels drop. But, why is that so important to us? Why do we like bass so much to begin with? I think that there are several potential explanations.

First, there seems to be a clear relationship between bass and rhythm. Drums in some form seem to have been a consistent aspect of virtually all cultures. And even primitive drums were able to generate lower frequencies than other early musical instruments could. Whether the drums consisted of hollow logs, or animal hides stretched over wood or gourd enclosures, drums of some sort have been used by almost all cultures as a method of communication across distances (low-frequency sounds travel), for ceremonial functions, and in the creation of music.

It's easy to see the last aspect of that in most contemporary classical, jazz, rock, world, hip-hop, rap, and other forms of music. Rhythm, typically created by bass sounds, provides the fundamental foundation for most music. And, although the specific rhythms involved (and the instruments used to create them--drums, a piano, or an upright bass or bass guitar, for example) may vary widely depending on the country or place of origin, or on the particular music genre, there is a kind of innate cross-cultural appeal to bass rhythms.

Bass rhythms may do something else besides lay the foundation for most music. To use a phrase that Gary @garygarrison who is a retired psychologist, likes to use, rhythmic bass frequencies may create "cortical arousal". Cortical arousal involves the stimulation of wakefulness, vigilance, muscle tone, and heart rate. There is a reason why military formations have traditionally marched to the rhythmic beat of drums, and why drums were often used to stimulate warriors entering battle. Compelling rhythms can be very stirring, and are often related to athletic and martial activities. I think a good example of that would be the drums used in the theme for the Olympic Games. I believe that we tend to react differently to rhythmic bass frequencies than we do to rhythms generated at higher frequencies.

So, we like rhythm, in general, and we especially like the sensations that bass rhythms may generate. And exclusive of rhythm, various bass sounds in music may simply be pleasing in and of themselves. But, if that is a pretty good general answer for why we like bass so much in music, how does that explain our preferences for strong bass in movies? We certainly have music in movies, and few movies would be as entertaining without the music. But, we also seem to enjoy strong low-frequencies in our HT's for non-musical aspects of the movie experience. Why do we enjoy the bass frequencies so much that many of us buy more powerful subwoofers specifically to be able to emphasize bass effects in movies?

I think the answer to that has something to do with low, loud bass sounds in nature. In nature, low and loud bass sounds are not a good thing. The roaring of lions or the sounds of other large predators; the sound and feel of a severe thunderstorm, or of a flash flood; the sound and feel of the earth trembling from an earthquake, or of a volcano erupting; are all natural sounds with loud, low bass that arouse atavistic feelings of fear and dread in us. That instinctive response may be hard-wired into our central nervous systems, or it may be an evolutionary adaptation. But, it is universal, and invokes what has been called the fight-or-flight response. It is another aspect of cortical arousal, but a less pleasant arousal, in this case.

Except that it is pleasant in movies, because we can experience that arousal without really being placed in danger. Where the special effects in movies are well done, I think that the sensations we feel can be a little like the way that we would be reacting to real natural disasters, or to real combat situations, but in a significantly diluted and safe format. We can enjoy the sensations of fear and dread, of increased vigilance and heart rate, and of stirred emotions that accompany the scene in the movie, while knowing that it's really just fantasy. We aren't actually trapped in an earthquake. A fire-breathing dragon really isn't about to devour us. It just looks, sounds, and feels like it. We crave those bass-induced auditory and tactile sensations in movies, and some of us probably crave them more than others.

That brings up an interesting thought. We often see people upgrading their subwoofers on the forum, and as previously noted, many forum questions involve how to obtain more bass. I wonder if, over time, some of us develop a tolerance for the sensation of bass-induced cortical arousal, and require even higher bass volumes to obtain the same levels of excitement that we have been accustomed to feeling? I would be surprised if that weren't a potential factor in the quest for more bass which so frequently seems to occur. The good news is that, for most of us, we probably eventually reach a point where enough subwoofage really is enough.


* Tactile Response and Low-Frequency Sounds:

Another aspect of our enjoyment of bass has already been alluded to in previous paragraphs. Bass frequencies can generate tactile physical sensations which are distinct from what we hear. We feel bass, as well as hear it, in a way that we don't with higher frequencies. Common examples of that are the thud of something heavy dropping to the floor, or the feel of lightning striking nearby and shaking the ground. The physical sensations, related to bass, are referred to as tactile response (TR), and TR is a little different from the bass we hear, although the sensations of hearing and feeling bass may be closely related, especially at very low-frequencies. Most of the tactile sensations we feel in our HT's are related to vibrations which we feel through the floor or through the air. They are caused by a combination of SPL and particle velocity--literally air moving due to bass sound pressure levels, and due to woofers (and ports) moving air in and out.

As with loud bass sounds, those tactile sensations can invoke feelings of fear and dread, or they can stimulate our emotions in positive and stirring ways, as the example of the drum introduction to the Olympic theme demonstrates. With a good recording, played on a good audio system, we can feel each drum strike. In contrast, a good example of the fear and dread that a tactile sensation can create would be from the first Jurassic Park movie, where the footfall of the Tyrannosaurus Rex coincides with the water trembling/rippling in a previous footprint. In that great scene, we can feel the same vibration that makes the water shimmer.


* There are several different types of tactile sensations which are associated with bass. One of the more popular tactile sensations is frequently called chest punch. That is a percussive resonance that we feel in our chests (an air-filled cavity) from an abrupt mid-bass sound, such as a gunshot, or the thump of a bass or kettle drum (the Olympic theme). Most people seem to feel that percussive resonance in a frequency range from about 50Hz to 100Hz.

There can be a good deal of individual variance with respect to all of our senses. Some people may be able to feel chest punch sensations as low as about 30-40Hz, or as high as about 150Hz. So, the 50 to 100Hz range is just an approximation, but most of us probably fall within that 'typical' range. At least one study suggests that the strongest peak for the test subjects seemed to occur at 63Hz. According to a similar study, it takes between 70db and 80db for most people to feel chest punch sensations.

Another important tactile sensation involves very low-bass frequencies, such as the one from the Jurassic Park example. We feel frequencies under about 30Hz in a completely different way than we do mid-bass frequencies. The low-bass feeling is more of a deep rumble, or a thud, than a mid-bass percussive strike. The deep bass vibrations typically seem to come more from the floor, rather than being transmitted directly through the air the way that chest punch is. (The vibrations are transmitted through the floor, as well as through the air, and they also go through walls and furniture. Room construction plays an important part in that, with suspended wood floors and drywall partitions having much more sympathetic resonance than concrete.)

The low-bass tactile sensations are generally more sustained in effect than the brief percussive chest strikes we feel. The low-bass sounds we hear are typically more prolonged, and the rumbling/thudding/vibrating sensations which accompany those sounds tend to linger as well. With very low-frequencies (below ~30Hz), it can be a little hard to separate what we are hearing from what we are feeling. It becomes even more difficult below 20Hz. That is one reason that tactile transducers (various forms of butt shakers) can be at least somewhat effective in augmenting bass sounds.

A third type of tactile sensation involves pressure that we may feel against our bodies, and especially against our eardrums. That sensation would be similar to what we might feel underwater, or on an airplane. That tactile sensation is associated with room pressurization, which may occur at very low-frequencies and at high volume levels.

(The feeling of pressure against our ears may also be associated with very high sound pressure levels, in general, and can lead to hearing damage if the sensations are prolonged. It should be noted that it can take a great deal of SPL to produce sufficient SPL to damage hearing under normal conditions. And, relatively few home theater systems can even reliably produce Reference volume levels, which can normally be used safely. It should also be noted that low-frequency SPL seems to be tolerated better by our ear drums than very high SPL at higher frequencies. For example, Reference levels of peak SPL's of 105db for the regular channels, and 115db for the LFE channel, are deemed to be safe. So, even at Reference volumes, bass frequencies are safely allowed to be higher in volume. Of course, those extreme peak levels in movies are also typically transient, and are rarely sustained.)

Smaller rooms, which are densely constructed and tightly sealed, will greatly enhance the pressurization effect which can occur from low-frequencies. Where bass frequencies are concerned, the sensation is connected to pressure vessel gain (PVG) which doesn't occur until below about 30Hz, in even the very smallest listening rooms. It is described in some detail in the following section on room gain. Room pressurization can also create cortical arousal, in terms of enhanced vigilance, and increased heart rate.

Where most of the tactile sensations being discussed in this section are the product of particle velocity, caused by the displacement of air, or which are the product of sympathetic resonance from a floor or other structure, the sensations associated with pressure vessel gain are the direct product of the room itself pressurizing--compressing with SPL and especially with low-bass frequencies. I believe that room pressurization also has to be related to the movement of air particles (as they are literally compressing), but it is easier for me to think of particle velocity as being a bit different from the phenomenon of room pressurization.

As explained in the next section, in order to really experience PVG, our bass transducers must be able to create sufficient SPL at the very low-frequencies where PVG occurs in our rooms. If they cannot, there will be insufficient SPL for PVG to amplify, and the room will not pressurize. It follows that smaller, tightly sealed rooms are much easier to pressurize with bass than larger rooms, or those which open to other parts of a house. (PVG is explained in more detail in the next section, but just to be clear about what I am saying, PVG will amplify very low-bass frequencies in nearly all rooms. If we have enough native low-bass SPL, we will get some additional ULF TR. But, whether we will feel a particular tactile sensation called pressurization will be highly dependent on room size and construction, and on total low-frequency SPL.)


** As with bass sounds, responsiveness to tactile sensations seems to vary somewhat among individuals. And, as with bass sounds, our preference for the different types of bass tactile sensations seems to vary as well. But the reason the phenomenon of bass tactile response is especially relevant to our discussion is because those tactile sensations may also influence our desire for more (or sometimes for less) bass SPL. I believe that it is partly the combination of bass sounds and bass sensations that makes bass frequencies so unique in nature, and which makes us want to emphasize them in a way that we much more rarely want to emphasize high frequencies. Among other things, TR adds realism and authenticity to the bass sounds we are hearing. We are accustomed to both hearing and feeling low-bass frequencies at the same time.

It should be noted that just as different individuals may be more, or less, aware of or sensitive to tactile sensations, so different individuals may also want to emphasize different kinds of tactile response. Some people are especially interested in enhancing the mid-bass TR (chest punch) and select subwoofers with stronger mid-bass capabilities. Or, they may add mid-bass modules, which have mid-bass port tunes, and which are bandwidth-limited to play no lower than about 40 or 50Hz. They are specifically designed to emphasize mid-bass frequencies. Other individuals may select subwoofers with deeper low-frequency extension, and more low-bass SPL, in an effort to experience stronger low-bass tactile sensations---especially the ULF rumbling or thudding sensations described earlier. And, they may add tactile transducers for the same reason.

There are a number of factors which can influence the tactile response that we get in our audio systems. Both our master volume and our subwoofer boost are important factors, with respect to particle velocity, as is physical proximity and the direction in which a driver, and ports for a ported sub, point. Some people employ nearfield full-range subwoofers in order to enhance tactile sensations. (Pointing drivers and ports directly at a listener may enhance tactile sensations, as both drivers and ports will be moving air.) Nearfield full-range subs can be especially helpful with ULF TR. Some people also use mid-bass modules (typically ported subs with tuning points around 50Hz or so) in the nearfield, to enhance chest punch sensations. The following thread examines the use of MBM's in detail:

https://www.avsforum.com/forum/113-s...-response.html

There are some physical factors which can also make a difference. In addition to physical proximity to a subwoofer, if we are on a suspended wood floor (wood above a crawl space, or on an upper floor), we will typically feel much more low-bass TR than if we are on a concrete slab. Concrete won't resonate, conducting vibrations from the floor to our listening chairs, in the same way that a suspended wood floor will. (A suspended wood floor can be a bit like a drum head vibrating from low-bass frequencies.) David Gage, of Deep Sea Sound, conducted tests on putting a couple of sheets of plywood or MDF under listening chairs in order to enhance tactile sensations. That can be effective at enhancing tactile sensations where HT's are on concrete.

https://www.avsforum.com/forum/19-de...tile-bass.html

Since higher bass SPL's will typically result in increased tactile sensations, the degree to which we perceive and enjoy those sensations may influence how much bass SPL we prefer at particular frequencies and at a particular listening level. For instance, if we wanted to feel more chest punch, we would want our subwoofers to emphasize mid-bass frequencies, or we might want more SPL at low-bass frequencies for more tactile ULF.

What I am suggesting is that bass is somewhat unique in that we both hear it and feel it. That combination of auditory and tactile stimulus affects many of us in ways that other frequencies do not. And, that same combination of sound and physical sensations may be a large part of the reason that we seem to like bass so much to begin with, and the reason that we may want to add boost to bass frequencies in a way that we may simply not care about with other frequencies.

This is an aspect of our interest in adding (or subtracting) bass SPL which can't be explained by referring to the Equal Loudness Contours, which describe our hearing acuity at different frequencies. We do hear different frequencies with variable perceived loudness, particularly as listening volumes change. But, we may be able to feel bass tactile sensations at much lower sound pressure levels than we can hear low-bass frequencies.

Again, the tactile sensations being discussed at the moment are caused by some combination of sound pressure levels and displacement of air. And, although displacement of air is related to SPL, it isn't exactly the same thing. Sound waves are vibrations which move through the air, vibrating back-and-forth as they move. In our rooms, they are created by our transducers. Particle velocity is not sound waves moving through the air, it is actual air particles being displaced by the backward and forward excursion of our drivers, and by the ports of ported subwoofers. (A great example of air displacement on a massive scale occurs during a thunderstorm, where lightning strikes fairly close by. The resulting thunderclap can shake even the sturdiest house.)

Brian Ding, the creator and designer of Rythmik Audio subwoofers, said something very interesting about tactile response and its use in movies. I think that it especially applies to low-frequency tactile sensations. This is a quote from a post he made in the Rythmik thread:

"Let me try here because we all experience something similar, just not as big as those in the movies which is created by CG (computer-generated special effects). We all play baseball and have experience of being almost hit by a ball without seeing the ball. How do you know a ball is there? It is the air disturbance (or sound, subsonic mostly, barely audible) caused by ball traveling at high speed. Any air disturbance will travel.... That is just the nature. With that in mind, when you are next a large object falls and lands next to you, your floor has much faster propagation speed than sound. So you feel vibration on your feet first, then you feel the air coming at you as "air blow" or as subsonic sound energy. Sound always arrives later. The difference of the two tells you how far away the impact point is from you, more or less. While it is not accurate, most people has this ability. Sound engineers do understand these principles. So they will create a sound energy first to simulate the ground thump (audible), and then the air ripple arrives later (subsonic)."

More SPL = louder volumes, and that same higher SPL typically means more tactile response. But, particle velocity (or air displacement) is also created by the physical movement of air particles (including those particles pushed by woofers, and those particles moving through a subwoofer port, particularly at the port tune where the port is out-of-phase with the driver) and is, therefore, somewhat distinct from sound waves which are carried through the air. We may be able to feel low-bass tactile sensations, which may be enhanced by resonances from the floor, even when we can't distinctly hear specific frequencies. I should note, however, that chest punch sensations seem to correlate much more closely to SPL than low-bass sensations do.

[I used the phrase "distinctly hear" in the last paragraph, because it is entirely possible that we could hear relatively soft low-frequencies if we could separate them from the complex sounds and harmonics (overtones of the fundamental frequency) which accompany them. Under normal listening conditions, we can't. But apparently, if individual test tones are played, some individuals may be able to hear very low-bass frequencies down to about 16Hz or so, at volume levels only about 20db (or slightly less than that) above the noise floor in our rooms.

The noise floor in most listening rooms is probably between about 35 and 50db, so hearing a sound in isolation, and in a relatively quiet room, we might be able to hear slightly lower than 20Hz sounds at only 55db or 60db. Of course, under normal circumstances, we won't hear low-bass tones in isolation. We will hear complex sounds, comprising multiple frequencies, and consisting of both fundamentals and harmonics of those bass frequencies. And, the higher-frequency harmonics will be easier to hear distinctly than the lower-frequency fundamentals. But, the fundamentals will still add bass weight which will help to color the sound. AVS member @unretarded linked the very interesting study below on another thread. Unfortunately, although a page link still works, the content to which I refer would have to be purchased now.]

The paper is linked here:

http://www.multi-science.co.uk/effec...-frequency.htm

[Another very interesting paper cites several studies which suggest that some individuals may be able to hear very low-frequencies (in the low single digits) with binaural headphones, and with sufficient volume levels. Apparently, even for those with excellent low-frequency hearing, about 100db (97db according to one study) would be required in order to hear a 10Hz sound. As noted above, we typically don't hear sounds in isolation, so separating a 10Hz sound from harmonics of that frequency, and separating the tactile sensations which accompany low-frequencies from the fundamental sound, can be very difficult. But, the very low-frequencies could still add weight to the sounds which we do hear more distinctly.

The paper also notes that we lose tonality in sounds below about 16Hz to 18Hz. Below those frequencies, sounds are just perceived as atonal noise, such as atonal sine waves. That suggests that the difference between a 10Hz sound and a 14 or 15Hz sound would be primarily related to the amount of tactile energy produced by the two sounds, since neither sound would have an audible difference in tonality.

Leaving pure sine waves aside, however, I suspect that a sound with a 10Hz fundamental frequency and a 20Hz first harmonic, might sound inherently lower than a sound with a 15Hz fundamental frequency and a 30Hz first harmonic, because we would be able to distinguish the tonality of the harmonics, even though we could not distinguish tonality in the fundamental frequencies. And, the 20Hz first harmonic would sound lower than the 30Hz first harmonic. This is just a theory of my own to explain part of what we may be hearing when we speak of differences in low-bass weight with subwoofers which can go lower and louder.]

The paper is interesting in describing the limits and diversity of human hearing, with respect to low-frequencies, and for much additional technical information on the subject of low-frequencies. The link is in PDF format:

https://www.researchgate.net/publica...nd_its_Effects


*** So, we may hear and feel bass frequencies in different ways at different volume levels. And, our setting preferences may be influenced by both our preference for bass sounds, and by our preference for bass tactile sensations. As we discuss and compare the amounts of sub boost which different people may prefer at a particular listening level, or the bass frequencies which we enjoy most, we are somewhat handicapped by an inability to also compare tactile response, since both SPL and TR can be factors in the quantity of bass we prefer.

Tactile Response can also be strongly influenced by the specific nature of our room and house construction, and by our proximity to our subwoofers, depending on whether we are on a suspended wood floor, for instance, and depending on the density of the construction materials used, and whether or not the room is tightly sealed. It should be noted, that the tactile sensations that I am discussing here are distinct from the effects that bass frequencies may have on other objects in our listening room, or elsewhere in or beyond our houses.

Things that shake in our houses, during intense bass scenes, vibrate sympathetically due to the same phenomenon of particle velocity that we feel in our bodies. (Transferred vibrations from the cabinet to a suspended wood floor, may also contribute slightly to those vibrations, but the primary effect will be from the floor reverberating due to particle velocity.)

But, what we feel as vibrations in our bodies, and what vibrations are transferred to other objects in a room, are two distinctly different issues. (For instance, something in a room could vibrate--resonate in harmony with a particular frequency, at a particular SPL, even if we didn't feel anything in particular happening. And, the sympathetic resonance which is causing something in the room to vibrate is not necessarily occurring at the very lowest frequencies.) It would be fair to assume that both excessive bass sounds, and excessive vibrations, either inside or outside our houses, could influence our bass listening levels and our desire for restricting our low-frequency extension.

[Since this issue comes up periodically on various threads, I decided to put in a brief comment about the use of acoustic pads or acoustic feet under subwoofers. If someone is on a suspended wood floor, he will get increased tactile sensations, especially at high volume levels. For people who really enjoy TR, that can be fun. But, as noted, not everyone likes those tactile sensations to the same degree. And, excessive vibrations transmitted through the floor may make other things in the room vibrate sympathetically at certain frequencies.

Putting something relatively inert, or absorbent, under the subwoofer to decouple it from the floor, can sometimes help to slightly reduce unwanted vibrations caused by direct contact between the subwoofer and the wood floor. It may, or may not, have a major impact on the overall low-bass TR, because that will primarily be caused by particle velocity, and not by direct contact between the subwoofer cabinet and the wood floor. What it probably will do, however, is to at least slightly reduce the vibrations which make other things in the room rattle. (It is often necessary to isolate rattles at the source of the rattle. For that, products such as Blu Tack can be very helpful.)

https://www.amazon.com/Blu-Tack-S050.../dp/B001FGLX72

There are a number of commercial products available to decouple subwoofers from a suspended wood floor, or people can easily DIY their own, based on the commercial offerings, if excessive vibrations transmitted through a wooden floor are thought to be a problem. It should be emphasized, however, that there will be no 'audible' benefit to doing this. It will just help to reduce unwanted vibrations transmitted through the floor.

And, there would be no acoustic-related reason to do it at all on something like a concrete or tile floor. Decoupling a subwoofer from a concrete floor really wouldn't be helpful, unless someone were on an upper floor in a condo or an apartment building, since concrete laid on top of soil doesn't transmit vibrations very well to start with. On some floors, there could be some benefit to putting something under a subwoofer to keep it from sliding slightly on a polished wood, or tile, or marble surface. But, in that instance, even something inexpensive like a carpet pad would probably work just fine.]

Something that may not be immediately apparent from the discussion of tactile sensations is that while room gain can amplify the SPL we hear at particular frequencies, it does not affect the tactile sensations we feel as chest punch, or as low-frequency vibrations. As noted earlier, those overt low-frequency tactile sensations are primarily created by the physical movement of air, and by corresponding floor resonances, and are not exactly the same thing as the sound pressure level in a room. Anyone interested in learning more about the more overt bass tactile sensations (as opposed to those where the room itself is pressurized), how they are produced, and how they can be measured in an HT, is encouraged to consult the following thread:

The VibSensor Accelerometer Test Thread



VII-B: Room Gain:

I thought that a brief discussion of room gain might be helpful in the context of this section. Room gain is a very complex subject which I don't fully understand. What I do understand is that the actual amount of gain a room will provide is something that can only be determined empirically (by listening and/or by measurement). Actual room gain, as heard or measured at the main listening position, is dependent on a number of factors, including the dimensions and construction of the room, the position of the bass transducers, and the location of the main listening position.


Bass Frequencies in a Room:

In order to discuss room gain, it is first necessary to talk about how bass frequencies behave in a room. Bass frequencies consist of long wave lengths, as explained in other sections. For instance, a 200Hz wavelength is about 5.5' long, an 80Hz wavelength is about 14' long, and a 20Hz wave length is about 56' long.

The length of the sound wave determines something about how it behaves inside a room. Below about 200Hz, bass frequencies are referred to as standing waves, as described in the subsection on room gain. Confined by a room's six surfaces (four walls, a ceiling, and a floor) bass frequencies below about 200Hz do three things.

First, some of the wave lengths reflect from nearby surfaces, causing boundary gain, and amplifying some bass frequencies.

Second, some of the bass frequencies bounce around the room until they run out-of energy, or they flex (bend) when they strike a room boundary, slowing down and colliding with other wavelengths. They may especially collect wherever two or more room surfaces meet, causing cancellation at some frequencies, and amplification at others.

[Frequencies below the transition frequency in a room are called "standing waves", due to their tendency to bend and collide with each other; unlike higher frequencies, which just keep bouncing-off surfaces in a room until they are absorbed or they run out of energy. As I understand it, when two bass wavelengths collide, superimposing themselves on each other, they may either amplify or nullify each other.]

Third, some of the bass frequencies go right through the walls, ceiling, or floor, causing bass sounds and/or tactile sensations to travel to other spaces beyond the room. The lower the frequency, the further that the wave lengths can travel to adjoining spaces. (That's why neighbors may complain about some low-bass frequencies, even when they can't hear the other accompanying sounds from a recording, that are in our more normal hearing range.)

When a room is tightly sealed, all three actions will still be taking place. Some of the bass wave lengths will be traveling through the physical structure of the room or house. Some wavelengths will be reflecting from nearby surfaces causing boundary gain. And, some wave lengths will be bending and pooling wherever two surfaces meet, causing cancellation at some frequencies and amplification at others.

[As noted in earlier sections, when we speak of a smooth frequency response, we are referring to a situation where some frequencies are not playing louder or softer than other frequencies. Where standing waves are concerned, this is more difficult to achieve, due to their tendency to both amplify and cancel various frequencies. As explained in the subsection on room gain, where amplification and cancellation occurs is determined by subwoofer placement in relation to room size and geometry.]

Overall, bass frequencies inside a room will be amplified by the six surfaces that they reverberate from. As explained in the subsection on Pressure Vessel Gain (PVG), once a certain low-frequency is reached, there will theoretically no longer be any cancellation occurring, and the room should only amplify bass below that frequency The frequency at which that occurs is determined by the physical dimensions of a room. The smaller the room, the higher the frequency at which PVG occurs.

If all of that is what happens when a room is tightly sealed, what happens when a room is open to other spaces? I believe that depends somewhat on the extent to which a room is open to other spaces. Some people say that bass will "expand" to fill adjoining spaces, or that subwoofers will "see" adjoining spaces. Neither of those statements is really accurate. Bass wave lengths will always continue to operate in the three ways described above.

If there are four walls, plus doors or openings to other spaces, some wave lengths will still ricochet from all six surfaces in the room, and some wave lengths will go through the openings in the same way that others are going through the walls, floor and ceiling. But, the bass wave lengths won't specifically seek-out the openings, or "try" to fill the spaces beyond the room. They will just randomly continue to rebound from one surface to the next, or they will bend and stop, or they will keep going straight through a boundary. And, it will just be random chance which wave lengths end-up passing through an opening; which ones pass through the wall beside the opening; and which ones bounce-off that same wall toward another boundary within the room.

To me, a good analogy might be to imagine a handball court, where the ball can bounce-off of all six surfaces inside the room. Now, imagine multiple (countless) balls, all bouncing randomly. Some of them keep bouncing back-and forth randomly, until they run out of gas. Imagine that some of them are randomly going straight through the room's walls, floor, and ceiling, instead of hitting at an angle that lets them ricochet-off to strike another boundary. Finally, imagine opening the door to the handball court. Some of the balls would inevitably bounce through the open door, and out of the room. But, it would just be random chance which dictates which ones do, and which ones don't.

Obviously, the more openings to the room there were, the more balls that would bounce out of the room. But, since there would still be countless balls remaining inside the room, the net effect of the number of balls leaving the room through those openings, might not be as dramatic as we would expect. I think that the key factor would be having a sufficient number of boundary surfaces for the balls to rebound from, to start with, and having those boundary surfaces close enough together to facilitate the ricocheting process.

I believe that the second part of that last sentence offers a simple layman's explanation for why smaller rooms get more room gain than large ones do. The room boundaries are closer together in a small room, allowing more bouncing, and colliding, to occur before bass frequencies lose energy, or randomly escape from the room. In thinking of this analogy, it is helpful to remember that bass frequencies radiate omnidirectionally from a bass source. So, in our analogy, the balls would be leaving the sources (our bass transducers) at every conceivable angle. And, that would add to the randomness of their actions.

* There are two important points here. First, low-frequencies can pass through nearly anything, including concrete block walls. The lower the frequency, the greater the ability to pass through walls, floors, and ceilings. But, I believe that, just as with the handball analogy, it is random chance which determines which wave lengths strike a room surface straight-on, and go right through, and which ones strike at just enough of an angle to flex and stop, or to ricochet off that surface and to continue to move around the room. That seems, to me, to be the most plausible layman's explanation for the three different ways that bass frequencies behave.

The second point is that, if our subwoofers are strong enough to play the frequencies to start with, there are always plenty of wave lengths left inside a room to create the bass sounds we hear, even after some wave lengths travel through walls and into adjoining spaces (or at some mid-bass and higher bass frequencies, even after some of them are absorbed by bass traps). And, there are plenty of wave lengths remaining in a room to create room gain, as long as they have opposing surfaces to rebound from. Of course, the larger the room, the greater the distance the wave lengths will travel in hitting a surface and returning to the listening position, and the sooner they will lose energy. The distances dividing the room's surfaces will determine the specific low-frequencies at which the greatest room gain occurs.

If a room only has three walls, and is entirely open to the space beyond, the situation is a little different. Now, there are only five surfaces within the immediate listening room for bass frequencies to ricochet from, instead of six. So, in that instance, relatively more bass wave lengths will enter the adjoining space and rebound from those walls. Some wave lengths will probably travel all the way to the far wall in the next room, rebound from it, and renter the HT room, causing some room gain at the lowest frequencies (due to the long distance that they travel), while others may remain in that room, or pass through walls in that adjoining space and escape both rooms.

A room which is completely open on one wall, may get less total room gain than a room which has four walls, even with large openings in the walls. The most important thing, from the standpoint of room gain, seems to be that the wall returns, at right angles, on all four sides, forming four corners. That is partly because the strongest room gain is created by wave lengths making a round trip from one diagonal corner to the other, and back again. This is explained in more detail in the subsection on Pressure Vessel Gain.

A room which is completely open on one side may get less room gain than a room with large openings, but with four corners. And, a room with large openings cut into into it may get somewhat less room gain than a room which is tightly sealed. How great the differences might be is something that would probably depend on a number of factors, including room size and construction density. But, the difference between a room that was completely sealed, and a room the same size, with four walls (and four corners) and openings to another space, would not be a night-and-day difference, according to my understanding of the way that bass frequencies behave.

[Where the discussion gets a little more complicated is when people speak of "pressurizing" a room with bass. This actually refers to creating a physical sensation of pressure against our ear drums and other parts of our bodies, similar to the cabin pressure we feel on our eardrums in an airplane flying at higher altitudes, or the uniform pressure we feel when we swim deeper under the surface of the water. As explained in the section which follows, that sensation is felt most strongly in a fully-enclosed and tightly sealed space, where the pressure surrounds us from all sides. But, that physical sensation of atmospheric pressure is not the same thing as the general amplification of bass SPL that occurs within a room.]

Later portions of this section will explain more about types of room gain, and something of the way that they occur. But, I believe that there is a lot of misunderstanding about the differences between sealed rooms and rooms which are somewhat, or largely, open to other spaces. I think that it is worth emphasizing that, all rooms, under about 20,000^3 in size, will obtain some degree of room gain.

It is also worth emphasizing that, the lower the frequency, the more that the room will amplify bass frequencies. Room gain isn't static. It is determined by the distance within a room that wave lengths travel, rebounding from at least two surfaces. It increases as frequencies go lower, with more amplification occurring in a room at 15Hz, for instance, than is occurring at 20Hz, and more occurring at 20Hz than is occurring at 25Hz.

(Those numbers are just used for illustration purposes. Actual SPL will vary both upward and downward, by frequency, depending on specific room modes within the room. There will be both peaks and dips in the Frequency response. But, in general, room gain increases as frequencies go lower.)

In a small room, room gain will start at a higher frequency than it does in a larger room, and there will be a greater additive effect to the gain as the frequencies go lower. The room gain in that small room will, therefore, be greater at lower frequencies than it would be in a much larger room. That has some relevance to our selection of subwoofers, which is covered in Section VIII.



Types of room Gain:


1. Boundary Gain:

There are three primary components to room gain. Boundary gain is the first component of room gain. (Room mode gain is the second, and pressure vessel gain is the third.) Bass transducers (including subwoofers) which are placed in an enclosed space, such as a home theater or a multi-purpose room, will be reinforced by proximity to boundaries, as sound waves reflect from those nearby walls. Closer proximity to a boundary (a wall, or even a partial wall) will typically enhance reinforcement in bass frequencies. And, the relative density of room construction can also affect the amount of boundary gain a room provides.

When speakers are placed in close proximity to multiple walls (a corner, for instance) the reinforcement increases, but may do so at the expense of some clarity. Determining the best locations for subwoofers (or speakers) in a particular room requires a trial-and-error process. But, all normal-sized rooms will provide some degree of boundary reinforcement.

Room gain (due to boundary walls) primarily occurs below the Schroeder Frequency, also called the transition frequency or modal frequency of a room. That transition frequency is typically below 200Hz, and is defined by the total volume of the room. The transition frequency is significantly lower in frequency in a very large room than it is in a very small room.

The transition frequency is defined as the frequency where "standing waves" are created due to interaction with boundary walls. (Standing waves are low-frequencies where some of the sound waves collect, standing or pooling where two room surfaces meet, rather than continuing to bounce around the room.) Above the transition frequency, the direct sound of the speakers dominates (although it is somewhat amplified and/or distorted by reflections). Below the transition frequency, resonances from the interaction of sound waves and room boundaries have more influence on the sound than from the direct sound of the speakers or subwoofers.

In this usage, "resonances" reinforce bass sounds due to reflections from room surfaces. Some of those reflections, which occur at close range, constitute boundary gain. In theory, boundary gain can add up to 3db of gain per boundary, which would mean 9db for a bass transducer placed entirely in a corner, as the floor and two side-walls would each contribute 3db.

In practice, boundary gain never seems to reach those theoretical numbers. That is partly due to walls flexing, and may also be partly due to some low-frequencies escaping through the boundaries. Denser walls may create more low-frequency boundary gain than very thin walls. Below is a somewhat technical resource for those wanting to learn more about boundary gain:

https://www.prosoundtraining.com/201...-loudspeakers/

There are on-line calculators which determine the transition frequency for a given room based on the room volume. In a small HT room of 1500^3, the transition frequency is about 200Hz. As rooms get larger, the transition point occurs at a lower frequency. For example, in a 3000^3 room, the transition frequency would be approximately 145Hz, and in a 4500^3 room, the transition frequency would be about 120Hz.

[It should be noted that different on-line calculators seem to differ slightly in their results with respect to the transition frequency. But, they should still give us a good general idea of where that transition frequency occurs in our rooms.]

Above the transition frequency of a given room, sound waves bounce around the room until they run out of energy, or until they escape through the room's boundaries. Below the transition point, bass frequencies (standing waves) continue to bounce-off solid surfaces, but they also collect and reinforce each other, increasing the total amount of the bass. (As noted above, some bass frequencies will also go through walls, floors, and ceilings, so some of the many standing waves will escape the enclosed space.)

As noted above, the extent to which bass frequencies in a given room will reinforce each other (or at some frequencies, cancel each other) is dependent on a number of factors. But overall, bass frequencies below the transition frequency are reinforced in our home theaters.


2. Room Mode Gain:

Room mode gain is the second component of room gain. As noted above, in an enclosed space, sound is reflected by the room's boundaries. At certain frequencies, two or more reflected sound waves can superimpose themselves on one another in such a way that some frequencies become louder or softer at certain locations within the room. Room modes occur at frequencies where those collisions of low-frequency sound waves are created by the room's fundamental geometry. (Boundary gain is enhanced by proximity to a boundary, such as a wall. Room modes occur irrespective of proximity to a wall, and can increase or decrease SPL at the affected frequencies.)

Significant room modes occur all the way up to about 200Hz, depending on the transition frequency of the room. And, they can cause both peaks and valleys in the frequency response. Room modes occur from low-frequencies striking walls (or ceilings and floors) and bouncing or pooling to collide with each other. Room modes above the transition frequency in a room are less significant in affecting the sound, as there are so many of them, and as they are so closely spaced together that they don't really affect the frequency response, or what we actually hear. However, below the Schroeder frequency in a room, where standing waves occur, there are fewer modes (spaced relatively further apart) and they have a greater effect with respect to creating audible (and measurable) peaks and dips in the frequency response.

Low-frequency sound waves traveling between any two parallel surfaces in a room are the strongest and are called axial modes. Tangential and oblique modes involve multiple surfaces (as low-frequency sound waves continue to ricochet around a room) and are weaker than axial modes. As noted above, room modes can both add and subtract bass at particular frequencies, but overall, bass frequencies will be amplified somewhat by room modes.

[An example of how room modes are calculated will probably be helpful. My room dimensions are: 28' long, by 24' wide, by 9' high. I have axial modes for each of those dimensions. The starting axial mode for the room length is ~20Hz which corresponds to a 56' wavelength. (28' x 2, because the wavelength has to make a round trip from one wall back to the other wall.) In theory, that axial length mode is my strongest room mode, and it should add 6db. My starting axial room mode for the room width of 24' is ~24Hz (based on 24' x 2). And, the starting axial mode for my room height is ~63Hz (based on 9' x 2). Interpreting room modes in a meaningful way requires some training and experience (which I lack). But, we may still benefit from understanding how room modes are determined when looking at on-line calculators.]

Together, the various room modes in a room define the low-frequency response of the room itself. As explained in other sections, subwoofer positioning, room treatments (bass traps) and room EQ can all affect the final frequency response. There are on-line calculators which can be consulted to understand where the various room modes will occur for a given room. The spacing of the various room modes will determine the relative evenness of the frequency response in the room, as room modes will both add and subtract bass at various frequencies. Oddly-shaped rooms, however, can be more difficult to calculate using on-line calculators.

The actual amount of room gain which we have in our rooms, due to both boundaries and room modes will vary depending on our room size and subwoofer placement, with smaller rooms receiving more overall room gain than larger rooms. (Room construction can also be a factor.) The use of multiple subwoofers, in good room positions, should theoretically allow for better utilization of room mode gain, with minimal cancellation. Subwoofer placement in relation to the main listening position (and with respect to other listening positions) is important in determining actual room mode gain.

Mark Seaton has stated that all rooms of about 20,000^3 or less should provide somewhere between about 6db and 18db of bass reinforcement, through a combination of boundary gain and room mode gain. (And, at very low-frequencies, from pressure vessel gain, which is described below.) That would be true even in rooms which are open to other parts of the house, although closer physical proximity to the subwoofers would be beneficial in that case. Small rooms would typically get much more room mode gain than larger rooms. In very small rooms (<1500^3, or so) total low-frequency room gain can be well in excess of 20db at very low-frequencies.

[It should be noted that, while smaller rooms benefit more from room gain than large rooms do, large rooms are typically more likely to exhibit a more uniform bass response than small rooms. In that case, the apparent trade-off is more bass, versus a somewhat more uniform frequency response. But, as with all such generalizations, the reality depends on the specific room.]


3. Pressure Vessel Gain:

The amount of overall room gain we get increases as frequencies go lower, with much more room gain occurring at 10Hz in a small room, for instance, than would be occurring at 20Hz. Slightly below the lowest modal frequency (the longest axial room mode), sound waves are too long to propagate (fully develop) in the room, and they theoretically only give back acoustically, amplifying the bass SPL at those low-frequencies. The phenomenon is called pressure vessel gain (PVG) and it happens with sound waves which are more than twice the longest dimension in the room.

[I have said that PVG "theoretically" only allows for amplification and not for cancellation. That is my understanding of how pressure vessel gain works. In practice, however, I have seen cancellation still occurring in a measured frequency response, at frequencies which should have been in PVG territory. So, as with most things in audio, theory and reality may not always be identical. A favorite expression I have heard is: In theory, theory and practice are the same, but in practice, they aren't.]

PVG can have both an audible and a tactile component. Where the content has low enough bass, and there is enough native SPL for PVG to reinforce it, we may hear more very low-bass content. If nothing else, PVG may add more bass weight to the sound. In a smaller sealed room, the amplification of those low-frequencies may literally create pressure against our bodies, and especially against our ear drums, similar to what we might feel on an airplane, or underwater. In some cases, at high sound pressure levels, the sensation may be fairly dramatic.

(Some people may enjoy that physical sensation of pressurization more than others. Personally, it's not a sensation I have ever particularly liked in airplanes and underwater. Even in a large sealed room, it may be possible to feel that sensation with sufficiently strong low-frequency SPL. For me, the sensation is typically preceded by a fluttering sensation in the ear drums, which I take as a warning to reduce the volume. That same sensation can sometimes accompany very high sound pressure levels at any frequency. It's never a bad idea to think about protecting our hearing, and physical discomfort is a pretty good indicator, in my opinion.)

People often speak of "pressurizing" a room with bass. But, the phenomenon of physical pressure against our bodies is actually a fairly rare phenomenon in most HT's. Having plenty of bass to satisfy an individual listener is not the same thing as "pressurizing" a room with bass. Even if someone wants to listen at Reference levels, with a bass boost in addition to that high volume level, the room may not "pressurize" with bass. A lot would depend on the size of the room, it's physical geometry, and the construction materials used. Denser materials could contribute to physical pressurization. Furniture within the room, and whether or not the room were sealed or open to other spaces would also affect the physical phenomenon known as pressurization. A small, densely-constructed and tightly-sealed room (such as a concrete bunker) should be much easier to "pressurize" with bass than a normal room constructed with sheetrock walls and ceiling.

But, regardless of whether a room is pressurized, PVG will amplify bass SPL below a certain frequency in our listening rooms. That frequency can be calculated by determining the longest diagonal dimension in the room, from corner-to-corner, and from floor to ceiling. The frequency which corresponds to that wavelength doubled (because the wavelength has to make a round trip) is the frequency below which PVG starts. In my room, for instance, the longest dimension in the room is 38' (28' squared + 24' squared + 9' squared = 1441. The square root of 1441 = 37.96.) If I round that to 38' and double it, the longest wavelength that can fully propagate (fit) within my room is the frequency which corresponds to that 76' wavelength. In my room, pressure vessel gain or PVG, starts at ~15Hz. The following table can be used to correlate wavelength to frequency.

http://www.soundoctor.com/freq.htm

Again, regardless of whether PVG causes physical sensations which we might describe as "pressurization" it will amplify the frequencies below that longest wavelength in a room. The theoretical gain associated with PVG would be 12db, at that 15Hz frequency, in my room. Below that frequency, there would be no further cancellation, due to room modes, and the room would only give back in a constructive manner. In my room, I could get a theoretical increase of another 12db at 7.5Hz for a total of 24db of room gain. I have deliberately used the term "theoretically" because actual room gain, due to PVG, is apparently never quite that full amount. Openings to other rooms, the relative density of construction, and even furniture and furnishings within a room, could reduce the actual gain attributable to PVG. A more realistic estimate of PVG would probably be in the neighborhood of 6db to 9db at the frequency corresponding to double the longest wavelength in most rooms.

In small rooms, PVG can start at a much higher frequency than the example used for my 6000^3 room, and the total amount of gain from PVG can be much greater. Let's take an example of a fairly small (but not completely atypical) room. It's dimensions are 16' by 12' by 8' and it is 1535^3. Performing the same calculation that we used for the larger room, the longest diagonal dimension for that room is 21.6'. Doubling that to 43.2' (for the wavelength to make a round trip in the room) and finding the frequency that corresponds to a 43' wavelength, gives us a frequency of ~26Hz.

So, in the 1500^3 room, PVG could add about 6db to 9db of room gain at ~26Hz. And, cancellation due to room modes would (in theory) not occur below that frequency. At 13Hz, PVG could add up to about 18db (double what it was an octave higher at 26Hz). So, PVG would be increasing its effect, in a symmetrical manner, below 26Hz in that small room. Contrast that with the 6db to 9db that the 6000^3 room may get at 15Hz. Small rooms can reinforce low frequencies a great deal more than is the case in large rooms, and an awareness of that may influence our choice of subwoofers.

Online calculators are available to calculate the transition frequency in a room, and to demonstrate where room modes will occur in a room. In the example of the small room above, significant amplification due to axial room modes can occur at much higher frequencies than they do in large rooms. For instance the longest dimension between two walls in that 1500^3 room was 16'. Doubled, that gives us a wavelength of 32' which corresponds to a frequency of 35Hz. So, the longest axial room mode could add up to ~6db at that 35Hz frequency. (It is worth noting again, though, that until the PVG frequency is reached, both amplification and cancellation will occur at various frequencies, due to room modes.)

A number of different online calculators can be used to determine at what frequencies room modes will occur, although the specific geometry of the room will have a definite bearing on their predictive accuracy. Those calculators obviously work much better for rooms which are either square or rectangular. Oddly-shaped rooms may actually have some advantages with respect to room modes not affecting the frequency response quite as much, but they are difficult to calculate for axial and tangential modes. This is an example of one online calculator:

https://www.bobgolds.com/Mode/RoomModes.htm


Conclusion:

To summarize this discussion of room gain, I should return to what was said in the introduction to this subsection. I don't fully understand how room gain works, and I suspect that I have only scratched the surface in my attempt to explain it to others. But, the important thing to take away from this discussion is that the room's specific geometry, combined with the placement of subwoofers, in relation to listening positions, will affect the amount of bass we hear.

Some degree of cancellation, at some frequencies, is probably inevitable, although we may not always hear any loss of bass in situations where the nulls are relatively narrow. Our brains do a pretty good job of compensating for occasional missing frequencies, and there will be harmonics of missing fundamental frequencies which may contribute to the impression of appropriate bass.. Overall, the room will amplify bass frequencies, and small rooms will amplify them much more than large rooms. With large rooms, the amplification will start at a lower frequency, and the amount of amplification will not be as dramatic, as illustrated in the large room, versus small room, examples that I used.


* Subwoofers which have very flat frequency responses, when measured in a quasi-anechoic setting, will always exhibit some peaks and valleys in the frequency response, whenever they are placed in a room. In extreme cases, those peaks and valleys can create a boomy, one-note sound, with peaks at some frequencies dominating what we hear. In other instances, where a subwoofer (or the listening position itself) is located in a wide null, bass frequencies cancel, causing the audible bass to be sharply attenuated. In either instance, where excessively boomy bass is the result, or in the slightly rarer instance where audibly attenuated bass is the result, subwoofer placement with respect to the main listening position is extremely important.

It is worth noting that good subwoofer placement is not limited to just the sub's overall position in the room. There is a tendency to assume that subwoofers need to point toward a listening position in the same way that speakers do. But, bass frequencies radiate omnidirectionally, so it isn't necessary to have a woofer pointed right at a listening position in order to get good bass sound. The center of a subwoofer cone (or the center point between two drivers in a multi-driver sub) determines it's frequency response. So, moving the location of the driver, with respect to a wall, or with respect to a listening position, may slightly change the frequency response.

In some cases, the best overall frequency response might be obtained by rotating the driver toward a wall, or at an angle to the listening position. So subwoofer orientation, at a particular room position, is also worth exploring. Sometimes, moving or rotating a subwoofer by even a few inches can have a beneficial effect. Optimal subwoofer placement is something that can only be determined by experimentation, and several factors including aesthetic appearance and tactile sensations may influence the final decision. There is additional information on subwoofer placement in the last section of the Guide.

Multiple subwoofers, properly positioned, can typically help listeners to achieve a more even and clearer-sounding frequency response. And, room EQ can also be very helpful with respect to bass frequencies. (Typically, using multiple subwoofers, combined with room EQ, is better than using just one method by itself. And, adding bass traps can also help with frequencies down to about 60 or 70Hz.) All of those measures may sometimes be needed to improve the frequency response at multiple listening positions, although the more widely the listening positions are dispersed, the more difficult it may be to get equivalent sound quality at every position.


** Just to clarify a question that comes up sometimes, room gain is not eliminated when room EQ is employed. Room gain can still reinforce very low frequencies, for instance, enabling subwoofers to play lower frequencies than they could have played on their own. Automated room EQ will not completely eliminate that, although room EQ may attempt to pull down a particularly high peak, caused by room gain, in an effort to achieve a smoother frequency response. Room gain is typically very helpful in assisting both sealed and ported subwoofers to play <20Hz frequencies. All of the potential headroom generated by room gain is still available to the user if he ever wants to tilt his bass frequency response upward, by using DEQ, by using an independent subwoofer boost, or by the implementation of a personalized house curve using the Audyssey app or via some other means. Any of those increases in bass would necessarily happen post-calibration.

As explained in earlier sections, what room EQ tries to do is to simply level the initial playing field inside a room, so that subwoofers are able to play closer to the flat frequency response that they started with when they were measured quasi-anechoically. After room EQ has accomplished that objective, it is up to the individual user to determine how much bass he wants to add, post-calibration, to what is now a somewhat smoother and more uniform frequency response, with fewer peaks and valleys. As with nearly every aspect of audio, the extent to which we prefer the results of room EQ with respect to bass frequencies, and the specific extent to which we prefer to add bass boosts post-calibration, are entirely individual questions.

For more information on how room gain may actually work in a particular room, and why it is so difficult to accurately predict in advance, an article written by Data-Bass is a very helpful resource.

https://data-bass.com/#/articles/5cb...e58a?_k=5a6bij


4. Room Gain Compensation:

Recognizing that different rooms may affect the low-bass frequencies more or less strongly, a number of subwoofer makers have a feature on their subs which can be adjusted to slightly increase or decrease the amount of low frequency SPL the subwoofer produces. This feature is generally an analogue dial on the back of the subwoofer and may be named something like Low Frequency Adjust (JTR subwoofers) or Room Size Control (PSA subwoofers). The max (clockwise) setting on that analogue control allows the subwoofer to achieve its maximum low-frequency SPL. A lower setting will reduce the SPL (most noticeably under about 30Hz) in situations where room gain is already strongly amplifying low-bass frequencies.

Not every subwoofer will have this feature. But, where this sort of control is available, determining which setting produces the most pleasing sound quality is a highly room-specific, and user-specific decision, and some degree of experimentation is generally required. But, a question that is often asked is where to set that analogue dial prior to running a system of room correction such as Audyssey. Several subwoofer makers including Seaton Sound, JTR, PSA, and Rythmik, recommend setting the control to about the mid-point, prior to running room correction.

Starting at about the mid-point allows the subwoofer to produce enough low-frequency SPL to enable a system such as Audyssey to EQ low frequencies, while still allowing for some upward user adjustment post-calibration. Using a higher dial setting would produce more low-bass SPL. But, that might be counter-productive, to start with the higher low-bass SPL, if room correction were to subsequently EQ it back to flat. For instance, if someone set that control to the maximum setting prior to running Audyssey, Audyssey might attempt to pull-down the low-frequencies in order to achieve a smoother frequency response. And, even if Audyssey, or YPAO, or whatever, didn't EQ out some of the low-frequency SPL, there would be no way to add a low-frequency boost after calibration.

It would always be possible to increase the total subwoofer SPL, post-calibration, but that boost would be the same at all frequencies. It wouldn't be possible to create a house curve by concentrating more boost in the low-frequencies, because the dial would already be at the maximum setting. On the other hand, if someone set the dial at its lowest setting prior to calibration (in an effort to preserve the maximum amount of low-frequency boost for later) the automated room EQ might not be able to measure and respond to the very low-frequency performance due to insufficient SPL. As noted, room EQ stops where low-frequencies roll-off by 3db. In that case, we might not be able to enjoy the benefits of room correction in the lower frequencies where they might be needed. (A circumstance in which we might want room EQ to leave low-frequencies alone is explained at the end of Section VIII-B: Comparing Subwoofer Performance.


* Starting with this setting in a middle or neutral position, prior to calibration, might be a good compromise for an initial system calibration. Among other things, starting with the control in a neutral position, prior to calibration, allows users the option to change the control either upward for more low-bass, or downward for less. Subsequent trial-and-error, however, might establish that a different initial setting works even better for a particular room and situation.

For instance, some people who have measured their frequency response have discovered that Audyssey was more effective in pulling up low-frequency dips when the room gain compensation was set to the max setting prior to calibration. In that case, starting with a high room gain compensation setting actually allowed them to have more audible bass post-calibration. As with other settings, experimentation can be very helpful in determining what works best with respect to frequency response, and with respect to specific user preferences. It is often necessary to run room calibrations more than once, with different room gain settings, to discover what actually works best in a particular room.



VII-C: The Equal Loudness Contours:

I have wanted to include a layman's description of the Equal Loudness Contours to the Guide in order to make some of the discussion of Reference, and of bass boosts, more understandable. The section which follows is intended to briefly explain how our hearing works, and the relevance of that to the previous sections.

Human hearing is not equally sensitive at every frequency. As noted in the section on DEQ, our hearing is most sensitive between about 2,000Hz and 4,000Hz, and is roughly equal in sensitivity in a range from about 500Hz to 5,000Hz. Frequencies above and below that 500Hz to 5,000Hz range require more loudness to be heard at the same level as frequencies within that average range. The Equal Loudness Contours graphically chart the SPL required to hear other frequencies at the same loudness that we hear 1,000Hz. That 1,000Hz standard is the basis of the Equal Loudness Contours.

People sometimes ask why we hear the frequencies we do, in the way we do? Why did our hearing evolve in the way it did? Anyone who is really interested can do some independent research on the subject. But, the explanation that I have read, which seems to be accepted by the scientific community, involves correspondence to the human vocal range. The fundamental tones of a female voice typically range from about a low of 350Hz to about 3KHz, with Harmonics up to 17KHz. The fundamental tones of a male voice typically range from about a low of 100Hz up to about 900Hz, with Harmonics up to 8KHz. (It has been reported that James Earl Jones' voice could hit ~85Hz.)

It makes sense that human hearing would have evolved to correspond somewhat to the range of the human voice, in order to facilitate communication, and to respond to warnings or calls for help. And, the correspondence of the human hearing range, and the human vocal range, are awfully close to be merely coincidental.

The Equal Loudness Contours which chart the way we hear different frequencies within our hearing range were empirically developed, in a number of studies, using young test subjects with "normal" undamaged hearing. Pure test tones were played through headphones in order to maintain a controlled environment without room influences. The Contours are, therefore, based on an average of healthy, normal hearing. It should be understood as we apply the Contours to discussions of our own audio systems, that room factors, and our own individual deviations from average, healthy, normal hearing will affect what we actually hear.

The Contours demonstrate several things. First, the Contours demonstrate that at at 1000Hz, 10db increase in SPL equals a doubling in perceived loudness. Second, the Contours demonstrate that our perception of loudness changes as frequencies change. For instance, as frequencies go down from about 500Hz, it takes more SPL for us to hear those frequencies at an equivalent volume.

"SPL" is a measure of sound pressure produced at a certain point in space. "Loudness" is a perceptual number--what we perceive when that sound pressure level reaches our ears. So, to restate this, as frequencies drop below about 500Hz, it takes more SPL for us to "hear" those frequencies at an equal loudness to 1,000Hz. Again, 1,000Hz is always the starting point for an Equal Loudness Contour.

If we examine the image below, which illustrates the Equal Loudness Contours, we can see the phon lines, each representing a doubling in perceived loudness. And, we can see that the center of the X axis is 1000Hz, where an increase of 10db of SPL represents a doubling in perceived loudness. Frequencies are shown on the X axis and decibels are shown on the Y axis. Using 80db as a starting point, we can see how much the SPL curves upward on the Y axis, as the frequency level on the X axis moves toward 16Hz.

So, for instance, at 125Hz (just about the start of what I earlier chose to call the mid-bass range) it would take 90db to hear a 125Hz test tone at the same volume as we can hear the 1,000Hz test tone. And, as noted above, an increase of 10db is twice as loud. By the time we get down to about 50Hz, it would take 100db to hear the same loudness as the 80db test tone at 1,000Hz. Another doubling in loudness compared to 90db. And, by the time that we get down to about 31.5Hz, it would take 110db. Still another another doubling in loudness compared to 100db.

Remember that we are comparing the SPL required in order to hear frequencies in equilibrium with those in our normal hearing range of about 500Hz to 5000Hz. So, in the example above, that's a 30db difference in the way that we hear equal loudness between the starting point of 1,000Hz at 80db, and 31.5Hz at 110db. This goes a long way toward explaining why we may need to add bass boosts to restore acoustic equilibrium as master volumes drop.

[It should be noted that something interesting happens to our perception of loudness, for bass frequencies, as frequencies go lower. The phon lines get closer together in a perceptible way, starting at about 250Hz. Below about 30Hz, the phon lines stop getting closer together. If you examine the Equal Loudness Contours, illustrated in the table below, you will see that the space between the phon lines is shrinking from 1000Hz down to about 30Hz. You will also notice that the space between the phon lines stays the same as frequencies go above the 1000Hz mark, which is the standard for how we hear increases and decreases in volume. (At 1000Hz, a 10db increase in SPL is defined as a doubling in loudness). The change in the spacing of the phon lines for bass frequencies affects our perception of loudness in a surprising way. That phenomenon is briefly explained at the bottom of this section.]





If we go up in frequency (starting above 5,000Hz) we see a similar pattern occurring with respect to our perception of loudness, but it occurs to a much lesser extent. Using that same 80db at 1,000Hz, we observe that by the time we get up to about 10,000Hz, we require about 92db to hear that 10KHz tone at an equal loudness to a 1,000Hz tone. Unlike what we see with the low-frequencies, however, as we go even higher in frequency, no further volume is required. In fact, the Contour starts to curve back down above 10KHz, indicating that a healthy young person, with normal hearing, can actually hear a 15KHz test tone at less volume than is the case with a 10KHz tone.

Of course, above about age 30 (or less) few of us have the same healthy, normal hearing than we would have had when we were much younger. Both damage and age take a toll on our hearing, and males typically experience more age-related hearing loss than females do. But, sort of irrespective of our general hearing, I think we can start to understand two reasons why we add bass boosts far more frequently than we do treble boosts.

First, as noted earlier, we may like the combined sound and feel of bass frequencies in a little different way than we do the sound of high-frequencies. And, we may want more bass simply for that reason. Second, we require much more SPL to hear bass frequencies, in equilibrium with frequencies in our normal hearing range, than we do with treble frequencies. For music, most low bass would only go down to about 50Hz. But, even there, we would need an increase of 20db, compared to just 12db at 10KHz, to hear everything in our audio recording in perfect equilibrium.

For movies, and only considering frequencies down to 31.5db (which is just the start of the very low-bass range) we would need an additional 30db, compared to 12db for frequencies of up to 10KHz. That is quite a difference. I believe it is also worth noting that the very-low frequency tactile effects, that were previously discussed, begin at about 30 or 35Hz. And, some movie fans may really want to feel those tactile effects in blockbuster/action movies.

To sum-up the brief discussion of the Equal Loudness Contours and their relevance to the way that we adjust our settings after calibrating our systems to Reference, I want to return to the issue of bass boosts. I think that there is still a third reason, besides the two listed just above for why we tend to boost our bass and not our treble frequencies. There are two parts to the explanation, and the second part is fairly complicated.


Part 1:

The third explanation for why we may add bass boosts, and not add corresponding treble boosts, starts with the sheer size of the frequency ranges involved. Most of us would tend to think of the range from 1000Hz to 10,000Hz as being much larger than the range from 10Hz to 500Hz, because the numbers are larger. But actually, the opposite is true. For instance, the range between 1,000Hz and 10,000Hz is just a little over three octaves. And, the range between 5000Hz, where our hearing is strongest, and 10,000Hz where we are down 12db in perceived loudness, only consists of one octave.

"Octave" in this usage is a musical term for a series of 8 pure notes or tones occupying the interval between two other notes, where one is twice the frequency of the other. For example, from 40Hz to 80Hz is an octave. From 1,000Hz to 2,000Hz is an octave. So, although we may need 12db more SPL to hear 10,000Hz than we do to hear 5000Hz, there are only 8 notes in that range. Contrast that with the frequency range below 500Hz, where our hearing is also dropping-off.

From 500Hz to 250Hz is one octave. From 250Hz to 125Hz is a second octave. From 125Hz to ~63Hz is a third octave. From 63Hz to 31.5Hz is a fourth octave. And, from 31.5Hz to ~16Hz is a fifth octave. (16Hz is a good nominal target for where we stop hearing bass fundamentals.) So, below 500Hz, we are dealing with a frequency range 5 times as large as the range above 5,000Hz, where the hearing of a healthy normal person would be in equilibrium with 1,000Hz.

That is one reason why we don't need to boost treble frequencies in quite the same way that we do bass frequencies. We are actually dealing with much more audible content below 500Hz, where our hearing drops-off, than we are above 5000Hz, where our hearing drops-off.


Part 2:

Part 2 is more complicated and has several components. To start with, most musical notes don't go above about 5,000Hz and very few go above 8,000Hz, although depending on the music and on the quality of the recording, there will be overtones (harmonics) of some notes which go higher than that. So, most of us can probably listen quite comfortably and appropriately to most music, in a frequency range up to about 5KHz or 6KHz, although 8KHz might be better. (As an aside, most traditional hearing tests only measure our hearing up to 8KHz.)

In addition to the fact that there is relatively little fundamental content above about 6KHz to 8KHz, our brains apparently learn to compensate for missing high-frequency information, such as harmonics of high fundamental frequencies. For instance, most adults, and especially men, unconsciously compensate for the deterioration in our high-frequency hearing that may occur over time, so that we are not aware of missing high-frequency information. Part of that compensation may also be related to the fact that with complex musical sounds, we hear both fundamentals and harmonics of most notes at the same time, which would not be the case with pure test tones (sine waves). So, even when some of the higher harmonics are missing, we may not ever be consciously aware of it.

Putting those ideas together, I would speculate that another reason we don't typically need to boost high frequencies very much, if at all, is because there is relatively little meaningful content above about 6KHz anyway, and because our brains can compensate pretty well, for the harmonics of higher frequencies which we may not hear. Individual room conditions may also contribute to the fact that many people may not prefer a high frequency boost.

As noted in the first section of the Guide, the prevalence of reflective surfaces in many rooms may create high-frequency distortion which can be very noticeable and very unpleasant to some people. (Earlier, I used the example of fingernails on a chalkboard.) I believe that this may be part of the reason that many Audyssey users seem to prefer the Audyssey (Reference) curve, for instance, which includes a high frequency roll-off. The general preference for a high-frequency roll-off would also be consistent with the research that resulted in the Harman Target Curve.

However, contrast the way we hear high-frequencies with the way we hear, and feel, five octaves of bass content. As bass frequencies drop, we are also not hearing those frequencies in equilibrium with the frequencies in that normal 500hz to 5000hz hearing range. So, in general, we will need to have more volume for the bass frequencies to hear them in equilibrium with the higher frequencies, and we will typically want to boost our bass frequencies in order to restore that equilibrium.

When we get to individual frequencies, however, the situation becomes slightly more complicated. As noted in the discussion of room gain, the room affects low-frequencies in ways that it doesn't affect higher ones, causing peaks and dips in our bass frequency response. Dips in frequency response, where the SPL of a particular bass frequency is softer, can also affect affect what we hear. But, we do have some things operating in our favor, even with low-frequencies. Bass frequencies have both fundamental frequencies and harmonics, too. For instance, it can be pretty hard (or impossible) to hear a narrow dip at let's say 60Hz. One reason that we can't hear it is because that narrow dip isn't wide enough to fully obscure even a single musical note.

Another reason that we may not hear some dips in SPL, at low frequencies, is that most recorded sounds contain complex combinations of frequencies, so there will typically be some sounds just below and just above the region of a dip. And, using the example of the narrow dip at 60Hz, with complex sounds, we are hearing some 40Hz and 50Hz content, and we are hearing harmonics of that content at 80hz and 100Hz. (This is an important point, because I have seen people chase a narrow dip which shows-up in an REW measurement that they freely admit that they can't actually hear.

But, as we go lower in frequency, where we really want to hear fundamental frequencies, at 20Hz and lower for instance, we may not be able to trick our brains at all, if there just isn't enough audible SPL under 30Hz, to produce a noticeable sound. As noted below, though, even that statement is a little more complicated than it seems. Among other things, we will tend to hear some additional weight or heft to the bass, more than we will hear distinct low-bass sounds.

As we think about how we hear bass frequencies, it may be worth talking about harmonics a little more. The fundamental of a sound is its lowest frequency. But, with the exception of pure tones, produced by sine waves which don't vary in pitch, all musical and other sounds will typically have harmonics. So for a fundamental bass frequency of 30Hz, for example, the Second Harmonic would be one octave higher at 60Hz, and the Third Harmonic would be two octaves higher at 120Hz, and so on. Except in the case of sine waves from test discs, or from something like the opening to Edge of Tomorrow, we will virtually always hear fundamentals in conjunction with those harmonics.

So, to some extent, our brains can be tricked into not missing bass fundamentals, which we don't hear, by the presence of harmonics of that fundamental frequency which are higher in pitch, and which are consequently much easier to hear. This is a complex subject, which I don't pretend to fully understand. It would be fair to say that even where very low-frequencies are not as loud as those occurring closer to our more normal hearing range, bass fundamentals can add weight to what we hear. That would be true even where our hearing is drawn more to the harmonics of low-frequencies which are much easier for us to hear, and which somewhat mask the very low fundamentals.

We sometimes wonder just how much SPL it actually takes to hear fundamental frequencies, considering that we don't hear them in equilibrium with higher frequencies? Conclusions vary on this subject (as they do with much in science) but according to some research I have read on the subject, we can hear pure bass tones at about 20db above the noise floor in a room. That was noted in the section on Bass Frequencies.

The noise floor is the normal volume level of the room itself, which can be a little louder than we may realize. Very quiet rooms can have a noise floor of about 30Hz, and rooms with more electronics, noisy AC systems, street sounds, and so on, can have noise floors in the low 50's. So, in some rooms, we might be able to hear a pure tone of 20Hz at about 50db or less, and in other rooms we might need to have about 70db for the same individual to hear that same tone. And, of course, our hearing will vary somewhat with low-frequencies just as it does with high-frequencies.

We know that low-frequencies give weight and foundation to complex sounds. How much SPL a given individual will need to hear enough of the low-bass weight or heft that he likes can vary considerably depending on that person's specific hearing and individual preferences, the noise floor of a room, and the content of a particular recording. I think it would be fair to say that the more low-bass is in a particular movie or music recording, the more that many people would want to give that low-bass some additional emphasis. How much emphasis we want the low-bass to have may strongly influence the selection and implementation of our subwoofers.

I believe the bass situation gets even more complicated when we include the way that we feel bass tactile sensations. As noted earlier, those tactile sensations are partly the direct result of bass SPL, although there are other factors that also influence what we feel. (It is worth noting, that low-bass tactile sensations may not require quite as much SPL for us to be able to feel low-frequency vibrations as is necessary for us to hear low-bass.) But, to strongly feel mid-bass chest punch, which is primarily airborne in nature, and especially to strongly feel ULF (<20Hz) tactile sensations, it may still take some fairly serious SPL to be able to do it. Depending on the individual listener, however, we may not want to increase our master volume above a level which feels comfortable to us in that 500Hz to 5,000Hz range, where our hearing is normally strongest. So, to both hear and feel what we want to, we typically boost our bass.

The good news with respect to very low-frequencies, particularly <20Hz, is that we feel those frequencies more than we actually hear them, anyway. And, even if we can't hear those frequencies as distinct bass sounds, they can still give some weight to the sound. Depending on some of the factors of subwoofer proximity and room construction mentioned earlier, it may be possible to feel low-bass (rumbling/vibrating) sensations with much less SPL than it would take to distinctly hear them. There is some research that suggests we may be able to feel low-bass tactile sensations at decibel levels as much as 20db below the levels required to consciously hear those same frequencies.

https://www.audioholics.com/room-aco...ation-of-sound


* The discussion so far helps us to understand more about our hearing in general, with respect to the Equal Loudness Contours, and why even the Contours may not fully describe why we add bass boosts. It is also important to remember that the Contours describe how "normal" hearing works at different frequencies. That is very useful information in terms of a generalized inclination to add bass to our audio. But, the Contours don't tell us anything at all about our individual preferences for bass or how much bass we might be inclined to add. Even if we could be certain that we had normal hearing, as defined by the audio tests that were used to develop the Equal Loudness Contours, we still wouldn't know anything about personal preference.

The young people who were used as test subjects listened to pure tones (sine waves) on headphones. They didn't listen to actual music or movie soundtracks, where individual notes have harmonics, and where musical passages and other sounds typically consist of complex combinations of individual frequencies, all with their own harmonics. And, the subjects weren't tested for their "preference" for certain frequencies, only for how they heard them with respect to equal loudness.

Put any of the test subjects, with normal hearing, or any of us with whatever deviations from normal hearing that we may have, in a given room and who knows what bass levels we would choose to use? Add in differences in sound content, from different recordings, which were mixed at different initial sound pressure levels. Add in different sources, and differences in the quality of the individual recording, or of potential compression from the source. Mix with differences in preferred listening levels for those recordings. Finally, stir with our individualized audio and tactile bass preferences, and we nearly all might like somewhat more, or less, bass than another individual with whom we are comparing settings. This idea is explored more fully in a succeeding section on selecting subwoofers. But, YMMV remains the most useful phrase on AVS.


** There is another thought with respect to bass boosts and the Equal Loudness Contours that I think is worth mentioning. I believe that recording engineers are very well aware of the fact that we don't hear all frequencies in perfect equilibrium. After all, in general, they hear frequencies the same way the rest of us do. So, I believe that they typically start with sufficiently-elevated bass for a track to sound the way they want it to sound, when a recording is played back at about the same sound level that they used in recording it. In my opinion, that is why we aren't all adding really excessive bass boosts to our movies, and to our music.

As noted in the first section of this Guide, most of us don't listen at the same Reference levels used to create the soundtracks for movies, and most of us are even less likely to listen at the same (unknown) levels used to create a particular music track. But, when we add bass during a listening session, a large portion of what we are adding probably will be to compensate for the difference between the original recording level and our current listening level. (Of course, with greater or lesser adjustments, depending on our individual hearing and our individual preferences.)

I believe that part of the reason that we aren't all adding even more bass, though, is because the recording engineer (at least on a good quality recording) will already have mixed the sound to compensate somewhat for the way that we hear bass frequencies. That would be true for music, and it would especially be true for movies with significant low-bass sound effects. A review of any number of movies with frequency response graphs of the low-bass will demonstrate instances where the low-bass is peaking at very high sound pressure levels.

The Ultimate List of BASS in Movies w/ Frequency Charts

The question has been asked on another thread whether the very low bass (below 20Hz or 30Hz) that we hear in 5.1 movies is ever put their intentionally by a film mixer? My own belief is that film mixers have monitoring equipment that will let them observe the frequency response of their mixes, even if the subwoofers in their recording studios don't go as low as some HT subs do. I don't think that mixers are typically looking at a graph and evaluating whether they hit a particular frequency, that they deliberately wanted to hit, at a particular SPL.

A somewhat pointless exception to this idea might be the 10Hz sine wave in the opening moments of Edge of Tomorrow. But, even if film mixers aren't deliberately trying to hit infrasonic frequencies, I believe that they would generally be aware of the low-bass in their mixes, and are either choosing to filter the low-bass out, or not. It makes sense to me that they will simply be using their artistry to reproduce or to fabricate sounds (and sound effects) which seem appropriate to them in both sound and feel.

As noted in earlier sections, in real life, we experience obvious ULF sounds and tactile sensations from-time-to-time: severe thunderstorms, airplanes or other loud engine noises, or heavy objects falling on the ground or on a hard surface indoors, are common examples from our normal lives. In attempting to provide a simulacrum of reality (or sometimes to exaggerate it for cinematic impact) I think it would be only natural for film mixers to allow low-fundamentals of bass sounds in their mixes, just as low-fundamentals exist in our daily lives. Whether that happens more in home theater mixes than it does in theatrical releases (for commercial movie theaters) is another question.


Conclusion:

I think that bass, and especially low-bass, frequencies represent an interesting puzzle for our HT systems. Some forms of room EQ (including the better versions of Audyssey) are designed to smooth-out uneven frequency responses caused by the interaction between bass frequencies and room modes. Overall, that is a good thing. But, in some cases, it is possible that room EQ may attenuate desirable peaks at very low-frequencies caused by room gain. Similarly, room EQ may attenuate some of the amplified low-bass which film mixers may have deliberately added in order to emphasize low-bass sound effects.

The LFE channel helps us in that respect, as that channel can have peak volumes up to 10db higher than the regular channels (115db versus 105db). But, even within the LFE channel, film mixers may choose to emphasize some very low-frequencies in relation to mid-bass frequencies, by including a low-bass SPL peak. In some cases, room EQ may work against that concept a little as it attempts to generate a frequency response with fewer peaks (or dips). Working in our overall favor, though, is our ability to feel very low-bass frequencies at lower sound pressure levels than we would need in order to actually hear them in equilibrium with other frequencies.

Putting all of those variables together may mean that having a frequency response which is both smooth-sounding enough to provide good sound quality, and which also allows very low-bass frequencies to be heard and felt in accordance with our individual preferences, can be a bit of a personal challenge. Adding to that challenge is the fact that not only will content sound different, depending on the source and how it is mixed, but also the fact that our own audio perceptions may change a bit with our moods. Speaking personally, my HT system simply doesn't sound exactly the same to me from day-to-day, although my general settings don't usually change dramatically.

Depending on the individual, Audyssey's DEQ may be helpful in restoring very low-bass frequencies, as it boosts very low-frequencies more than it does mid-bass frequencies. And, DEQ is implemented after the room EQ filters are set. Audyssey's new app may also be helpful in tweaking specific frequencies to suit user preferences. Other room correction systems, and room gain compensation controls which are built-into some subwoofers, may also offer features which can accomplish a similar objective of emphasizing (or de-emphasizing) low-bass frequencies.

Ultimately however, whatever audio system components may be employed, I believe that the development of an "appropriate" bass boost, or house curve, which allows bass frequencies to sound (and feel) the way that a particular individual wants them to, in a particular room, for some particular listening material, on a particular day, is very much a YMMV proposition.


[I mentioned earlier that there is something interesting about the way we hear bass frequencies. On the one hand, it may confuse things a little more, but on the other hand, it may have direct relevance to the next section regarding bass preferences and subwoofer selection. It was noted several times that as frequencies get lower, we don't hear them as well as those in our more normal hearing range. But, in considering that aspect of the Equal Loudness Contours which define normal hearing, we are only comparing changes in volume between lower frequencies and those in our normal hearing range. What happens when we compare changes in volume among the bass frequencies themselves?

Something curious happens as the Contours (the phon lines which define equal loudness) begin to bunch-up below about 70Hz, or so, and especially below 30Hz. Starting a little below 1000Hz, and becoming especially noticeable below about 120Hz, we can see the Contour lines visibly beginning to move closer together. As the space between the Contour lines decreases, it takes less increase in SPL to hear bass frequencies at equivalent loudness. Somewhere around 30Hz, it only takes about a 5db increase in SPL (Linkwitz says 4.8db at 20Hz) for us to perceive loudness as having doubled. Looking at the ELC graph above, which illustrates normal average hearing, it is easy to see how the Contour lines become very close together below 31.5Hz.

Remember that the standard definition of loudness is based on 1000Hz. At that frequency, a 10db increase in SPL equals a doubling in loudness. But, a little below 30Hz, it only requires approximately a 5db increase in SPL for a doubling in loudness. For those who are especially interested in low-bass in movies, that is valuable information. Subwoofers which have more SPL below about 35Hz may be able to produce proportionally much more perceived loudness, at those frequencies, than subwoofers which do not. Of course, it is also important to factor in room size, because as noted in the section on room gain, small rooms may already be significantly amplifying frequencies under about 25Hz or 30Hz.

This also has some significance as we increase the volume of our subwoofers in relation to our other channels. As we increase the volume of a low-tuned ported subwoofer, or as we add a low-bass house curve to a sealed subwoofer, using what is sometimes called a Linkwitz Transfer, the low-bass increase in volume is perceived as greater than the mid-bass increase in volume. That lets us notice the low-bass frequencies more. This is one reason why ported subwoofers may sound heavier than sealed subwoofers, and why lower-tuned ported subwoofers may increase that perception, compared to higher-tuned ported subwoofers. Again, below 30Hz, an increase in SPL will sound louder to us than the same SPL increase does at 70Hz or at 120Hz. This idea will be explored a little further in the following section on subwoofer selection.]

The phenomenon explained above is graphically illustrated at the bottom of this page:

http://www.linkwitzlab.com/thor-measmt.htm



Section VIII: Bass Preferences, and Subwoofer Selection and Placement:

Previous sections have attempted to address some of the issues in HT and subwoofer calibration and some of the reasons for our bass preferences. It would be highly desirable to read Section VII, and especially the first two subsections on bass frequencies and room gain, before reading Section VIII.

But, people are constantly starting new threads to ask for advice on buying subwoofers, or they are inquiring about a particular brand on subwoofer owner's threads. In reading and attempting to assist with hundreds of those inquiries, I have observed that many prospective subwoofer buyers are asking the same sorts of questions. The following comprehensive sub buyer's guide may give buyers a good basic foundation for selecting subwoofers. Thanks go to @gene4ht for originally suggesting this portion of the Guide.

There are already some very good resources available for people wanting to buy subwoofers. For subwoofers costing under $300, I would particularly direct readers to Jim Wilson's @Jim Wilson long-running thread: https://www.avsforum.com/forum/113-s...-300-less.html. But, what about people who have slightly larger budgets? What sorts of things should they be considering and what sorts of choices are available to them? How can they better understand their own rooms, their own bass preferences, and how different subwoofers will relate to room size and bass preferences?

This section will attempt to help readers to answer those questions. It will approach the issue of selecting subwoofers from a slightly different perspective, and will try to address issues such as: some of the differences between sealed and ported subs, how powerful a subwoofer should be for a particular situation, and the choice between single versus multiple subs. Some general discussion of some of the major ID (internet direct) sub makers will be included, and there will also be some discussion regarding subwoofer placement in a room.

For readers who don't have the time or patience to read the entire section, some general rules are offered in this introduction. Readers wanting to understand more about how subwoofers work, and who want to understand the backstory behind the recommendations, are encouraged to read through the entire section when they have the time to do so. Sections VII and VIII would ideally be read consecutively, as the discussions in VII lay the foundation for much of the discussion in VIII. The general rules which follow are repeated at the end of Section VIII-C.


General Rules To Consider In Subwoofer Selection:

The subsection on ID subwoofers discusses some of the other factors which buyers might wish to consider in selecting subwoofers. But, concentrating just on performance objectives, there are a few general rules that might also be worth considering. It is important to note, that there may always be exceptions to any of these general suggestions.

1. The first general rule is to try to buy the most powerful subwoofer you can afford. You will see that advice repeated often on the forum. The reason for that is that most of the people shopping for a subwoofer are probably already upgrading from one that isn't powerful enough. You won't want to repeat the process multiple times, as many of the people who are advising you have. And, even if this is your first subwoofer, you may be surprised at how much difference a good subwoofer will make in your audio system.

It's not just that we may overestimate the power of a particular subwoofer, or pair of subwoofers, when we make a purchase. It's also the fact that we may underestimate how much we are going to enjoy good low-bass when we hear it in our HT's. We may find that, as time passes, we may want more of the mid-bass and low-bass sounds and tactile sensations that we are enjoying. If we find that it's the lower frequencies we like, we may especially wish that we had started with a more powerful subwoofer.

2. Try to define your own listening preferences. It is important to understand that your preferences are unique to you, and that your room size may not be the most important factor in determining your bass preferences, or your subwoofer requirements. Is this going to be mostly for movies, or for music, or about 50/50? That will influence the advice you get, and may influence your decision regarding the subwoofer you select. If you can define your room size and your typical listening levels, you will find information in this section which will help you to determine how much subwoofage you need. Others can advise you on this subject, but ultimately you will need to follow your own judgment.

3. Try to decide as early as you can whether you are looking for sealed subwoofers or for ported subwoofers. Subsequent general rules may help to refine that decision, but understanding the differences between ported and sealed subs, and trying to align those differences with your own goals, is probably a good starting point.

Subsection VIII-A gives good general guidance as to the differences between sealed and ported subwoofers. Briefly, ported subwoofers typically produce more SPL below 50Hz, and much more SPL below 35Hz, than comparable sealed subwoofers do. But, the ported models are usually significantly larger and heavier, and may also be more expensive, than the sealed models.

People often recommend sealed subs for music listening, and ported subs for movie viewing, because most music doesn't require extremely low-frequency SPL and TR (tactile response), while many movies do. Depending on the individual and on the room, it may be more complicated than that, but the generalization does offer us a starting point in our selection process.

4. Try to listen to some subwoofers somewhere that will help you to define what 80Hz, and 40Hz, and 20Hz frequencies sound and feel like. Most people believe that the frequencies they are hearing are much lower than they actually are. For instance, a bass guitar chord at 60Hz or 80Hz may seem very low to us, when in fact it is only a mid-bass frequency. Try to find out for yourself some of the differences between mid-bass frequencies and low-bass frequencies. There may be an AVS member in your community who will help you with an audition of his system, and who can help to define bass frequencies for you.

Understanding the difference between hearing very low-frequencies, and feeling very low-frequencies, is also important. Mid-bass frequencies allow for more distinct separations between bass sounds and the accompanying tactile sensation--chest punch. Below about 30Hz, or so, the low-bass tactile sensations we feel are more difficult to separate from the low-bass sounds we hear. They tend to blur together. It may also be important to understand how much difference the frequencies below about 30Hz, with their accompanying TR, may make for 5.1 movies in our HT's. For music, those very low-frequencies will be much less important. Also understand that the lower you want to go with significant SPL, the more subwoofage it will take, and the more expensive it will be.

5. You can try to determine how much overall SPL you are looking for to accommodate your preferred listening level. I think it is fair to say that the most important goal is to have sufficient undistorted SPL from about 120Hz down to about 20Hz. (Depending on the capabilities of the speakers in a room, and how crossovers are implemented, the critical range could be even be from about 80Hz down to 20Hz.) So, I would recommend concentrating on that general goal. Within the overall 20Hz to 120Hz frequency range, however, I think that it would always be fair to emphasize the middle of that range, from about 40Hz to 80Hz, first. An inability to play fundamental mid-bass frequencies loudly enough, would make a subwoofer essentially ineffective for virtually any kind of use.

Try to define your actual SPL goals if you can. What is the loudest you play, and have you ever tried to measure your bass with a calibrated SPL meter? (There is a method shown for trying to calculate your probable bass SPL needs, if you can't measure them, that is illustrated in VIII-C.) Do you want to achieve 110db at ~63Hz (with an additional 3 or 4db of headroom) or do you want 120db--or even more? Starting with the mid-bass range, and comparing subwoofer capabilities in that range, to our normal listening volumes and subwoofer boosts, probably makes the most sense. Remember that dual identical subs will net +6db averaged across the subs' entire frequency range. Also remember that it is the combination of master volume and subwoofer boost which determines how loudly a subwoofer plays. So, you need to understand your preferences for both variables.

6. Once you believe that you understand your overall SPL requirements, consider low-frequency extension. If you are sure that you will have enough undistorted SPL throughout the mid-bass frequency range, pay special attention to the low-frequency extension you are hoping to achieve in your room. Are you looking for 20Hz, or 15Hz, or even lower extension?

It's important to try to identify your own goals, and it's important to be realistic about this, especially if you are in a large room. In a really large room, extension into single digits may be an unrealistic goal for anyone who is not prepared to add something like multiple JTR Cap 4000ULF's. Many people are content to have reasonably good bass SPL down into the low-20's. And, 20Hz is still the THX standard.

Even on AVS, and on most of the subwoofer threads, most people would be very happy to shoot for about 15Hz, or so, in their HT's. Again it's important to remember that you won't normally want to play your subwoofer at max output levels, so when you try to calculate your room gain and compare that to max output levels for subwoofers, you will probably want to leave an allowance of 3 or 4db of undistorted headroom.

It's very helpful to try to define your low-frequency goals in advance. Most of the upgrades I see are to get more low-frequency bass, and not just more bass in general. Selecting ported subs, with lower port tunes, may be an important factor in your selection process. If you don't select ported subs with sufficiently low port tunes, it may be very hard later to get sufficient low-frequency extension, simply by adding multiple identical subs.

(With sealed subs, we can add more of them and achieve significantly lower extension, due to their more gradual roll-off characteristics. Adding several ported subs, on the other hand, may not help nearly as much with respect to really low-frequency extension. The ported subwoofers' tuning points will probably always be the single most important factor with respect to how much low-frequency extension they can achieve. That is because ported subwoofers roll-off so quickly below the tuning point. So, having multiple subwoofers which have a 20Hz port tune, for instance, probably won't help you to produce significant SPL at 10Hz, in even fairly small rooms.)

7. Another general rule is that it is a good idea to compare the low-bass performance of a subwoofer, to the mid-bass performance, using graphs of their respective frequency responses, as described in subsection VIII-B. Those graphs may tell us something important about the relative "bass weight" of the subwoofers we are considering. Depending on room size (which is a factor with respect to low-frequency room gain) and on our own specific bass preferences, some of us may want more low-bass weight, and some of us may want to emphasize mid-bass frequencies more. And, it will be important to pay attention to the native frequency response of the subwoofers we are considering, and not just to their max output capabilities.

8. Remember that dual subwoofers will add more than just a doubling (+6db) in output. Dual subwoofers (or ideally even more than two) will typically provide better frequency response, and better bass envelopment, than a single subwoofer can. Proper subwoofer positioning is important with dual subwoofers, just as it is with individual subs. But, in general, dual subs will provide advantages that most users will benefit from. So, having a long-term plan to obtain dual subs will be important in most (although not necessarily all) cases.

9. It may be helpful to recognize that ported subs will not only provide more low-frequency SPL, they will also generally provide stronger low-frequency tactile response than sealed subs will. (Below about 30Hz, it can be much more difficult to distinguish between bass sounds and tactile sensations. So, low-bass TR may be an important factor in the overall movie experience.) And, there may be differences in the amount of TR produced by different brands of ported subwoofers.

If the stronger low-bass tactile sensations are important to someone, especially if the HT is on a concrete floor, that is something to keep in mind with your subwoofer selection. The reverse is also valid, if someone is on a suspended wood floor, or if he isn't looking for as much TR in general, because suspended wood floors conduct low-frequency vibrations much better than concrete can. Suspended wood floors can amplify the overall low-bass that we perceive, especially in a small room.

10. Another general rule is that smaller rooms (under about 2500^3) will typically create room gain which may significantly amplify low-bass frequencies. (Room gain won't amplify low-bass TR, the way a wood floor was described as doing, but room gain will amplify low-bass SPL.) In many cases, larger, more powerful, ported subwoofers may not be as good a fit in a very small room (especially under about 1500^3 or so). In those instances, ported subwoofers which don't emphasize low-bass frequencies quite as much as mid-bass frequencies, may be a better choice. And, where sufficient low-bass is available, due to room gain, many people in smaller rooms may enjoy the slightly less overt tactile sensations of sealed subwoofers. Very strong low-bass SPL and tactile sensations, at close range in a small room, are not for everyone.

11. Looking at it from the perspective of a larger room, rooms over about 3000^3 may take a lot of sealed subwoofers to create sufficient low-frequency SPL to satisfy some listeners, as the room will not amplify low-frequencies nearly as much as it will in small rooms. As rooms get even larger than 3000^3, the advantages of powerful ported subwoofers, which can generate significant low-bass SPL, and significant low-bass tactile sensations, may become more-and-more important.

12. Understand, that room size notwithstanding, personal preference is as important with bass as it is with most other things. Take your time and weigh your options carefully. It is likely that more than one subwoofer model, from more than one company, will satisfy you. And, it is unlikely that there is only one perfect subwoofer which can meet all your performance objectives. Once you believe that your performance objectives have been met, consider any other factors you believe are important, including cost, aesthetics, features, customer service, etc. (We may start with cost as our primary objective. But experience has shown, that if performance objectives are not met, most people will soon be back, trading-up for more powerful subwoofers, and probably losing money in the process.)

With all of the generalizations being made, however, there will always be exceptions which depend heavily on specific circumstances, and on the specific preferences of the individual. Our ability to understand our own performance goals and preferences may be the single most important factor in subwoofer selection.


* People who are buying subwoofers sometimes question whether a particular subwoofer will be fast, or slow; clean and tight sounding, or loose and boomy? Among very inexpensive subwoofers, that can be a legitimate concern. Among the subwoofers being discussed in this section, it is not a concern, although there may be some qualitative differences between the starter subs mentioned, and the more expensive ones. For the most part, though, all good subwoofers, when properly located and implemented in a room, should exhibit good sound quality.

People are also sometimes concerned that larger subwoofers won't sound as good as small ones. That is not something to be concerned about either. It may be helpful for someone who is thinking of buying a subwoofer, to read an article on subwoofer myths by Data-Bass, a highly respected source of information on subwoofers in general. The Data-Bass tests of subwoofers represent the gold standard for the industry. Some of the discussion on woofer excursion may seem a little technical. But, the discussion regarding small subwoofers (those with 8" or 10" drivers) compared to subwoofers with 15" and 18" (and larger) drivers is relatively easy to understand and well worth reading. (In reading the article, it may help to know that "Q" and "BL" are car audio terms. Q = sound quality, and BL = SPL.)

https://data-bass.com/data?page=knowhow&type=1


The Subsections in Section VIII are as follows:

VIII-A: Sealed Versus Ported Subwoofers

VIII-B: Comparing Subwoofer Performance

VIII-C: Selecting Single Versus Multiple Subwoofers

VIII-D: Internet Direct Subwoofers

VIII-E: Subwoofer Placement


VIII-A: Sealed Versus Ported Subwoofers:

Some people believe that there are significant differences in the sound of sealed subwoofers, compared to ported (also called vented, or bass reflex) subwoofers. My own belief is that subwoofers from the same subwoofer maker are likely to sound more similar than not, and that audible differences among sealed and ported subwoofers, in general, are less likely to be very significant given comparable quality. (A potential exception to this general statement is explored in a few paragraphs.)

Nathan Funk (of Funk Audio) is an extremely well-respected subwoofer maker. He has pointed out that sound quality differences among different subwoofers, where terms such as "fast" and "slow" are used, are most often the result of hearing distortion or compression in a sub which is either poorly made to begin with, or which is being driven beyond its capabilities. (For reference purposes, audible distortion or compression is much more likely to be heard--noticed--above about 50Hz, and much less likely to be heard below that.)

Good subwoofers, properly deployed in a room, should sound at least somewhat similar. (And, if they are employed properly, they should not distort or compress bass frequencies.) The similar sound would especially be the case if sealed and ported subwoofers from the same manufacturer were compared. And room EQ could further enhance the similarity by mitigating some of the room's negative influences on the subwoofers' native frequency responses, making the subs sound even more similar throughout their frequency ranges. Astute listeners might hear differences in head-to-head comparisons of good subwoofers, but as a general rule, the differences should be fairly subtle.

(Saying that room EQ might make ported and sealed subwoofers sound more alike, isn't to suggest that ported and sealed subs should be mixed together in a room, unless someone has the ability to measure and independently EQ their individual frequency responses. Automated room EQ may not be able to help at all when ported and sealed subs are mixed together in a room. But, if you put sealed subs in a room and EQ them, and then put ported subs in the same room and EQ them, it may be hard to tell the sound of sealed and ported subs apart. At least, that has been the conclusion of a number of blind listening tests.)

I believe that we are far more likely to hear significant differences among comparable quality speakers than we are among comparable subwoofers, irrespective of the effects of room correction. The most important reason for that is simply that our hearing is not as acute at <120 Hz, and <80Hz, and <40Hz frequencies, where we are likely to be comparing subwoofers. Absent overt distortion or compression, good subwoofers may sound much more similar to each other than comparable speakers playing higher frequencies would.


[This is just speculation on my part, but there is an aspect of the difference between sealed and ported subwoofers that I believe could be audible to some people, with some content. As explained at the end of the subsection on the Equal Loudness Contours, subwoofers which have more low-frequency extension, and which have stronger low-frequency SPL, may sound relatively heavier in the low-bass, with content extending down to about 40Hz or so, even at moderate listening levels. (That is because the basic frequency response of a subwoofer doesn't change, regardless of volume level, although at very low volume levels, low-bass frequencies may become harder for us to hear.)

Since ported subwoofers inherently produce relatively more low-frequency SPL than sealed subwoofers do, typically starting at or just below about 50Hz, it makes sense that the ported subs could have a proportionally "heavier" sound. The difference might be subtle, but it seems to me that there could be an audible difference to some people. If we were accustomed to hearing proportionally less low-bass from a sealed subwoofer, we might notice the relatively heavier bass sounds from a ported subwoofer. And, vice-versa, if we went from a ported subwoofer to a sealed one. I think it's possible that some of us might notice that the sealed subwoofer sounded relatively "lighter" for some content than what we were accustomed to hearing.

If this were a factor in what we heard, it would be somewhat dependent on content, and the amount of subwoofer boost we were using could also influence our perceptions. Remember than an increase in volume would sound relatively louder for 30Hz and lower frequencies than it would for mid-bass frequencies. I believe that for most content, the differences would be fairly subtle. But, our perceptions vary so widely that I wouldn't rule it out as something that some of us could notice. Whether a particular individual liked having more, or less bass weight to the sound, would be an entirely different question.

This wouldn't, however, have anything to do with "fast" versus "slow". Sound waves all move at the speed of sound, so very low-frequencies wouldn't be moving slower than mid-bass frequencies. And, ported subwoofers wouldn't be playing those frequencies slower, any more than larger drivers would. But, the extra weight in the low-frequencies could convey a different impression to some listeners, versus the relatively lighter sound of a sealed subwoofer, which wasn't able to play low-frequencies with as much SPL. This could, in part, account for the reason why some listeners say that they prefer sealed subwoofers for music, and ported subwoofers for movies.

(This could also help to explain why people who are used to hearing music played only by 8" or 10" woofers may be surprised by the extra bass weight of a 12" or larger woofer. And, people hearing that extra bass "weight" from the larger woofer could, in turn, have helped to create the myth that small woofers are "faster" than larger ones. In that particular case, what people might really be hearing is simply a subwoofer playing more low-bass content, that was always there, but which could not previously be played loudly enough by the small woofer to be noticeable to the listener.)

Getting back to the music/movie issue, most music (with the exception of bass-enhanced music) has very little content below 30Hz, and not a great deal below 50Hz. (And, some music recordings won't go even that low, regardless of what the instruments are capable of playing.) Ported subwoofers are typically stronger than their sealed counterparts below 50Hz, and much stronger below 30Hz. That extra bass "weight" might be something that some listeners could be aware of with some content, and which they might react to either positively or negatively, on a strictly individual basis. It is just something else to keep in mind when selecting subwoofers.]


Leaving aside the question of whether there are significant differences in the sound qualities of sealed and ported subwoofers, however, there are some important design differences. Sealed subs are typically able to have much smaller cabinets, and they are also usually less expensive than equivalent ported models. The larger cabinet volume required for most ported subs is due to the ports themselves, which require space, and to the need for larger cabinets to increase low-frequency extension. The cabinet volume can be a very important component in a subwoofer's ability to achieve significant low-frequency extension. The larger cabinet, and the design of the ports, add to the cost of ported subs.


[It should be noted that a number of factors go into determining the output capabilities of a subwoofer. Although this discussion focuses primarily on the difference between sealed and ported subs, some basic principles of overall subwoofer design are probably worth repeating. One which was stated above, involves cabinet volume. A doubling in cabinet volume results in a 3db increase in SPL across a subwoofer's entire passband. Cabinet volume is especially important with respect to low-frequencies as the larger cabinet size allows for more air displacement (and resonance) within the cabinet.

A second factor is driver diameter or driver number. For instance, doubling the number of drivers (woofers) within a given cabinet results in an increase of 3db of output. Third, amplifier power is a factor. Doubling the RMS amplifier power of a sub results in an increase of 3db of output. But, just considering these last two factors of driver number/diameter and amplifier power, in a vacuum, without knowing something about the motor strength of a particular driver, or its excursion capabilities (its ability to move in-and-out, compressing and displacing air) won't really tell us much about a subwoofer's output capabilities. Nor will raw specifications tell us much about how linear (flat) a subwoofer is, or how much distortion it may produce at different frequencies.

I need to emphasize this point a little more strongly, because it comes up all the time in individual "help me choose a subwoofer threads". Larger drivers can be more efficient than smaller ones, but that doesn't mean that someone should always choose the subwoofer with the larger driver. More RMS amplifier power may also be a helpful way to compare subwoofers, but the output difference between a sub with a 500 watt amp and one with a 700 watt amp would be completely negligible.

It is the combination of cabinet volume; driver size; motor strength; excursion; amplifier power; DSP (digital signal processing) which directs amplifier power to specific frequencies; and other factors, such as the tuning point for ported subs, which will determine a subwoofer's actual output capabilities. That is why measured output comparisons are more useful than just comparing woofer size or amplifier power.

The DSP used to even-out a subwoofer's frequency response, and to emphasize any specific areas of the sub's passband (such as distributing SPL more evenly, or concentrating it more in low-frequencies) are proprietary to the particular designer. And, they can affect both a subwoofer's total output, and its overall performance. This is why people who are focused on output need to have access to max output comparisons, when they are comparing subwoofers from the same sub maker, or from different sub makers.

Max output alone will not tell us how a subwoofer will sound, and it is not the only metric that is worth considering. For instance, looking at THD (total harmonic distortion), particularly for the more audible frequencies above about 50Hz, would also be extremely useful. And, looking at compression graphs, or other ways of determining a subwoofer's native frequency response, at below max levels, can be very helpful as described just a little later. But, comparing max output at least gives us a starting point in differentiating among different subwoofer types and models.]


Sealed subwoofers, which are smaller in size than their ported counterparts, offer greater placement flexibility in tight spaces. In situations where acoustic music content is the primary listening goal, sealed subs may be able to offer owners exactly the bass support required in a compact and cost-effective package. And, for owners who prefer sealed subs in general, some very large models are also available. In small rooms, sealed subwoofers may be able to benefit from sufficient room gain to hit very low-frequencies in movies with ample SPL. Of course, that depends somewhat on how much low-bass a particular individual wants. Where really high sound pressure levels are required, even in a smaller room, it may sometimes take several good sealed subwoofers to reach frequencies under about 35Hz at sufficiently loud volumes to satisfy their owners.

Sealed subwoofers typically have higher SPL above 50 or 60Hz than equivalent model ported subs, but they typically can't produce nearly as much SPL, below about 35Hz, compared to those same ported subs. They have a smoother slope (generally 12db per octave) so that, as they lose volume at lower frequencies, it happens more gradually. Buyers who are particularly interested in stronger mid-bass SPL may want to consider sealed subs specifically for that reason. One thing that it is important to note about sealed subwoofers is that they will typically extend lower in frequency than ported models, but they may not have much SPL left when they do.

This part may be a little confusing. Ported subwoofers have larger cabinet volumes than sealed subs, and the cabinet volume helps them to achieve lower extension. But, it is the ports (their length and their diameter) which really determines a ported subwoofer's low-frequency capabilities. They act a little like the pipes in a pipe organ to help the subwoofer produce low-frequencies as air passes through them and across the mouth of the port within the cabinet. More of the amplifier power of a ported sub is also specifically concentrated into a relatively narrower frequency range. Ported subwoofers can be very powerful from about 35Hz down to about 15Hz, or a little lower depending on the tuning point. Once a ported subwoofer gets a few Hz below its tuning point, however, the SPL drops-off very quickly.


[It may be worth taking a moment to explain part of the difference in the way that sealed and ported subwoofers operate. Sealed subs have tightly-sealed, air-tight cabinets. As the driver (woofer) moves in-and-out, it creates pressure within the resonant chamber of the cabinet. That pressure produces sound, and sound pressure levels which are dependent on the capabilities of the subwoofer.

As noted above, a ported subwoofer operates a little differently. The diameter and length of the ports determine how much the ports can reinforce bass frequencies, within about an octave, or an octave and a half, above the tuning point of the ports. (For instance, a ported subwoofer with a 17Hz tuning point might be much stronger than a comparable sealed sub up to about 35Hz, and a little stronger up to about 50Hz.) As frequencies come closer to the port tune, the driver does less work, and the ports do more--producing almost all of the sound from the subwoofer. Meanwhile, the driver hardly moves at all due to back-pressure from the cabinet.

A little below the port tune, however, the ports stop contributing at all, and the driver has to work completely on its own. But, it can't push against air in the cabinet now, because the cabinet is open to the outside (through the ports) and there is no longer any air pressure inside the cabinet to push against, because the ports have completely stopped pressurizing the cabinet. At this point, the subwoofer typically starts to make port chuffing, or gasping sounds, as the driver continues to try to move in-and-out, and as air simply rushes out of the ports.

Both sealed and ported subwoofers have high-pass filters (HPF's) implemented in their DSP (digital signal processing) to protect the driver from over-excursion (too much forward and backward movement), and to prevent voice coils from overheating. Sealed subwoofers can continue to play lower than where they are strongest in SPL, since the driver is always pushing against air pressure inside a sealed cabinet. So, the slope of the HPF is typically 12db per octave.

Ported subwoofers lose air pressure completely, a little below their port tune's, and start to make noises, as described above. So, a steeper slope is implemented for their HPF's to prevent as much of that as the subwoofer designer chooses. It is typically 24db per octave, and it starts right around the port tune. That is why ported subwoofers roll-off so much faster than sealed subs, and it is why multiple ported subwoofers won't give a listener a lot of additional low-frequency extension below the tuning point of each individual subwoofer.]


Sealed subs have their available amplifier power distributed more evenly, and they keep rolling-off much more gradually. So, they can typically go lower in frequency than a comparable ported sub, but with less-and-less SPL. As noted in the next paragraph, whether it will be enough SPL to be meaningful at a specific frequency is the question? Both subwoofer types have virtues, and which one to pick is a personal choice which depends on the room, the type of listening material, and the preferences of the individual user.

Both sealed and ported subs benefit from room gain to exactly the same degree, in that the room gain available to amplify low-frequencies is not dependent on the type of sub. At frequencies several Hz below the tuning point of a ported sub, however, sealed subs may sometimes be able to benefit more from room gain, as ported subs have a much steeper drop-off (generally 24db per octave, as noted above) than sealed subs do. On the other hand, the ported subwoofer may be producing SPL around it's tuning point (and a little below it) that is equivalent to what several comparable sealed subs can produce at that frequency.

The key with either type of sub is to still have enough SPL remaining, at a particular low-frequency, for room gain to augment that SPL enough for it to make a real difference in the bass we hear. Sometimes, it's just a matter of being able to add a little more weight to the sound, even if specific bass frequencies can't actually be distinguished. But, at some point, the low-frequencies may have so little SPL that they don't add anything at all to the sound.

It is also important to note that very large rooms may have very little low-frequency room mode gain, as illustrated by the example of my room in the section on room gain. And, the pressure vessel gain may start at too low a frequency to be very useful, unless our subwoofers can generate enough SPL at very low-frequencies for the PVG to amplify it sufficiently to satisfy us. In that case, where a large room is involved, room gain cannot be relied upon to strongly augment low-frequencies for either sealed or ported subwoofers. In situations where that applies, having subwoofers which can develop really significant low-frequency SPL on their own may become a higher priority. Understanding where room modes may actually augment low-bass frequencies, in a particular room, can be an important key to understanding the type and number of subwoofers which may be required to satisfy a specific individual.


[This next point may seem a little abstract at first, but just allow the concept to sink-in gradually, because it may be important somewhere down the road. Each subwoofer has a native frequency response which can be measured quasi-anechoically (at 2 meters outdoors, away from any large objects or structures, for instance). That's what's happening with the graphs shown on Data-Bass, in the individual measurements sections of the various subwoofers. And, that natural slope (the shape of the graph line, as a subwoofer starts to roll-off naturally at low-frequencies) doesn't change regardless of the volume at which the subwoofer is played. If a subwoofer is played at 75db, that will be the slope, and if a subwoofer is played at 105db, that will still be the slope. The subwoofer will simply roll-off when it rolls-off. (At very high volume levels, compression will occur, and that will influence the slope, especially for the lower-frequencies.)

Once a subwoofer is placed in a room, the smooth quasi-anechoic graph line will become ragged, as the room creates random peaks and dips in the frequency response. Now, the formerly smooth graph will represent a jagged line instead of a relatively flat one. And the shape and specific start of the roll-off, at low-frequencies, will change as room gain lifts some of those lower-frequencies. But, the new shape of the frequency response will also not be dependent on listening volume. The room-influenced slope will remain the same unless a new factor, such as a different subwoofer placement, or multiple subs, or some form of room EQ is introduced.

So, when we are selecting a subwoofer, we are also selecting it's native frequency response--its slope. Part of what this means from a practical listening standpoint, is that a subwoofer which has relatively more <35Hz, or even <20Hz SPL, will have a little more bass weight to the sound than a subwoofer which has relatively less SPL at those frequencies. Depending on content, that extra bass weight may be audible to some listeners even at moderate listening levels, and may become more so at higher listening levels. Of course, content really matters in this discussion. The more that the content emphasizes low-frequencies, such as the special effects in action movies, the more that the additional bass weight may be noticeable.

Conversely a subwoofer with more SPL in the mid-bass will always sound relatively louder there. The room gain compensation feature that many ID subwoofers offer can be used to subtract some of the low-frequency bass weight where it is excessive (either due to room gain, or to personal preference) and some subwoofers offer their own DSP which can boost some mid-bass frequencies relative to the low-bass. Some forms of automated room EQ, which go low enough to affect deep bass frequencies, can also help to somewhat equalize differences in mid-bass and low-bass in subwoofers. (Remember, however, that automated room EQ stops setting control points where the subwoofer starts to naturally roll-off by 3db, so some bass-weight might be removed by room EQ, but probably not all of it.) And, listeners can also employ measures such as the cascading crossovers discussed in Section III, if they want to slightly emphasize mid-bass frequencies.

But, even knowing that there may be opportunities to somewhat tailor a subwoofer's sound to our room, and to our listening preferences, selecting subwoofers with an understanding of a sub's natural frequency response still makes sense. The earlier discussion of potential differences between a relatively heavier sound of ported subwoofers, versus a relatively lighter sound of sealed subwoofers might be one example. And, individual room size might be another. For example, in a larger room, the extra low-bass weight may be very welcome, and in a very small room, it may be much less so. Deciding where in the bass frequency range we want subwoofers to be strongest, what that means with respect to our room size, and where we are willing to accept a subwoofer's low-frequency roll-off, are all potential performance factors to consider in selecting a subwoofer.]


Ported subwoofers typically have larger cabinets, which allow deeper resonance, and as explained above they also have ports to move air and to create resonance at lower frequencies. That combination allows ported subwoofers to create higher sound pressure levels (SPL's) from about an octave or so above their tuning point, to a few Hz below the port tune. Again however, below the tuning point, ported subwoofers lose SPL fairly quickly. In general, the more that a driver can move air by excursion (moving forward-and-backward), the more SPL it can generate. Ported subs are selectively designed to move (displace) more air at specific frequencies. And, full-range ported subs are designed to concentrate their additional SPL on lower frequencies. (I'm using the phrase "full-range ported subs" because, as noted in a previous section, there are also types of ported subs which are designed only to play mid-bass frequencies.)

Depending on the subwoofer, full-range ported subs may be tuned anywhere from about 30Hz down to as low as 10Hz. They use the combination of their larger cabinets, their ports moving air, and their specific port tunes to achieve much greater low-frequency SPL than equivalent model sealed subwoofers. In fact, depending on the specific subwoofer, some ported subs may have as much SPL below 35Hz as two or more comparable sealed subs. By the time frequencies below 20Hz are compared, it may take three or four sealed subs, of a similar driver size and model, to generate as much SPL as a single ported subwoofer.

But, as noted above, the cabinet size of the ported subwoofer is likely to be much larger, and some ported subs may give up a little mid-bass SPL in return for the extra SPL at low-frequencies. It is also important to note that slightly below the tuning frequency of a ported subwoofer, the driver is no longer loaded by the enclosure, and acts as if it is in free air. With both sealed and ported subwoofers, it is enclosing woofers inside a cabinet that allows them to compress air, and to produce low-frequency SPL.

And, it is air pressure, created within the subwoofer cabinet, which allows the ports to contribute to the SPL. As ported subwoofers near their tuning point, the ports do more and more of the work, and the driver's excursion reduces so that it is no longer moving in and out as much. A little below the port tune, the driver (woofer) of a ported subwoofer quits compressing the air inside the cabinet anymore, and it acts as if it isn't inside a cabinet at all. And, the SPL consequently drops like a rock.

Sealed subs, on the other hand, can go on compressing air inside a cabinet longer, as the driver continues to move in and out. However, as the amplifier power is used up, and the excursion capabilities of the driver are reduced, the amount of SPL produced at very low-frequencies may be entirely inconsequential. But, put enough sealed subs together, all playing those same low-frequencies, and there may be enough combined SPL to make a real difference. That wouldn't be the case with ported subwoofers. Several ported subwoofers, with the same port tune, wouldn't be able to go much lower than a single one could. Individually or collectively, once they get a little below their port tune, they lose their ability to compress air inside a cabinet, and they can no longer produce low-frequency SPL at all.

Readers who are interested in understanding the difference between the relative low-frequency SPL responses of comparable model ported and sealed subs are encouraged to consult the Systems List at Data-Bass, where sub models from some of the same makers can be compared by frequency. That comparison can help readers to determine whether sealed or ported subwoofers will be more suitable for their rooms, and for their specific low-frequency goals. In some cases, such as with some HSU, Rythmik, and SVS subs, the same subwoofers offer both sealed and ported modes, which makes direct comparisons of sealed and ported SPL even easier. That comparison on a single page will give a reader a good general sense of the difference between sealed and ported subwoofers, which can then be applied to other brands and models.

Remember, though, that it is the combination of air moving through the ports, acting in conjunction with specific port tunes, and with the corresponding DSP applied, which really gives the ported sub its ability to create more low-frequency SPL. Once all the ports are plugged, even on a subwoofer which is designed to be convertible to sealed use, a ported subwoofer's extra cabinet volume may not actually enhance its output, at every frequency, compared to a smaller sealed subwoofer of the same model. (This question comes up from time-to-time.)

A good example of that is a comparison of a ported SVS PB13, operating in sealed mode, compared to the normal operation of the much smaller sealed SVS SB13. They are both illustrated on the Data-Bass Systems page, linked below. In this context, a "system" is a built subwoofer rather than an individual component, such as a woofer, which can be put into a cabinet selected by the user. Detailed measurements for each subwoofer are available by left clicking on that particular subwoofer.

https://data-bass.com/#/systems?_k=am5rgi


* As was pointed out in several earlier sections, 5.1 movies and bass-enhanced music can have substantial low-frequency content. In deciding between sealed and ported subs, potential buyers should consider their primary listening goals and their own bass preferences. Room size is often regarded as the primary factor in determining subwoofer selections. And, it certainly is an important factor with respect to room modes and perhaps also with respect to listening distances. (SPL decreases with distance by ~3db per doubling of distance indoors, so the distance from a subwoofer to the MLP may tend to be greater in some large rooms.)

I believe, though, that listening volumes and preferred bass boosts are at least as important as room size. The more that a particular individual wants to be able to listen at high volumes, and the more that he wants to boost his bass in relation to other frequencies, the more subwoofer SPL he will need. If he wants to emphasize the very low-frequency content in 5.1 movies, or in bass-enhanced music, the more he will need subwoofers that can produce high SPL at those specific frequencies. Data-Bass and other measurement sources (such as Audioholics) are good resources for determining subwoofer capabilities at specific frequencies.

** Another aspect of the potential difference between sealed and ported subwoofers is in the tactile sensations that they produce. Speaking in very general terms, ported subwoofers will produce much more low-bass tactile response (TR) than sealed subs. Depending on the individual, that may or may not be an advantage. As noted in earlier sections, different people are more aware of some tactile sensations than others. And, different people may like more intense tactile sensations than others.

Ported subs move more air at low-frequencies than comparable sealed subs, because the ports themselves are pulling air in, and pushing it out. With sealed subs, only the excursion of the driver is moving air, and it's moving air forward and backward within a relatively smaller cabinet volume. The additional movement of air through the ports increases particle velocity, especially at very low-frequencies, and that increases tactile sensations. (Air movement through the ports should typically be strongest within about an octave of the port tune, although it continues to occur even above that. Air movement and TR should be at the maximum very near the tuning point of a subwoofer.)

People who enjoy those low-bass tactile sensations (thunder, explosions, the stomp of a T-Rex) in movies may want to consider ported subwoofers for that reason. That might particularly be the case where someone's HT is on a concrete floor, as concrete doesn't conduct low-frequency vibrations very well. Alternatively, someone on a suspended wood floor, which does conduct more tactile energy through the floor, and up through a chair or couch, may not wish to have quite as much overt low-frequency tactile response as some ported subs can provide.

This issue of low-bass TR may be worth exploring a little further in this context. The Guide states several times that it can be difficult to distinguish between sounds and tactile sensations below about 30Hz. And, it becomes even more difficult below 20hz. I believe that sympathetic resonances from a suspended wood floor (a wood floor, above a crawl space, or on an upper floor) can greatly affect how we perceive low-bass sounds. A powerful ported subwoofer, tuned below 20Hz, may produce much muddier-sounding bass on a suspended wood floor, in a small room, than the same subwoofer would produce on a concrete floor, or in a larger room. As noted earlier, concrete just doesn't resonate the way that a relatively thin wood floor over 2 x 4's does.

I have seen several examples of situations in which a wood floor, or even a significant wood riser on concrete, could affect the apparent low-bass sound of low-tuned ported subwoofers, even for experienced listeners. That could be another factor that accounts for differences which some people hear between ported and sealed subwoofers, because sealed subwoofers would not be able to produce nearly as overt low-bass TR as low-tuned ported subwoofers could in the same room. And, there would not be as much floor resonance to muddy the sound of the low-frequencies. I would not expect this to be a factor with most music, but it could be a factor with the low-bass special effects in some movies, where much more ULF TR could be involved.

This is not to say that low-tuned ported subwoofers (let's say 17Hz or lower) couldn't be appropriate in a room with a suspended wood floor. But, the combination of a small room (under ~2000^3) and a suspended wood floor, would make me more cautious about buying large ported subwoofers with low tuning points. Larger rooms, or rooms with concrete floors, might be a better match for those subwoofers, in that case.

Room size; the importance of focusing on low-bass frequencies for movies, versus more mid-bass frequencies for music; and the specific tactile response which a user prefers, are all factors to consider in selecting subwoofers. And, there is rarely a one-size-fits-all solution to the question of which type, size, or number of subwoofers will best fit a particular buyer's personal requirements.


Passive Radiators:

There are probably more speakers which utilize passive radiators than there are subwoofers, although DefTech is one of several companies that offers them. But, a question came up on the thread, so I decided to include a brief description of passive radiators here. A passive radiator subwoofer has a second woofer, which is usually slightly larger than the primary one. It resembles an active woofer in appearance, but lacks a voice coil or a magnet, and it is not connected to the subwoofer's amplifier.

The real driver has a voice coil and a magnet, and is powered by the subwoofer's amplifier. The passive radiator simply resonates due to the pressure within the cabinet, and its diameter and mass determines the frequencies to which it can contribute. In that respect the passive radiator acts just like the port on a ported sub, where the port diameter and length determines it's tuning frequency.

Passive radiators allow for some additional low-bass augmentation in much the way that ported subwoofers do. The slope of the passive radiator is even sharper than that of a comparable ported sub, and it seems to be just slightly less powerful than a comparable ported subwoofer. The passive radiator lacks the same low-frequency tactile energy that a ported sub can supply, via air moving through the ports, or from the out-of-phase condition which exists between the ports and the driver near the port tune. So, passive radiators will not produce as much ULF TR as ported subs can.

Advantages of a passive radiator, however, include the capability to increase low-bass SPL, within a smaller cabinet volume, and the inability to experience any potential port chuffing, due to air leaving the ports of a ported sub, at extreme volumes near the tuning point. It would be interesting to hear some of the major ID subwoofer makers discuss the overall advantages and disadvantages of passive radiator designs versus ported designs. But, in the absence of that discussion, we may have to take the scarcity of passive radiator models as a kind of statement in itself. According to one source, passive radiators are only slightly more complex to design, but may be more costly to build than ported (also called bass reflex) subwoofers.




VIII-B: Comparing Subwoofer Performance:

In the last subsection, I mentioned using max output comparisons along with THD and compression graphs in order to compare subwoofer performance. Lately, however, as described in several paragraphs just above, I have started to see a subwoofer's native frequency response as another extremely valuable way to make comparisons. Like most people, most of my subwoofer comparisons in the past have been made on the basis of max output, and I have also paid attention to Total Harmonic Distortion (THD) in the process. Once we are certain that we will have enough total SPL in our rooms, however, I think it is worthwhile to turn our attention to a subwoofer's native frequency response.

In some respects, it is the native frequency response of a subwoofer that really determines how it performs, and which may affect differences in the way it sounds. And, that native frequency response doesn't really change with changes in volume, unless we push the subwoofer into compression. The room will influence the FR in both predictable and unpredictable ways, but those influences will also not be dependent on changes in listening volume. The in-room frequency response of a subwoofer will remain whatever it is, unless we intentionally manipulate it with some form of DSP, which is either internal to the subwoofer itself, or external, via an AVR or other external device.

When we look at the native (quasi-anechoic) frequency response of a subwoofer at 90db, for instance, we see how the subwoofer plays low-frequencies in relation to mid-bass frequencies. And, that has an influence on the way the subwoofer sounds at virtually any volume level. So, understanding a subwoofer's native frequency response (in a graph format) may actually be a much more important way to compare subwoofers than by simply comparing max output numbers on a table. Looking at a graph will allow us to see where the subwoofer rolls-off, and how much SPL it maintains at low-bass frequencies, in relation to mid-bass frequencies. We still need to look at max output numbers, so that we will understand the total SPL limitations of a particular subwoofer, or pair of subwoofers. But, we would then want to avoid those max output levels, in any case.

Assuming that we would not typically want to play a subwoofer at max output levels anyway, the max output wouldn't necessarily be a completely relevant indication of its performance in our rooms. For instance, if we play a subwoofer at max output, several things will happen. First, some frequencies will compress quite strongly, meaning that lower frequencies (and eventually some others) will simply stop getting louder as other frequencies continue to increase in volume. If mid-bass frequencies stop getting louder, as the overall volume level in an HT system is increased, that may be very noticeable to some people. THD will also increase to very high levels, and at some frequencies we may hear that distortion. If the subwoofer is ported, it may chuff at this point, and that might also be very noticeable. Clipping (squaring off the top of a sound wave) may occur and that can also be audible to some of us. In short, we really want to have enough headroom to avoid running our subwoofers at absolute max output.

[It may be worthwhile to spend a moment talking about distortion and compression. Mechanical noises from a woofer, or port chuffing, may be easy to hear and to recognize as unnatural. Distortion and compression may sound more subtle. Subwoofers which become somewhat boomy or muddy sounding, when the volume is increased beyond a certain level, may be distorting. On the other hand, most well-made subwoofers will stop getting louder in the lowest frequencies first, if they are compressing (literally, pushing frequencies together) and we may not be able to hear that happening at all. In that case, the mid-bass frequencies would just keep getting louder in relation to the low-bass frequencies, which would stop getting louder. At some point, all bass frequencies would stop getting louder, once a subwoofer's extreme limits were reached.

Based on my review of a number of Data-Bass compression tests, allowing about a 3db to 4db cushion between a listening level and the max burst output is a good idea to prevent both distortion and compression. It should be remembered, though, that subwoofer/room interactions can be unpredictable, and optional features such as room EQ and DEQ may make specific subwoofer boosts more difficult to calculate, so some listener vigilance (careful listening) may be required to avoid any unwanted influences on our audio quality.]

We will still want to know something about max output, so that we can back down from it somewhat, and we will still want to know something about where significant compression may start. We can use both of those metrics in choosing subwoofers with enough total SPL. But, assuming that we intend to have enough subwoofage for our rooms, and for our listening levels (including subwoofer boosts), to play at something less than max output, how then should we compare subwoofer performance? I am starting to see the subwoofer's native frequency response as more-and-more important for this purpose. That native response can be determined by looking at a frequency response graph, or a compression graph on the sub maker's website, or we can look at compression graphs for subwoofers on sites such as Data-Bass.

When we compare those graphs of subwoofers' native frequency responses, we can see something about the way the subwoofers can be expected to actually perform in our rooms at most normal listening levels. What we are looking for is the general shape of the frequency response. We are particularly looking for where the SPL starts to roll-off, at any volume level, and how steeply the subwoofer rolls-off the low-frequencies. Understanding that will help us to predict how loudly the subwoofer will play some frequencies, in relation to other frequencies. And, that can be very important information to have.

The comparison that follows provides an interesting example of how that works. Unfortunately, I can't easily create my own graphs to overlay the frequency responses of the various subs which I compared, so I will simply describe where the graphs can be found, and what I observe when I study them. I believe that the statements I make will be reasonably accurate, and if someone sees something I missed, I hope that he will point it out.

I'm going to compare native frequency responses down to ~16Hz. That is a good number to use, in my opinion, because we typically have good data down to about that frequency, and because most of us can't hear distinct tonality of sounds below about 16Hz or 18Hz, anyway. Subwoofers which can get into about the low to mid-teens in-room, with good SPL, are typically sufficient for about 99.9% of HT systems even on a site like AVS Forum.

Let's compare some large ported ID subwoofers in a somewhat similar price range, although the price of the first subwoofer has recently dropped quite a bit, since that model is no longer being made. Let's compare the SVS PB13 (in extended mode), the PSA V3611, the JTR Cap 1400, the SVS PB16 and PB4000 (both also in extended mode), and the FV18 (in 16Hz mode). The use of extended mode for the SVS ported subs would be comparable to the use of max room gain compensation for the PSA and JTR subs. Users could have more or less SPL at low-frequencies by manipulating either the tuning point, or the room gain compensation, depending on the specific subwoofer model. But, using full extension for all of the subwoofers (except the FV18) allows apples-to-apples comparisons.

Sources for the graphs of frequency response are Data-Bass compression graphs for the PB13, the Cap 1400, and the FV18; the sub maker website for the V3611 (a compression graph); the sub maker website for the PB4000 (FR graph at 90db); and an Audioholics review for a graphed frequency response for the PB16. I think the comparisons are extremely interesting, and I think that they tell us something about how the subwoofers will perform in normal listening use.

[If someone is using Data-Bass to compare subwoofers, he can start on the Systems page, linked in other places in the Guide, and linked again below. Left click on the subwoofer you want to examine. That will take you to a page about that particular subwoofer. Then click on "Measurements". That page will illustrate max output and THD. Finally, click on a header called "Extended Charts". That will take you to a page showing, among other things, a compression graph. In the compression graph, the native frequency response of the subwoofer will be clearly displayed at lower volume levels, and as more volume is added and the frequencies compress, you will be able to see how the specific shape of the frequency response changes. In a room, the subwoofer will be able to achieve somewhat higher volumes prior to compression.]

https://data-bass.com/#/systems?_k=am5rgi

All of the measurements of frequency response, which we are comparing, were performed quasi-anechoically. That means that the measurements were taken outdoors, away from any other objects or structures, with the subwoofers sitting on the ground. Starting with the PB13, with the Sledge amp in extended mode, we can see that the subwoofer's native frequency response starts to roll-off slightly at about 30Hz. By 20Hz, the native response is down about 2db (about a 40% difference in perceived loudness at that frequency) and it then recovers a decibel (bumps up slightly) out to about 16Hz. Below 16Hz, which is really about the port tune, it rolls-off more rapidly.

Looking at the V3611, we can see that the subwoofer starts a slight roll-off at 40Hz. By ~30Hz, the response is down by ~1db, and by 20Hz, the response is down by about 3db (about a 60% difference in perceived loudness at that frequency). Below 20Hz, the subwoofer has a steady roll-off. By about 16Hz, the subwoofer is down 7 or 8db (well over a 100% difference in perceived loudness) compared to its native output at 40Hz and higher.

That is an enormous difference, compared to the PB13, and I think that the difference could be audible in most rooms, to many listeners, with the PB13 sounding relatively (slightly) heavier in the low-bass, and the V3611 sounding relatively more impactful in the mid-bass, at virtually any listening levels. The reason that I think that the difference in low-bass heaviness might be slight compared to the difference in mid-bass impact is just that there is more mid-bass content, and we might be more likely to notice differences there.

It's important to emphasize that I am not talking about total SPL here--the V3611 can play most frequencies much louder than the PB13 can, if we turn-up the volume on the V3611 to higher levels than the PB13 can achieve to start with. I'm just comparing the relative loudness of the low-bass and mid-bass frequencies, because they would remain consistent with each other, regardless of the total volume level. Based on the anecdotal experiences of subwoofer users, the lower bass weight of the PB13, compared to the V3611 is audible with a good bit of listening material. Apparently, low-frequencies (below 30Hz, and especially <20Hz) provide a foundation for sounds that matters to some listeners.

(At least, the relative loudness of mid and low-bass frequencies would remain consistent until max output and resulting compression were approached. As noted earlier, compression would start to squeeze the low-frequencies first with any subwoofer, making the subwoofer with less mid-bass SPL run out of audible gas first. I think that we would generally want to avoid pushing our subwoofers quite that hard, in any case, though. The real goal, in my opinion, is to have sufficient subwoofage that we don't have to approach max output, with the resulting degradation in sound quality that could accompany that. So, this whole exercise once again presupposes that we intend to have sufficient subwoofage to avoid any significant compression to begin with.)

Note that in making this comparison, I am not saying which subwoofer would sound better to a particular listener, and I can't. That would be partly room-dependent and partly listener-dependent. To me, it's a lot like comparing the chocolate content of two candy bars. We can determine which has more chocolate, and which has less, but no one except the person tasting the candy bars can determine which tastes better to him. And, our individual tastes will vary quite a bit. To continue what may be a useful analogy, just think of the difference between a dark chocolate candy bar and a milk chocolate bar. They might have very different amounts of chocolate, and while both may taste good, some of us may notice more distinct differences in the taste than others would, and some of us may like one taste more than another.

In the same context, I think it is worth noting that all listeners may not all hear bass frequencies in the same way. Most of us are aware that high-frequency hearing can be affected by age and damage-related hearing loss. But, low-frequencies can also be affected by those factors. Irrespective of age and damage, some of us may also have had slightly better, or worse, low-frequency hearing to start with. Our physical attributes typically seem to be distributed in some sort of bell curve, with most of us falling toward the middle of that curve, and there is no reason to suppose that our low-frequency hearing is different. So now, we have potentially different low-frequency hearing and potentially different low-frequency preference to consider. That's why even objective comparisons will only tell us part of the story. We may still have individually subjective responses to both low-bass and mid-bass SPL, just as we did with our dark chocolate/milk chocolate analogy.

Continuing our comparison and looking at the Cap 1400, we see a very different frequency response compared to the last two, and especially compared to the V3611. The Cap 1400 is very flat all the way out to about 23Hz, at which point the response rises slightly, by about 2db at 17Hz (about a 40% increase in perceived loudness at that frequency) before starting a roll-off at 17Hz, which continues out to 15.5Hz. At 15.5Hz, the Cap 1400 would have the same SPL as it did at 30Hz. That's quite a difference from what we saw with either of the first two subwoofers.

The discussion of room gain in Section VII suggested that boundary gain would slightly amplify frequencies under the transition frequency in a room, so both mid-bass and low-bass frequencies would be amplified equally in a room. Below about 30Hz or 40Hz, room modes and pressure vessel gain could amplify the bass somewhat in smaller rooms. And, room gain would amplify them even more below 20Hz, even in very large rooms, such as my 6000^3 room.

So, in some rooms, a subwoofer which produces relatively more mid-bass than low-bass might still be in a good listening equilibrium, for a particular listener, due to the influence of room gain. (In a larger room, that equilibrium would be harder to achieve with subwoofers which produce relatively less low-bass.) But in both larger and smaller rooms, I believe that the Cap 1400 could always potentially sound slightly heavier than the PB13, although some of us would notice it more than others. And, the Cap1400 could potentially sound much heavier to some listeners than the V3611 would.

In a smaller room, with significant room gain, the differences in the low-bass sound could be equalized in such a way that bass weight would be a non-factor; or for some listeners the extra bass weight of some subs might actually be perceived as a negative. In a very large room, on the other hand, where room gain was providing less amplification of low-frequencies, the extra bass weight might be much more noticeable. And, it might be much more likely to be perceived as a positive.

This is all somewhat hypothetical, however. As with the comparison of the first two subwoofers, whether that relative low-frequency heaviness would be considered a good thing, or a bad thing, would be entirely up to a particular individual to determine. I believe that the room size, the typical listening content, and the specific listener preference with respect to low-bass versus mid-bass emphasis, could all be significant factors in determining the ultimate subwoofer selection. We also shouldn't discount the importance of simply liking what we are accustomed to hearing, if we are more familiar with subwoofers which produce relatively more, or relatively less, low-bass weight.

The final two SVS subwoofers, the PB16 and the PB4000 are actually somewhat similar in performance, when I compare the native frequency responses, although the PB16 gets a little louder at the lowest frequencies. The PB16, in extended mode, is relatively flat out to about 30Hz before starting a gentle rise which peaks at about 2db at about 17Hz (about a 40% increase in perceived loudness at that frequency). At 16Hz, the PB16 is down about 1db, compared to 30Hz.

In theory, this subwoofer should sound slightly heavier than the PB13, due to the boost in the 20Hz range, and much heavier than the V3611. (Anecdotally, a number of owners who have compared the PB16 to the PB13 have commented on the difference in bass weight. I had both in my room, at the same time, and could hear a difference with some content.) I think that the PB16 and the Cap 1400 would be fairly neck-and-neck into the high teens, although the Cap 1400 might have a slight edge in perceptible bass weight starting about 17Hz, and continuing below that. (That would be due in part to the motor strength of the JTR driver.) And again, the difference in the relative heaviness of the low-bass sounds would not be dependent on the listening volume, but it might be somewhat dependent on both the listener and the room.

[At very low listening levels, the relative bass weight of various subwoofers would be much less a factor, in my opinion, because the lower bass frequencies would all fall-off much faster anyway, in relation to those in our more normal hearing range. But, if we were listening at master volume levels of about -30 or higher, and particularly if we were employing any sort of subwoofer boosts, the natural sound of the subwoofer could still be relatively heavier or lighter in the low-bass, depending on the subwoofer's native frequency response. I should also add that content matters. If the listening material contains very little bass below about 30Hz, then the relative heaviness of one sound in relation to another will be much less noticeable. All of these comparisons would necessarily be subjective, both with respect to what we as individuals could actually hear, and with respect to what we would like if we did hear differences in low-bass weight.]

I decided to include the new PB4000 in this comparison, and I was a little surprised when I did. I know from max output comparisons that the PB4000 has about 1db more max output than the PB13, distributed across the passband. That is due to a combination of a slightly larger cabinet volume, 200 watts more power, and different DSP. But, it was the shape of the frequency response that I found interesting. To all practical intents, the subwoofer is completely flat out to about 17 or 18Hz. It doesn't exhibit the slight roll-off (~2db at 20Hz) that its predecessor, the PB13 did, and it doesn't have the slight rise, starting at about 30Hz, that the PB16 does.

(As an aside, I went from a PB13 rear nearfield, to a PB4000, and imagined that the PB4000 sounded slightly heavier in the low-bass than the PB13 did in exactly the same spot. I couldn't really account for that by comparing max output. But, I can account for it, if I consider the 2db roll-off at 20Hz, that occurs with the PB13 at all listening levels. That's about a 40% difference in perceived loudness at that frequency. Below 20Hz, there would even be a slightly greater difference, although by 16Hz, it would level-off to just about 1db. In this case, saying that the PB4000 only averages about 1db more SPL than the PB13, at max output, doesn't tell the whole story with respect to what someone may actually be able to hear.)

A final subwoofer that I decided to compare is at about the same price point as the PB4000. It is the new Rythmik FV18. The FV18 has both a 16Hz tuning point and a 12Hz tuning point. From my study of the measurements, I believe that the 16Hz tune might be the slightly better one of the two, overall, so that is the one I am using for comparison purposes here. The FV18 can go a little lower with the 12Hz tune, but the slope changes dramatically with that port tune.

If we examine the various graphs for the 16Hz mode, we see a somewhat similar pattern that we did with the SVS PB13. The FV18 maintains a very stable frequency response down to about 20Hz, and then begins a gentle roll-off down to about 16Hz. At 16Hz, the FV18 is down about 3db in SPL compared to 40Hz. That would make it reasonably comparable to the PB13 with respect to its general roll-off characteristics, and might make it somewhat comparable with respect to its audible bass weight. The use of the rumble filter with Rythmik subwoofers could also be a factor in how much the low-bass is emphasized.

(I decided to add an amendment regarding the FV18. There is a new model available with larger ports, 4" rather than 3.5", and with a paper cone. The paper cone gives the subwoofer about 3db more mid-bass than the metal cone driver that was tested on Data-Bass. In addition, the new model with the larger ports appears to generate more tactile energy than other Rythmik ported subwoofers. The FV18 was already producing more SPL below 16Hz than the PB13. The combination of higher SPL, below 16Hz, and increased low-bass TR, may enable the newer Version 2 model to produce more apparent bass weight than the PB13 with which it is compared above.)

I think that it's important to note, once again, that we aren't comparing max output numbers in this exercise. The point of this is not to determine which subwoofer plays the loudest overall, at max output, or even which subwoofer plays the loudest at some specific frequency. We can determine that by looking at max output comparisons, such as the table on Data-Bass. What we are trying to determine in this exercise is whether there might be a difference in the relative bass weight of the various subwoofers, if we compare their low-frequency roll-off characteristics, because those roll-off characteristics are native to the subwoofers' inherent frequency responses at all volume levels short of compression.

All of the subwoofers will benefit from room gain to the same degree, so what we are really trying to determine is whether some subwoofers may produce more low-bass weight than others, relative to their mid-bass frequencies. We already know anecdotally from any number of subwoofer GTS's (get-togethers) that some subwoofers do sound relatively heavier or lighter than others, and this exercise helps us to understand why that might be the case. Some of the subwoofers will definitely produce more, or less, low-bass SPL, proportional to their mid-bass SPL, and that may influence the way that the subwoofers sound with some low-bass material.

It is fair to assume that subwoofer makers are all very well aware of the specific frequency response characteristics of their subwoofers, and that they are all assuming that room gain will augment the very low-frequencies to some degree, depending on the size of the room. But, to the extent that we can see trends in subwoofer design, I think it is also fair to say that there has been a very clear trend, over the last several years, among most ID (Internet Direct) subwoofer makers, toward pushing more SPL into the very low-frequencies, compared to the mid-bass frequencies. Whether they are responding to specific market demands, or simply pushing the envelope with respect to subwoofer development is another question. Buyers who wish to perform the exercise illustrated above can determine how the particular subwoofers they are considering compare in that regard.

I also want to point out that the exercise above dealt only with low-bass SPL in relation to mid-bass SPL. Subwoofers with more SPL concentrated in the low-bass may sound proportionately heavier than some other subwoofers, with certain types of listening material. But, that doesn't necessarily mean that they will also have more low-frequency tactile response. As discussed in the subsection on Sealed and Ported Subwoofers, low-bass TR seems to be the product of more than just SPL at low-frequencies. There is a subjective comparison of low-frequency tactile sensations for several ID brands in a later subsection titled Internet Direct Subwoofers. Total SPL comparisons (covered in some detail in the next subsection), frequency-specific SPL (which we have been discussing here), and tactile response (covered in the ID subsection), are all factors which prospective buyers may wish to consider in selecting subwoofers.

Continuing a theme that I started in the previous subsection, I believe that once we are satisfied that we are going to have enough total SPL from a particular subwoofer (or class of subwoofers), it is actually the native frequency response of the subwoofers that we might then want to compare. I believe that we absolutely want to buy subwoofers which have enough overall SPL, throughout their frequency range, so that we don't approach their max output levels. And, it's possible that a number of different subwoofers could serve us well in that regard.

But, when we factor in our presumed room gain (based on the size of our rooms); our own listening preferences (including our desire for more or less tactile response at mid-bass and low-bass frequencies); and the native response of the subwoofers we are comparing (looking at mid-bass and low-bass in comparison to each other), I believe that we have a much better basis for making informed subwoofer selections. What we actually choose at that point will be entirely dependent on the individual.


Native Subwoofer Response and Room EQ:

Something that I have not addressed up to this point is the extent to which automated room EQ may change a subwoofer's native frequency response? And, it is an important question. Not everyone reading this Guide will have automated room EQ, and not all forms of room EQ will be equally effective with bass frequencies. But, where room EQ is able to affect both mid-bass and low-bass frequencies, what does that mean with respect to the discussion of native frequency response? The honest answer is that I don't know. I believe that a lot would probably depend on the individual room and on the specific subwoofer placement.

Here is what I know about Audyssey. First, Audyssey attempts to achieve about a +/- 3db variance in frequency response across the entire frequency range from 10Hz to 22KHz. In a smaller room, where bass frequencies are being strongly affected by room modes, even a +/- 5db variance might be considered successful. To achieve its objectives, Audyssey can pull down peaks by up to 20db, and it can raise dips by up to 9db. The disparity in adjustment is deliberate, as Audyssey is trying not to raise frequencies in a way that will reduce total output capabilities.

The theory is that equalizing larger peaks at some frequencies will help to equalize smaller dips at other frequencies, leaving subwoofers with about the same amount of total headroom, while evening-out the frequency response. But, that's just theory, and a lot depends on the actual location of peaks and dips. Very low-frequencies require more amplifier power than mid-bass frequencies, so where in the frequency response the bass peaks and dips are distributed matters. In practice, it is very difficult to know exactly how automated room EQ will affect mid-bass and low-frequencies, both with respect to total headroom, and with respect to mid-bass/low-bass equilibrium.

FWIW, I think that one reason subwoofer shootouts may not reveal slightly more dramatic differences among subwoofers is that level-matching subs and EQing them separately, for comparison testing, really does help them to sound more similar than different.

(I actually think that it would make more sense to compare subwoofers in the same room, without EQ, if we want to understand what their essential qualities and differences are like. We can always use room EQ to try to make subwoofers sound more like each other if we want to. But, there will probably always be subtle differences, even after the operation of room EQ, which might be more difficult to detect in a subwoofer shootout. Hearing the subwoofers in their natural state might actually be much more revealing, as we may not know what post-calibration adjustments we might want to make to the frequency response, depending on the native capabilities of the subwoofers.)

It follows from the above, that EQing subs in our listening rooms would tend to somewhat equalize the disparities in native frequency response that the earlier discussion about comparing subwoofer performance concentrated on. So, there might not be quite as much difference in bass weight, among different subwoofers, once automated room correction had somewhat evened-out the frequency response.

I believe that the statement above is generally correct. As noted earlier, most good subwoofers should sound more alike than not. But, I also believe that the specific distribution of room modes in a particular room, and the way in which those modes create peaks and valleys in the frequency response, is somewhat unpredictable even for many audio experts. And, it is certainly going to be unpredictable for most of us who are selecting subwoofers.

So, I think that it still makes sense to understand the native frequency response of the subwoofers we are considering. And, it still makes sense to determine whether we are looking for subwoofers with relatively more mid-bass SPL, or with relatively more low-bass SPL, because we may still be able to hear some differences in relative bass weight, even after room EQ has done its work. I believe that the room size, and the amount of low-bass gain that the room was providing, could also be factors in how much bass weight difference we could hear after room EQ had attempted to even-out the frequency response.

Some forms of automated room correction such as Audyssey XT-32 and Dirac Live may be especially successful in flattening-out a subwoofer's native frequency response so that neither mid-bass, nor low-bass, is emphasized. (Audyssey can pull down peaks by as much as 20db.) Or, depending on the specific room and subwoofer placement, they may be less successful. But, it makes sense to me, that starting with the frequency response we want, gives us a better chance to end-up with the frequency response we want. That is especially the case if we ever want to implement some sort of house curve to change the mid-bass/low-bass emphasis.

[Most modern subwoofers have built-in DSP capabilities that make it easy for us to create house curves, if we want to emphasize low-frequencies, with respect to mid-bass frequencies, in order to create more bass weight. Some forms of DSP are quite specialized, but almost every subwoofer has something that would compensate for room gain. For instance, many subwoofers such as Seaton, PSA, and JTR subwoofers will offer an analogue control to modify the influence of room gain, as discussed in Section VII-B: Room Gain Compensation. Starting with that RGC control in the middle, during calibration, allows users to add low-bass post-calibration by turning the control upward toward the max setting. That will emphasize the frequencies below about 30Hz or 35Hz relatively more than the mid-bass frequencies, and the effect will typically increase below 20Hz.

Increasing the RGC can sometimes have a pretty dramatic effect, for reasons explained earlier. As we increase the volume of both very low-frequencies (<30Hz) and other bass frequencies, the lowest frequencies will be perceived as relatively louder than the mid-bass frequencies. If we specifically emphasize the lowest frequencies more, that difference in perceived loudness will become even more dramatic.

The earlier discussion of room gain compensation, in Section VII-B, mentioned that starting with the analogue knob at a very low setting prior to calibration might not allow room EQ to set control points for very low frequencies, since the SPL would have rolled-off too much for room EQ to be operational at that frequency. But, there could be circumstances, especially in larger rooms, where that roll-off might be beneficial. For instance, where measurements or listening demonstrated that room EQ was pulling down a very low-frequency peak that the listener wanted to keep, he could deliberately set his RGC knob at a very low setting prior to calibration. That way, room EQ couldn't pull down a peak he wanted to preserve, and he could further accentuate that peak by turning-up RGC after calibration.

A similar technique can be employed by owners of ported subwoofers which have variable port tunes. Examples would include some SVS models, along with HSU, Rythmik, and Monoprice subwoofers. Performing a calibration with a 20Hz port tune, for instance, and then changing the port tune to 16Hz, post-calibration, would be an effective way to add bass weight from 20Hz down, without having room EQ subtract it during calibration. And, the lower port tune would be occurring at such a low-frequency, in most rooms, that there would be no reason to worry about room modes. 20Hz and under would be pressure vessel gain (PVG) territory in almost every room, so there typically wouldn't be any cancellation at those very low-frequencies--only increased bass weight and TR, which a particular listener might find appealing. Whether this would be an appropriate thing to do in a specific situation could only be determined through trial-and-error, and the use of REW would facilitate that process.]

I also think it is worth noting that AVR's, along with their respective forms of room EQ, may come-and-go over time, while the really good subwoofers we select may be more likely to be with us for much longer. For that reason, considering our subwoofers' native performance, irrespective of room EQ, may always be worthwhile.



VIII-C: Selecting Single Versus Multiple Subwoofers:

Previous sections have explained some of the advantages of multiple subwoofers in terms of improved frequency response. But, a question that is frequently asked on the Forum is whether it is better to buy one more powerful sub, or two less powerful ones. As with most such black-and-white questions, the best answer is that it depends. My own preferred advice is for buyers to buy as much subwoofer as they can reasonably afford, because the various subwoofer threads are constantly filled with people looking to upgrade their subs. Starting with the most powerful subwoofer (or with the most powerful pair of subwoofers) we can afford, means that the next step in the evolution of our bass systems may simply involve adding similar subwoofers, when we either want an improved frequency response, better bass envelopment, or more bass SPL.

The alternative to starting with subwoofers which are inherently competent to begin with is to pivot, selling our existing subwoofers in order to be able to buy the more powerful models that we now realize we want. Compounding this tendency is the fact that many of us who experience serious low-bass SPL realize that we like the bass sensations even more than we initially thought that we would. And, we want more of them.

Subwoofers tend to depreciate in a fashion similar to the way that cars do in the first year, although used subwoofers typically stabilize a little more in price than cars do, after that initial depreciation of 20 or 30%. So, starting with a more powerful (and expensive) subwoofer may actually turn out to be a better long-term solution from a financial perspective. It should, however, be noted that some companies such as SVS and PSA offer full-value trade-ins on subwoofer upgrades made within the first year. (With PSA, you have to ask for a trade-in, but it will typically be granted.) The buyer has to pay the return shipping, but that is usually not more than about $150. So, the upgrade possibility can also be a factor in deciding how much subwoofer to go with initially.

With respect to the original question about buying one more powerful subwoofer or two lesser ones, I think that there is a component of instant gratification involved with buying two subwoofers. If we buy two lesser subwoofers now, we will immediately have the benefits that come from having dual subs. And, that is hard to argue with. There is also the chance that the extra 6db that the second sub will provide (averaged across the subs' entire frequency range) will be enough. So, there can be valid reasons to start out with dual subs rather than a single more expensive one. That might be particularly true for people who are just starting to add subwoofers (or better ones) to their HT systems, and who don't really know exactly what their low-bass and SPL requirements will be in the first place.

In that case, though, I might recommend that they start with a sub maker offering a long-term upgrade path, such as the one-year trade-up mentioned above. Nearly all of the ID subwoofer makers (which are the only subs involved in this discussion) will offer a free-trial period ranging from about 45 to 60-days. That may be enough time for a particular individual to decide whether a subwoofer is powerful enough. If not, the most he is out is for return shipping.

But, if our bass interests didn't increase gradually over time, and if 45 to 60-day trial periods were enough to fully appreciate our own future bass requirements, I don't believe that there would be nearly so much subwoofer turnover occurring among sub owners. Consequently, my own advice is typically to start with the single most powerful subwoofer a person can afford, and then to upgrade to another, if and when it is required and feasible to do so.


* An additional thought on the choice between one more powerful subwoofer, or two less powerful ones, involves subwoofer size. I often see people on subwoofer threads who are extremely concerned about the size of the subwoofers they are considering. That can definitely be a legitimate concern, especially where space is tight, where aesthetics are a major factor, or where the preferences of a significant other are also involved. The idea that two smaller subwoofers would be easier to unobtrusively position in a room is sometimes a deciding factor in a person's subwoofer selection.

But, over a period of several years of participating in numerous subwoofer threads, I have seen a fairly consistent pattern emerge. The vast majority of people who were initially concerned that a subwoofer they were buying would be too large have not found that to be the case. It simply becomes another piece of furniture in a mixed-use room, for instance. And, in the case of people who chose smaller subwoofers out of concern for size, many were back on the forum within a few months, or a year, looking to upgrade to larger models.

I think that it may be much easier for most of us to visually adapt to a larger size subwoofer, than it is to adapt to what we may perceive as inadequate bass performance. That is especially true if our preference for bass increases when we add more capable subwoofers, and experience better bass for the first time in our own listening rooms. And, so many buyers buy a subwoofer only to second-guess themselves later, and wonder what if they had gone with the larger ones? If this were not such a consistent pattern, I believe that there would be far fewer people upgrading subwoofers on the forum. The issue of subwoofer size, in relation to performance preferences, is simply one more factor to carefully consider in making a subwoofer selection.


** Returning to the idea of what constitutes appropriate subwoofer power is the tremendous range of potential variation involved. There are people whose HT rooms are smaller than 1,000^3, on a suspended wood floor. And, there are people whose HT rooms are larger than 10,000^3, on concrete. As noted in previous sections, the 1000^3 room would get much greater room gain than the very large room, and the low-frequency tactile sensations would be much stronger on a suspended wood floor. Our listening distances are also different, with some of us located very close to our subs, and some of us 15' or 20' away. Both SPL and tactile sensations decrease with distance. SPL probably decreases by an average of about -3db for each doubling of distance. (It is -6db in an anechoic or quasi-anechoic measurement, but about half that in-room.)

We also listen to different types of content, some of which may have dramatically different amounts of low-bass. Low-bass SPL requires a great deal more effort for the subwoofer to produce. We listen at different SPL's, ranging from less than -30 MV, up to Reference (0.0 MV) and beyond. And, we use subwoofers with anywhere from zero boost up to +15 or +20db, depending on our listening volumes and bass preferences.

For all of the differences in our rooms and listening habits, there are equivalent differences in the subwoofers available to satisfy our specific requirements. For instance, if we consult the Data-Bass systems list, we can see something like the SVS SB-12 NSD, which is a nice little sealed subwoofer, producing a max burst output of 91.9db at 20Hz. And, we can see something like the ported JTR Cap 4000ULF producing 119.3db at that same frequency. That's a difference of 27.4db. At frequencies below about 30Hz, each +5db increase in SPL is approximately equivalent to a doubling in perceived loudness. The difference in those perceived loudness levels is so great that the numbers don't really have a meaning anymore.

But, in some rooms, for some listeners, enjoying their music or movies at their preferred listening levels, with their preferred subwoofer boosts, a single SB-12 NSD will be perfectly adequate. In other rooms, perhaps not a great deal larger, an owner may have dual Cap 4000's. And, neither subwoofer owner will be "wrong" in his personal audio preferences. The differences in our personal range of preference for bass SPL and tactile sensations can be an even more important factor than room size, in my opinion. That is why I have emphasized bass preference as such an important factor throughout this Guide.

For most of us, I believe that the only way we can know exactly how much subwoofage is enough for our needs is through experimentation. We try something, and then we add to our systems, as required, over a period of time. But, I think it is always better to have a little more on tap than we may ever need to use--particularly if we want to keep distortion levels low and our sound quality high. And, we can always control the volume levels of our subs. For that reason, I would typically encourage people to err on the side of more total bass output than they initially think that they may need. And, that is consistent with the recommendation to buy the most powerful subwoofers(s) that we can afford, and believe that we will reasonably need.


*** One of the ways that we could try to guesstimate our subwoofer requirements is by starting with our listening level. Let's take a hypothetical example of someone in a 2000^3 to 3000^3 room. That would probably be pretty close to an average (or slightly larger than average) size. Let's say that the listener uses a master volume of -15. That would also be about average. And, let's say that the listener wants to be able to hit about 20Hz for 5.1 movie and music content. That would be near the bottom of our hearing range, and would be right on the THX standard for Reference.

Given reasonable subwoofer positioning, we can probably expect to get at least about 6db to 12db of room gain in that size room, at about 20Hz. Let's be conservative and go with 6db of total room gain. (I'm not going to try to factor in subwoofer distance at this point, although at a distance of 12' to 14' that could require another 3db. But, let's assume that the total boundary and room mode gain covered us for the listening distance.) Now, if we start to put some things together, here is what we know so far. Reference for the .1 LFE (low frequency effects) channel is 115db peaks, and we are -15 from that. So, we only need 94db from our subwoofer at that frequency. (100db -6 that we are already getting from room gain.)

But, what if we are using a sub boost? If we have Audyssey and are using DEQ, it is boosting our subwoofer by +6.6db at -15 MV for frequencies under 30Hz. If we are using an independent sub boost on top of that, as most people do, we are actually putting an even higher demand on our subwoofer. Let's say we are boosting +3.5 in addition to DEQ and we will round the boost to an even +10db. Now, our subwoofer needs to be able to produce 104db at 20Hz, at a master volume of -15. (If we don't have Audyssey, some of us may very well still be using comparable sub boosts.)

If we get a new subwoofer, our master volume level probably won't go up--at least not unless we really wanted to listen to louder volumes, to start with, and were holding back because of our sub, or for some other reason. But, what may be more likely to happen is that we will want to add even more subwoofer boost than we had been using. If we didn't want even more mid and/or low-bass than we already have, we probably wouldn't want to upgrade. (If our master volume did go up, that would have the same effect as boosting our subwoofer.)

So, now we might need to add in a cushion so that we can have more bass boost than we had before. Let's add another 3db. Now, we are at 107db at 20Hz, at our preferred listening level of -15 MV. And, we want to be careful about taking it right to the limit on the max output of our subwoofer, because we don't want to throw in too much distortion, or have to worry about compression, or port chuffing if it's a ported sub. So, we probably need to add about another 3db of headroom for a total of ~110db at 20Hz. And, we can use Data-Bass, or a similar resource, to see how close we are at that frequency with the sub or subs we like. (Dual identical subs should allow us to have about +6db at that hypothetical 20Hz frequency.)

[I want to add a caveat to the example above. I have assumed equivalent SPL for mid-bass and low-bass (20Hz) frequencies in that example. And, based on the Equal Loudness Contours, that is an appropriate assumption to make. But, as with almost everything, the reality is a little more complicated than that. First, low-frequencies don't need to be exactly at the same SPL as other frequencies in order to add some bass weight to the sound. How much bass weight is enough will depend heavily on the individual. Second, as noted in other sections of the Guide, we may be able to feel low-bass tactile sensations at lower SPL's than we can hear them. And, below about 30Hz, it is harder to separate what we hear from what we feel. So, as hard as I may try to create a hypothetical way to predict how much low-bass SPL someone may need, it is only a very general guideline. There will still be a strong YMMV component to it in actual practice.]

As noted, the above is a purely hypothetical exercise, based on some assumed listening levels and sub boosts, and based on some assumptions regarding equal loudness levels. But, I believe that the assumptions made may somewhat fairly represent a potential real-world scenario. The exercise itself may seem a little complicated at first pass. But, short of measuring our actual in-room frequency response, it may be a way to help some of us to have a general idea of what we are looking for, based on our own room size and preferred listening habits. The reality certainly won't exactly correspond to the mathematical exercise, but it's much better than a wild guess. And, for those of us who already have subwoofers, and who are planning to upgrade, it may give us a starting point in our search for subwoofers that match our personal performance objectives.

[In the interest of thoroughness, I should add another component that wasn't included in the hypothetical exercise shown above. I stated that the LFE channel requires peaks of 115db at a Reference listening level. But, the subwoofers in our HT systems also have to provide bass support for the regular channels, as well. And, they can have significant low-frequency peaks too. Those peaks will only reach 105db, at 0.0 MV, but they will still exert some additional demand on the subwoofers. I have seen varying estimates of how much additional demand the bass in the regular channels may exert, but they vary based on room size, and on the number of channels employed in an audio system. (Most estimates suggest allowing 1db of additional headroom for each bed channel up to a maximum of 7.) Whatever the correct number may be, the idea that the other channels will also require subwoofer support is another reason to have slightly more subwoofage than we think that we might need, based solely on mathematical calculations.]


**** A seldom-discussed variable, that makes comparisons of listening levels and subwoofer boosts difficult, is the noise floor of a room. The noise floor of a room is the SPL, measured in decibels, when the room is "quiet". "Quiet" is an ambiguous term, but I would define it to mean that there is no one moving, or talking, or playing music or other content in the room. There would be normal external noises from the street, or from other parts of the house. There would be noises produced by electronics in the room, or from the HVAC system. But, there would be nothing added to, or subtracted from, the typical noise level in the room.

The noise floor would be measured with an SPL meter set on C-weighting Slow. According to what I have read, the noise floor could range from a low of about 35db to a high of about the low 50's. Most rooms would probably fall into a range of about 40db or 45db. Factors which could influence the noise floor could be the relative density of the room's construction, and any room treatments or softening influences within the room, as well as the relative loudness of electronic, HVAC, and other normal room noises.

I think that this is an important variable, which is often overlooked in discussions of listening levels, subwoofer boosts, and system headroom. Someone who is in a room with a noise floor of 40db might be able to listen comfortably at a somewhat lower volume level than someone who had a noise floor of 50db. Even if the master volume level were exactly the same in both rooms, since automated calibration routines should compensate for the noise floor in calibrating systems to Reference, the relative demand on the audio system could be significantly higher in the room with a high noise floor. (The room correction microphone would have to "hear" sounds clearly, above the noise floor, and I believe that it would have to set trim levels higher than it would if the noise floor were lower.)

In a room with a high noise floor, if it takes overall higher system volume levels to hear sounds in our normal listening range, and especially to hear low-bass sounds, then the noise floor could be an important factor in determining the overall system headroom, and especially the subwoofer headroom, we require. This is an additional variable that I think could be important in determining the amount of subwoofage required in a particular room. And, it could certainly be a factor in determining the amount of sub boost we might choose to add. (The noise floor can be determined with an SPL meter set to C-weighted slow. A calibrated meter would be the most accurate, but an inexpensive one or a smartphone app, could still be helpful for this specific purpose.)

I believe that for someone who doesn't have a subwoofer now, or who is making a major jump in quality and output, understanding his existing noise floor might be beneficial in selecting a subwoofer (or pair) which will have sufficient headroom. Someone who already has a good benchmark might already know that he is just upgrading for 6db more SPL, or for slightly lower extension. But, for someone making a major jump, that type of benchmarking might not be possible. I don't know how important noise floor is in the relative scale of things, but I think it may be worth measuring and considering, in some subwoofer selection processes.


Conclusion: Deciding what subwoofer to buy, and whether to start with one more powerful sub, or two smaller ones, is a very individualistic decision. Buyers often start threads to solicit suggestions, and they often consult with subwoofer makers to seek their advice. I am honestly not sure which method is more effective. Threads will elicit suggestions that range from those that are focused on the prospective buyer's stated needs and preferences, to random endorsements of subwoofers which may not correspond at all to that buyer's situation.

In my experience, ID submakers tend to be very objective in the advice they give, and they try to be very careful not to oversell prospective buyers. But, I very frequently see buyers upgrading from the original purchase, where the sub maker was consulted, so I suspect that the sub maker recommendations are often quite conservative.

(Ironically perhaps, I also believe that some subwoofer designers are not really strong bass enthusiasts in the same way that many of their customers are. I know of some specific exceptions to that, but it is another reason that subwoofer makers may tend to give very conservative advice regarding subwoofers, because they don't personally require as much subwoofage in their rooms as many of their customers do.)

I think that for most of us, selecting the right subwoofers will be a process, and we may not get it exactly right the first time no matter how hard we try. The fact is that we have to get the subwoofer(s) in the room to decide how things actually sound and feel, and to decide what we like. And, as noted earlier, even then, our preferences for more bass may continue to evolve for a time. That is why most subwoofer advice starts with the suggestion to get the most powerful subwoofer(s) the buyer thinks he is likely to want, and to be able to afford.


With all of those factors in mind, here is a repeat of the general rules for prospective subwoofer buyers that was offered at the beginning of Section VIII.


General Rules To Consider In Subwoofer Selection:

The subsection on ID subwoofers discusses some of the other factors which buyers might wish to consider in selecting subwoofers. But, concentrating just on performance objectives, there are a few general rules that might also be worth considering. It is important to note, that there may always be exceptions to any of these general suggestions.

1. The first general rule is to try to buy the most powerful subwoofer you can afford. You will see that advice repeated often on the forum. The reason for that is that most of the people, shopping for a subwoofer, are probably already upgrading from one that isn't powerful enough. You won't want to repeat the process multiple times. And, even if this is your first subwoofer, you may be surprised at how much difference a good subwoofer will make in your audio system.

It's not just that we may overestimate the power of a particular subwoofer, or pair of subwoofers, when we make a purchase. It's also the fact that we may underestimate how much we are going to enjoy good low-bass when we hear it in our HT's. We may find that, as time passes, we may want more of the mid-bass and low-bass sounds and tactile sensations that we are enjoying. If we find that it's the lower frequencies we like, we may especially wish that we had started with a more powerful subwoofer.

2. Try to define your own listening preferences. It is important to understand that your preferences are unique to you, and that your room size may not be the most important factor in determining your bass preferences, or your subwoofer requirements. Is this going to be mostly for movies, or for music, or about 50/50? That will influence the advice you get and may influence your decision regarding the subwoofer you select. If you can define your room size and your typical listening levels, you will find information in this section which will help you to determine how much subwoofage you need. Others can advise you on this subject, but ultimately you will need to follow your own judgment.

3. Try to decide as early as you can whether you are looking for sealed subwoofers or ported subwoofers. Subsequent general rules may help to refine that decision, but understanding the differences between ported and sealed subs, and trying to align those differences with your own goals, is probably a good starting point.

Subsection VIII-A gives good general guidance as to the differences between sealed and ported subwoofers. Briefly, ported subwoofers typically produce more SPL below 50Hz, and much more SPL below 35Hz, than comparable sealed subwoofers do. But, the ported models are usually significantly larger and heavier, and may also be more expensive, than the sealed models.

People often recommend sealed subs for music listening, and ported subs for movie viewing, because most music doesn't require extremely low-frequency SPL and TR, while many movies do. Depending on the individual and on the room, it may be more complicated than that, but the generalization does offer us a starting point in our selection process.

4. Try to listen to some subwoofers somewhere that will help you to define what 80Hz, and 40Hz, and 20Hz frequencies sound and feel like. Most people believe that the frequencies they are hearing are much lower than they actually are. For instance, a bass guitar chord at 60Hz or 80Hz may seem very low to us, when in fact it is a mid-bass frequency. Try to find out for yourself some of the differences between mid-bass frequencies and low-bass frequencies. There may be an AVS member in your community who will help you with an audition of his system, and who can help to define frequencies for you.

Understanding the difference between hearing very low-frequencies and feeling very low-frequencies is also important. Mid-bass frequencies allow for more distinct separations between bass sounds and the accompanying tactile sensation--chest punch. Below about 30Hz, the low-bass tactile sensations we feel are more difficult to separate from the low-bass sounds we hear. They tend to blur together. It may also be important to understand how much difference the frequencies below about 30Hz, with their accompanying TR, may make for 5.1 movies in our HT's. For music, those very low-frequencies will be much less important. Also understand that the lower you want to go with significant SPL, the more subwoofage it will take, and the more expensive it will be.

5. You can try to determine how much overall SPL you are looking for to accommodate your preferred listening level. I think it is fair to say that the most important goal is to have sufficient undistorted SPL from about 120Hz down to about 20Hz. (Depending on the capabilities of the speakers in a room, and how crossovers are implemented, the critical range could be just from about 80Hz down to 20Hz.) So, I would recommend concentrating on that general goal. Within that overall 20Hz to 120Hz frequency range, however, I think that it would be fair to emphasize the middle of that range, from about 40Hz to 80Hz, first. An inability to play fundamental mid-bass frequencies loudly enough, would make a subwoofer essentially ineffective for virtually any kind of use.

Try to define your actual SPL goals if you can. What is the loudest you play, and have you ever tried to measure your bass with a calibrated SPL meter? (There is a method shown for trying to calculate your probable bass SPL needs, if you can't measure them, that is illustrated in VIII-C.) Do you want to achieve 110db at ~63Hz (with an additional 3 or 4db of headroom) or do you want 120db--or even more? Starting with the mid-bass range, and comparing subwoofer capabilities in that range, to our normal listening volumes and subwoofer boosts, probably makes the most sense. Remember that dual identical subs will net +6db averaged across the subs' entire frequency range. Also remember that it is the combination of master volume and subwoofer boost which determines how loudly a subwoofer plays. So, you need to understand your preferences for both variables.

6. Once you believe that you understand your overall SPL requirements, consider low-frequency extension. If you are sure that you will have enough undistorted SPL throughout the mid-bass frequency range, pay special attention to the low-frequency extension you are hoping to achieve in your room. Are you looking for 20Hz, or 15Hz, or even lower extension?

It's important to try to identify your own goals, and it's important to be realistic about this, especially if you are in a large room. In a really large room, extension into single digits may be an unrealistic goal for anyone who is not prepared to add something like multiple JTR Cap 4000ULF's. Many people are content to have reasonably good bass SPL down into the low-20's. And, 20Hz is still the THX standard.

Even on AVS, and on most of the subwoofer threads, most people would be very happy to shoot for about 15Hz, or so, in their HT's. Again it's important to remember that you won't normally want to play your subwoofer at max output levels, so when you try to calculate your room gain and compare that to max output levels for subwoofers, you will probably want to leave an allowance of 3 or 4db of undistorted headroom.

It's very helpful to try to define your low-frequency goals in advance. Most of the upgrades I see are to get more low-frequency bass, and not just more bass in general. Selecting ported subs with lower port tunes may be an important factor in your selection process. If you don't select ported subs with sufficiently low port tunes, it may be very hard later to get sufficient low-frequency extension simply by adding multiple identical subs.

(With sealed subs, we can add more of them and achieve significantly lower extension, due to their more gradual roll-off characteristics. Adding several ported subs, on the other hand, may not help nearly as much with respect to really low-frequency extension. The ported subwoofers' tuning points will probably always be the single most important factor with respect to how much low-frequency extension they can achieve. That is because ported subwoofers roll-off so quickly below the tuning point. So, having multiple subwoofers which have a 20Hz port tune, probably won't help you to produce significant SPL at 10Hz, in even fairly small rooms.)

7. Another general rule is that it is a good idea to compare the low-bass performance of a subwoofer, to the mid-bass performance, using graphs of their respective frequency responses, as described in subsection VIII-B. Those graphs may tell us something important about the relative "bass weight" of the subwoofers we are considering. Depending on room size (which is a factor with respect to low-frequency room gain) and on our own specific bass preferences, some of us may want more low-bass weight, and some of us may want to emphasize mid-bass frequencies more. And, we will want to pay attention to the native frequency response of the subwoofers we are considering, and not just to their max output capabilities.

8. Remember that dual subwoofers will add more than just a doubling (+6db) in output. Dual subwoofers (or ideally even more than two) will typically provide better frequency response, and better bass envelopment, than a single subwoofer can. Proper subwoofer positioning is important with dual subwoofers, just as it is with individual subs. But, in general, dual subs will provide advantages that most users will benefit from. So, having a long-term plan to obtain dual subs will be important in most (although not necessarily all) cases.

9. It may be helpful to recognize that ported subs will not only provide more low-frequency SPL, they will also generally provide stronger low-frequency tactile response than sealed subs will. (Below about 30Hz, it can be much more difficult to distinguish between bass sounds and tactile sensations. So, low-bass TR may be an important factor in the movie experience.) And, there may be differences in the overall amount of TR produced by different brands of ported subwoofers. If the stronger low-bass tactile sensations are important to someone, especially if the HT is on a concrete floor, that is something to keep in mind. The reverse is also valid if someone is on a suspended wood floor, or if he isn't looking for as much TR in general, because suspended wood floors conduct low-frequency vibrations much better than concrete can. Suspended wood floors can amplify the overall low-bass that we perceive, especially in a small room.

10. Another general rule is that smaller rooms (under about 2500^3) will typically create room gain which may significantly amplify low-bass frequencies. (Room gain won't amplify low-bass TR, the way a wood floor was described as doing, but room gain will amplify low-bass SPL.) In many cases, larger, more powerful, ported subwoofers may not be as good a fit in a very small room (especially under about 1500^3 or so). In those instances, ported subwoofers which don't emphasize low-bass frequencies quite as much as mid-bass frequencies, may be a better choice. And, where sufficient low-bass is available, due to room gain, many people in smaller rooms may enjoy the slightly less overt tactile sensations of sealed subwoofers. Very strong low-bass SPL and tactile sensations, at close range in a small room, are not for everyone.

11. Looking at it from the perspective of a larger room, rooms over about 3000^3 may take a lot of sealed subwoofers to create sufficient low-frequency SPL to satisfy some listeners, as the room will not amplify low-frequencies nearly as much as it will in small rooms. As rooms get even larger than 3000^3, the advantages of powerful ported subwoofers, which can generate significant low-bass SPL, and significant low-bass tactile sensations, may become more-and-more important.

12. Understand, that room size notwithstanding, personal preference is as important with bass as it is with most other things. Take your time and weigh your options carefully. It is likely that more than one subwoofer model, from more than one company, will satisfy you. And, it is unlikely that there is only one perfect subwoofer which can meet all your performance objectives. Once you believe that your performance objectives have been met, consider any other factors you believe are important, including cost, aesthetics, features, customer service, etc. (We may start with cost as our primary objective. But experience has shown, that if performance objectives are not met, most people will soon be back, trading-up for more powerful subwoofers, and probably losing money in the process.)

With all of the generalizations being made, however, there will always be exceptions which depend heavily on specific circumstances, and on the specific preferences of the individual. Our ability to understand our own performance goals and preferences may be the single most important factor in subwoofer selection.



VIII-D: Internet Direct Subwoofers:

I mentioned earlier that the Guide is only discussing ID (internet direct) subwoofers. There are two reasons for that. First, although I am not any kind of an expert on different subwoofer makes, I at least have some general familiarity with the ones I am discussing. Second, I personally believe that ID subwoofer makers generally represent cutting-edge subwoofer technology and subwoofer performance. For most people, the ID subwoofers offer a superior performance-for-value proposition. The specific subwoofer brands I will list are those I feel I have enough general familiarity with to even comment about. If I don't comment on a subwoofer brand (ID, or otherwise) it isn't for any negative reason. It is just due to some comparative lack of familiarity with the brand.

I think it is important to add that I would like for all of the sections of the Guide to be as objective as possible. But, that is particularly important, in my opinion, when I mention specific brands. If I say anything that can be in any way construed as negative, please point it out to me, because that is not my intention. I honestly believe that there are so many really good subwoofers made today that a buyer can expect to be pleased with any number of different subwoofers. My purpose in including a very brief synopsis of some of the ID makers is simply to help a prospective buyer to get started in his search. Independent research, and the application of independent judgment by the buyer, will still be required.

I don't want to duplicate the good work that Jim Wilson has already done with subwoofers costing $300 or less, so I will start with what I would consider a slightly higher entry-level. SVS offers subwoofers starting at around $500 in their Outlet store. Older models can sometimes be obtained in unused condition for even less than that. SVS also offers intermediate and high-end models, and the available features and performance qualities go up as the price increases. Easy model availability, good quality, and strong customer service are hallmarks of the SVS brand.

Entry-level HSU subwoofers are probably about comparable in price to the entry-level SVS subs and they also have good reviews and good customer service. Specific performance, in terms of max output and extension, and specific features, could be compared to determine which might be more suitable for a buyer. Rythmik subwoofers also start in that same general price range and have good reviews. Both brands have in-stock subs which go up into the intermediate range and higher. Rythmik subs offer the widest performance range of the three companies. Rythmik Audio also has excellent customer service.

For subwoofers costing up to about $1000, including shipping, the HSU VTF-15 MK2 ported subwoofer may offer the most low-frequency performance and overall versatility (with both ported and sealed modes). The Rythmik F12 and the Seaton JS-12 are very good sealed subwoofers at a somewhat similar price (the Seaton is a bit more).

Monoprice THX subwoofers are a very new entry into the Internet Direct market. It is too soon to evaluate their customer service, but their pricing seems very competitive for the quality and performance they deliver. They are offered in three ported models: the Monolith 10, the Monolith 12, and the Monolith 15. All three are THX-rated, which requires that they have very low distortion. They also have strong SPL with excellent low-frequency extension for their respective driver sizes.

Seaton Sound does not offer entry-level subwoofers, nor do the other sub makers which follow, although all of them offer subwoofers at several price points. The various Seaton subs are mostly sealed, where the other brands mentioned offer a wide selection of both sealed and ported models. Seaton subs tend to concentrate on low-distortion and good sound quality, even more than they do on max output, and Mark Seaton is considered a guru in the subwoofer industry. Model availability is largely based on demand, as most of the subs are made in response to specific orders, and there can be some wait times for both orders and communications.

PSA subwoofers offer a wide choice of sealed and ported subs. According to PSA owners, both the ported and sealed models sound somewhat similar, with a more significant amount of low-bass tactile energy from the ported models. Most of the ported models appear to have a port tune of about 20Hz or higher, and they deliberately concentrate more SPL in the mid-bass range than is the case with some of the other subwoofer brands.

(As of April 2019, PSA has introduced the TV36 subwoofer. It is a large, dual 18" driver, ported subwoofer with a 13.5Hz port tune, and several driver options. The iPal driver may be the best 18" driver commercially available. I would expect the TV36's low-frequency performance to rival that of a Cap 2400 down to about 12.5Hz, with potentially much more mid-bass SPL than the comparable JTR sub, depending on the PSA drivers selected by the buyer. The driver selection will also affect the price, with the iPal version being the most expensive.)

PSA may make the most powerful sealed subwoofer commercially available with its S7201 model. As might be expected, it is a very large subwoofer. PSA is also known for exceptional customer service, and strong customer loyalty. PSA normally has in-stock models, and some refurbished subs as well.

JTR subwoofers also offer a wide choice of sealed and ported subs, but the brand is increasingly associated with ported subs. The JTR port tunes range from 18Hz down to 10Hz, depending on the model. Compared to the earlier vented PSA subs, for instance, JTR subs deliberately concentrate much more SPL in the low-bass frequencies, particularly from about 30Hz down, offering what used to be a fairly clear choice between the two different design philosophies. (The new JTR Cap 118HT is a slight exception to that, with an 18Hz port tune, and a deliberately greater concentration on mid-bass SPL. Other current models offer a 10Hz port tune, although the older Orbit Shifters with higher port tunes and enormous mid-bass SPL's, are still available by special order.)

The Captivator 4000ULF is the most powerful ported subwoofer currently available. JTR ported subwoofers are known for having extreme low-frequency tactile energy, due to powerful drivers with strong excursion capabilities, and due to the quantity of air moved through the ports at low-frequencies. These subwoofers are made in response to specific orders, and there can be some wait times.


[Almost all comparisons of subwoofers are at least somewhat subjective, but sound quality is especially so. Rather than trying to address something quite that subjective, I would prefer to try to compare performance attributes. Some examples of performance attributes which can be compared include max output at specific frequencies, or across the full bandwidth. Whether a sub concentrates more SPL, in a specific portion of the frequency range which appeals to the buyer, would be another way. And, that concentration of SPL, as noted in an earlier subsection, could have an impact on a subwoofer's subjective sound characteristics. As explained in some detail in the previous subsections, Data-Bass, and other sources of measurements, would facilitate those performance comparisons.

The amount of perceived tactile energy produced by a subwoofer would be a third way to compare subs. This one would also be somewhat subjective. But, TR (tactile response) can be measured, as noted in Section VII, and anecdotal reports of TR seem to be pretty consistent for various brands of subwoofers. As noted earlier, ported subwoofers typically produce more low-frequency tactile response than sealed subwoofers do, so that may be something to keep in mind when selecting a subwoofer.

As a general rule, I would suggest that comparing TR among sealed subs would be closely related to the SPL that they produce at mid-bass (chest punch) frequencies, or at low-frequencies. More SPL equals more potential TR. But, as noted in other sections, mid-bass TR and low-bass TR are entirely different feeling sensations. Ported subwoofers typically produce much more low-frequency TR than sealed subs, in part due to the greater SPL they produce at very low-frequencies, and in part due to the action of the ports which produce more particle velocity. So, comparing low-frequency TR (the deep rumbling or thudding sensations) among ported subwoofers may be a useful exercise for someone interested in having either more TR, or less.

For instance, Rythmik ported subwoofers, including the very powerful FV25HP, are deliberately designed to more closely resemble sealed subs when it comes to low-frequency tactile sensations. (The servo mechanism limits the excursion of the woofer, and it is partly the forward movement of the driver, pushing air ahead of it, that creates low-frequency particle velocity.) I believe that it would be fair to say that on a continuum, they might represent the smoothest feeling ported subs, with PSA subs more in the middle, and JTR subs at the more aggressive end. JTR subwoofers, by all accounts, have the strongest low-frequency tactile sensations of all the ported subs.

(According to early anecdotal reports, the new PSA TV36 with the 18" iPal drivers and a 13.5Hz port tune produces very significant low-bass TR. So, the above statement about PSA ported subwoofers having less low-bass TR than JTR ported subwoofers may not apply to the new TV36's.

Again, based mainly on various anecdotal reports, there may be a correlation between a port tune in the low to mid-teens and increased low-bass TR. For instance, the PB16 Ultra, with a 16Hz port tune, can produce very strong low-bass TR. And, some listeners report that the Cap 1400, with a 17Hz port tune has more perceived <20Hz TR than the Cap 2400ULF, with a 10Hz port tune. It makes sense if we consider that low-bass TR would be strongest near the port tune, and that there is much more content in the roughly 14 -18Hz range than there is in the 10-12Hz range.)

SVS subwoofers would also be more toward the strong end of the TR continuum. I would expect large SVS subwoofers to have more aggressive low-frequency TR than the earlier vented PSA subwoofers, for instance, based on their lower port tunes and more emphatic low-bass SPL. The earlier model V3611 PSA subwoofers would in-turn have proportionately more mid-bass chest punch in comparison to SVS subs, due to their higher overall SPL. The new iPal TV36 and Neo TV36 might conceivably have more of both kinds of TR.

HSU ported subwoofers and Monoprice subwoofers might be somewhere more toward the middle of the two extremes created by Rythmik and JTR (and the other subwoofer models mentioned above). As noted above, I have chosen to emphasize ported subs in this comparison, since tactile sensations may be a fairly easy way to differentiate among some of the different subwoofer makes. But, I need to emphasize that these observations are anecdotal, based on user reports, and that this may be a YMMV situation in terms of what a given individual feels, in a given room, and in terms of how much TR he likes in either the mid-bass or low-bass frequencies.]


As I have moved up my list of ID subwoofers, the most expensive (and largest) subs have been the S7201 and the Cap 4000. (And now, the new iPal TV36.) And, that is no coincidence if we remember the dictum that there is no replacement for displacement. But, I have saved another ID maker for last. Funk Audio subwoofers are not known for being large. In fact, some of them are remarkably compact for their relative performance capabilities. And, they are primarily offered as sealed subs, although Nathan Funk can custom-make ported subwoofers upon demand.

What they are known for is beautiful cabinetry and overall audio quality. Some of them achieve significant SPL in comparatively small cabinets, via custom-made and expensive drivers, extremely powerful amplifiers, and innovative DSP. They are all custom-made to order, and depending on circumstances, the waiting time may be measured in months rather than in days or weeks. They can also be very expensive, depending on the specific model and finish option selected. But, where budgets are less constrained, they can be a very good choice.

There are many factors which can potentially influence buying decisions, and I have tried to touch on a few of them. Another factor that should be mentioned is subwoofer flexibility. Some subwoofers offer variable tuning points. HSU, Rythmik, and SVS subwoofers have that feature on some of their ported models. A subwoofer control which allows users to emphasize, or de-emphasize, low-frequencies (room gain compensation) is offered by most of the ID makers on their nicer models. That helps to allow users to better align the room gain they are getting with their own low-bass preferences.

Some sub makers offer programmable DSP that goes well beyond port tuning options or room size controls. HSU, Rythmik, SVS, and Funk Audio, all offer very sophisticated user controls to implement variable filters, boosts, or slopes, depending on the overall system and room interactions. Those features are often model-specific, especially with SVS, with more features offered only for the more expensive models. Individual users have to decide for themselves how valuable those additional features are to them.

In my opinion, those features are likely to be easier to use where someone is dealing with a single subwoofer, as the use of automated room correction, through an AVR, may make multiple setting changes more complicated where multiple subs are concerned. But, even there, the use of variable DSP can help to fine-tune subwoofers either before, or after, running automated room correction. As with everything else involved in subwoofer selection, the amount of user-controlled DSP offered is something that different individuals will see as more or less important.

I have deliberately tried not to say too much about any of the specific sub makes, as I don't want this Guide to strongly influence someone else's decision. As I said at the beginning of this section, I consider subwoofer selection to be very individualistic. Some of us will have specific space constraints, or may need to economize, and may not care about additional features, or about aesthetic issues.

Others will want smoothness, or will perhaps value output above everything else. And, those buyers wanting more output may have a clear idea of where in the frequency range they want to have the extra SPL. (Mid-bass versus low-bass, for instance.) Still others may be drawn to specific features, or to aesthetic qualities, in certain makes or models. Perceptions of responsiveness to questions and of overall customer service may also influence buying decisions. In my personal opinion, there is no right or wrong answer to the question of which subwoofer to buy, as long as the subwoofer selected satisfies the individual buyer's requirements.

Prospective buyers who want to use this Guide as a reference are encouraged to use the very brief synopses above just as a starting point, and to do their own research. Reviews, and particularly those which include measurements, can be helpful in making comparisons among subwoofers from different companies. Where those are not available, it may be possible to do extrapolations that provide some basis for comparison. And, other forum members can usually help with those output extrapolations.

Subwoofer owner's threads can also be very helpful in defining levels of customer satisfaction and customer service. Most owners are more than glad to help prospective buyers. In fact, many owners are inclined to say: "The water is fine. Come on in." But, where the satisfaction level of existing owners is high, that can be very encouraging for a prospective buyer. Regardless of the enthusiasm of any particular owner's threads, however, the fact remains that there are a lot of good subwoofers on the market, and there are plenty of satisfied owners for every brand.

I would suggest that prospective buyers attempt to approach their search in the way that this section is organized: first deciding whether they are interested in sealed or ported; as part of that also trying to determine how powerful their new sub (or subs) should be and doing output comparisons--paying particular attention to where in the frequency range they want to have the SPL; and then moving-on to specific features and aesthetics. Of course, it is also understandable if features and aesthetics happen to rank high in a prospective owner's mind. Particularly in a mixed-use room, or in a living room, the subwoofers can sometimes be important pieces of furniture.

Obviously, budget will be a factor at all times. But, in my experience, shopping for new subwoofers is a little like shopping for a new car. We may start with a budget in mind, and yet end-up in a very different place. That's just something to expect and to prepare for. If you really need to hold the line, then hold it. But, it will take some real self-discipline, and you will probably need to avoid as many subwoofer threads as possible.

Finally, don't be afraid to trust your own instincts (or judgment) in what you like. In my opinion, you probably won't really be able to discover what works for you, from a performance standpoint, until you can get something in your room and try it out. As a general rule, a number of different subwoofers, with similar output characteristics, would probably perform in a very similar fashion. For most people, the differences in sound signature or sound quality, among comparable subwoofers, would probably be relatively modest. And, several different subwoofer makes and models might be equally satisfactory to a prospective buyer.

If you did happen to try a particular subwoofer and weren't really satisfied, the very worst that would happen is that you might need to send that one back and try again with a different make or model. That is, however, a good reason to start with performance characteristics, when you are comparing subwoofers, so that you will at least have a general idea of the amount of output you will be getting, and whether that output will align with your own preferences for overall volume, or for mid-bass or low-bass. Then, if the subwoofer you selected has enough inherent performance capability to satisfy you, the only question will be the extent to which you like its overall sound quality, appearance, and features. And, as noted earlier, most people are usually satisfied with their subwoofer choices in those respects.

It is worth noting again, that sound quality differences among subwoofers are usually fairly subtle, and that our hearing is not terribly acute at subwoofer frequencies. Many people might have some difficulty even distinguishing among different subwoofers in a blind listening test. So, for most people, any really good subwoofer will probably be satisfactory from a sound quality standpoint, and once we have defined our performance requirements, other factors such as appearance, features, customer service, and cost, may be more helpful in informing our final buying decisions. It is also worth reiterating, that many of us on AVS seem to be in a perpetual process of upgrading subwoofers. Starting with the largest subwoofer we can afford, and then adding multiples as required, is probably the best way to avoid that.

But, too much research on too many different threads can also lead to indecision, as we become overwhelmed by the sheer number of choices. I have definitely seen paralysis by analysis go on for months before someone finally pulled the trigger. And, those individuals may not have been any more sure, when they finally tried a subwoofer, than they would have been months earlier. I would say to take all the time you need, but to not be afraid to take the plunge when you are ready to actually try something, because trying a subwoofer at specific locations, in your specific room, is ultimately the only way to discover whether it will satisfy your personal performance requirements.



VIII-E: Subwoofer Placement in a Room:

I decided to put a brief explanation of subwoofer placement in this section. Readers may be able to find other articles, elsewhere, which expand on the subject, but this should cover some of the basics. Subwoofer placement is highly dependent on the specific room, and the relationship to the main listening position (MLP). As with all of the subjects covered in the Guide, I don't consider myself to be an expert. But, perhaps I can distill some of what I have learned from others, including some who are experts on the subject.

First, where low-frequencies are concerned, the room plays a dominant role in determining bass performance. That point was made in the earlier section on Room Gain. Placement of subwoofers within a room can positively or negatively impact overall bass performance. The temptation to put subwoofers where they look most symmetrical, and where they will be aesthetically pleasing, is a natural one. Everyone has to make his own decision about how much he is willing to compromise form in order to achieve function, and vice-versa.

Having at least one subwoofer at the front of the room can make sense from a functional standpoint, if subwoofer localization occurs with the sub in other locations. The great majority of the content in movies, and in music, comes from the front soundstage. According to what I have read, the center channel is carrying content in movies for more than 80% of the time. And, it is often carrying the most meaningful content. Other speakers, and particularly the front speakers are supporting that content produced by the CC. But, the CC is virtually always in operation, and when it's not carrying as much content, the front speakers almost always are.

So, under most circumstances, a subwoofer located somewhere on the front wall will prevent localization of bass sounds. Although some bass sounds and special effects will come from the surround channels at the same time that they come from the front soundstage, much of the low-bass in movies will be occurring on screen. And, with a subwoofer located in the front of the room, the bass sounds, and tactile sensations, for much of the content will simply seem to be coming from the place they should be--the front soundstage where the screen is located.

On the other hand, where 80Hz and higher crossovers are employed, localization of bass sounds may not be an issue to begin with, as bass may simply seem to come from everywhere. Low-bass tactile sensations, however, won't be dependent on crossover settings, so potential localization of tactile sensations is another variable which may need to be considered. As with so many things in audio, there is a trial-and-error component to localization, which is highly dependent on both the room and the individual. I believe, for instance, that bass localization is much more likely in large rooms where people are seated at a greater distance from a single bass source, or where a single bass source is to one side of a room.

In understanding this issue, it may be helpful to remember that the subwoofers are playing all of the bass content in the regular channels below the crossover point, and they are also playing the entire LFE content in a movie (which goes up to 120Hz with a default LPF of LFE setting). I have sometimes wondered if the LFE channel could be a potential source of localization for some people. We usually think of localization of bass sounds only in the context of our 80Hz or higher crossovers. But, the LFE channel may be playing up to 120Hz, at a volume level 10db higher than the bass in the regular channels. In any event, I believe that it can sometimes be important to have at least one subwoofer located somewhere in the front of the room.

Generally speaking, most people probably can't localize the direction that bass sounds are coming from below about 80Hz. And, for some of us, the threshold may be even higher. But, this depends somewhat on the individual, as does sensitivity to TR, so depending on the room, the crossovers used, on the LPF of LFE employed, and on the specific user sensitivity to bass localization, some degree of experimentation may be required.


Doing a Sub Crawl:

The best way to pick the optimum room placement for a subwoofer is probably by means of a sub crawl. The way that the sub crawl is typically described is that a subwoofer is placed in the listening chair, while some steady bass content is played. And, the listener crawls around to different locations in the room while keeping his head at about subwoofer height. That puts the subwoofer where the listener would be, and allows the listener to be where the sub would be. Doing that maintains the same acoustical relationship between subwoofer and listening position. Wherever the bass sound is loudest and clearest (either by ear or by measurement) is the best position for the sub.

The advantage of doing it this way, is that the listener only has to move the subwoofer once--to his chair, in order to find the best position to permanently locate the sub. Lifting a large, heavy subwoofer onto a nice chair or sofa, however, may not be something that everyone wants to do. (It would be alright to move the chair or sofa out of the way, putting the subwoofer on top of something else in that same position. For bass frequencies, there would be no difference at all whether the sub was on the listening chair or on top of something else.) There is an excellent video which shows how to perform a sub crawl:

https://www.youtube.com/watch?time_c...&v=R4o13mNmlaA

It is also possible to move the subwoofer, with furniture sliders, to some likely positions while coming back to the MLP to listen each time, or relying on someone else to move the subwoofer while the listener remains in the chair. The same increase in volume, without excessive boominess, would be the goal. The first method is quicker, but either method can work. If you do decide to move the subwoofer(s) around to test placement options, some furniture sliders will be very helpful. They come in a variety of sizes and shapes for carpet, for hardwood floors, and for both (with interchangeable covers). Something like this would be available on-line, or in stores such as Home Depot:

https://www.amazon.com/SuperSliders-...ardwood+floors


Single Subwoofer Placement:

What are some likely positions for a subwoofer? A corner would be one. Corners provide the most boundary gain, which will make the subwoofer play louder. Corners also allow the subwoofer to interact with multiple room modes, which may yield a better frequency response. But, corner locations may also generate may also generate more audible distortion, which some users characterize as a boomy sound. So, we definitely want to listen for increases in volume, but we also want to be objective about the quality of the sound we are hearing, if it starts to sound boomy. The ideal scenario is one in which the SPL is reinforced, but all frequencies are heard more-or-less clearly and distinctly. (When we describe bass sounds as boomy, that is typically our ears telling us that we are either hearing distortion, or that a strong peak, at some bass frequency, is dominating the other bass frequencies.)

Moving the sub partway out of the corner, or rotating it might be helpful if a corner position is attempted. Although it seems counterintuitive to turn the driver away from where we want the sound to go, the driver doesn't have to face in any particular direction, as bass frequencies radiate omnidirectionally. (I have emphasized that for emphasis--subwoofers can face in any direction.) If a subwoofer is ported, and the port is facing a wall, it is advisable to keep ports approximately one port diameter away from a wall. Let's say about 4" for a typical ported subwoofer, even if it has a slot port. Small adjustments in position can make a significant difference. Sometimes, adjusting the position or direction of a subwoofer, by only a few inches, can change the sound quantity and/or quality.

(It may be important to understand that a subwoofer's frequency response is determined by the center of the driver (the woofer itself), or if a subwoofer has two drivers, by a point midway between the two drivers. That means that a subwoofer which is 30" deep, for instance, could have a somewhat different frequency response if the driver faced out from a wall, compared to the more unusual position of the driver facing a wall. I would not expect dramatic differences in FR from changes such as that one, but it could certainly be worth attempting in some cases.)

Another likely location for a subwoofer would be at about 1/4 or 1/8 of the wall length. So, if the front wall is 16' wide, moving the sub 2' to 4' out from a side wall might be a good position. Being at the 1/2 wall position can also work, especially for side-wall placement with dual subs. But, it is strictly trial-and-error to determine what will work well in a particular room. There are some on-line calculators that can help to determine proper speaker placement, but I don't know of a reliable one for subwoofer placement. The REW measurement software includes a sub placement calculator that seems to be helpful in predicting likely locations, in some cases. But, trying various placements and listening critically (and/or measuring) are probably still the best methods we have.

If subwoofer localization is not an issue, then the best position for a sub might be on a sidewall, or at the back of a room. In that case, the same corner, or 1/4 wall, or 1/2 wall experimentation might be required. Placement of a sub in a nearfield (~ 3') or very nearfield (<18") arrangement with respect to the MLP can also be very effective. Not only does the close proximity magnify SPL (since loudness decreases with distance) but it also increases tactile sensations. That can be good or bad, depending on the individual.

Some people find that a single nearfield subwoofer can also provide very enveloping bass. Others seem to find that a single nearfield sub makes it easier to localize. YMMV! The other thing that nearfield placement does, though, is to somewhat take the room out of the equation, so that room modes have less influence on the sub's native frequency response. NF or VNF subs can be very helpful in that respect.


* I have mentioned the relationship of the subwoofer to the MLP several times. It can be very difficult to have an equal bass response throughout a room. To do so typically requires a lot of bass sources. But, what most people usually can develop is good bass within a limited listening area. It can, therefore, be important to identify where we want to have the best bass response and to concentrate on that. Generally, that will be the MLP.

It is also important to realize that where we place the MLP to begin with can have a significant impact on our bass. For instance, placing the MLP at the exact center of a square or rectangular room will almost certainly produce a null at the MLP that no amount of subwoofer positioning or room EQ can cure. In that case, moving the MLP forward or backward of that center position by even a couple of feet can frequently solve the problem.


Dual subwoofer Placement:

Once we have one subwoofer properly located with respect to the MLP, we can start to try positions for a second one. We would use essentially the same procedure that we did for the first sub, listening (or measuring) for locations where the combined sound of the two subs is the loudest, and where it blends the best. When we find the right location for the second sub, the bass may seem to come from everywhere, rather than from one general direction within the room. That is the sensation of bass envelopment that was referred to earlier.

Occasionally, a symmetrical arrangement of dual subwoofers along the front wall actually produces the best audible results. More often, placing subwoofers on opposing walls excites room modes differently, preventing cancellation (nulls) at some frequencies, and random peaks at others. Placing subs in front and rear diagonal corners is often a very effective arrangement, as is 1/4 walls on front and back walls, or on 1/2 side walls. And, the use of a rear nearfield sub can also be a very effective solution, particularly if one sub is located on the front wall. If two subs have slightly different SPL capabilities, placing the weaker sub nearfield helps to equalize their output.

It should be noted that placing subwoofers on opposing walls such as front-and-back, or side-facing walls, can allow the subwoofers to engage more room modes. Engaging more room modes will help to even-out the frequency response in a way that may not be possible if both subwoofers are on the same wall. But, subwoofers on opposing walls are often out-of-phase with each other, when the phase control on both subs is set to 0.

If subwoofers are placed on opposing walls, and the sound gets softer than it is with either of the subs playing independently, some phase adjustment will be required. Typically, some steady bass content or test tones would be played, while the phase control ( or distance control) of one subwoofer is adjusted. The phase of the sub being adjusted should be set to the position where the combined sound of the subs is strongest. Most often, it would probably end-up at 180, which would equate to reversing the polarity on that sub, although it might work best at some other setting. The other sub's phase control would remain at the default setting of 0.

As the different potential arrangements listed above suggest, experimentation is the key to finding a workable arrangement in a particular room. And, when we find subwoofer locations that please us, we can stop experimenting if we want to, rather than searching for the "perfect" placement. That is up to the individual.

We can also experiment with changing distance or phase settings on a subwoofer, once we have determined our final room placement. As noted above, that can be especially helpful where subs are on directly opposing walls. But, it may also be helpful to experiment with distance or phase settings, even with a single subwoofer, in order to obtain better integration with our center channel for HT purposes, or with our front speakers for a music-only system.

The Sub Distance Tweak, briefly explained in a subsection of Section III-C, labelled: Dealing With Cancellation is linked below. It illustrates some methods for changing phase or distance on one subwoofer in order to reduce a null. In general, phase and distance tweaks would be performed after calibrating with automated room EQ. As with everything in audio, however, informed experimentation can be the key to superior performance.

https://www.avsforum.com/forum/113-s...nces.html#IIIC


* One final thought on positioning subwoofers involves our own perceptions of where subwoofers can be located in a room. Sometimes, we need to look at our rooms from a fresh perspective, in order to see placement opportunities that we didn't know we had. This is something that I have seen other people on the forum experience, and something that I have experienced myself. We get so used to certain room arrangements that we literally can't see other alternatives.

I found that to be the case in my own large mixed-use room. Once I decided that I really needed multiple, large subwoofers to achieve the low-frequency performance I wanted, I started to look at my room from a new perspective. And, with relatively modest rearrangements I was able to position four large subwoofers just where I needed them to be. Room aesthetics were very important to me when I started, and they were very important to me when I finished. I just needed to be a little more imaginative about where and how I was positioning my subwoofers. I have seen that same scenario play out with many other people on the forum. Where there's a will, there's a way, with subwoofer placement as with almost everything.

Once single or multiple subwoofers are properly positioned, we can follow the advice in the first and second sections of the Guide to calibrate, and to tweak the subwoofers, to achieve our preferred frequency response and overall sound quality.





Regards,
Mike


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GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.

Last edited by mthomas47; 06-15-2019 at 10:35 AM.
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post #3 of 1339 Old 02-20-2018, 10:22 AM
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Looks like a good read. I will dive in after work. I keep adjusting sub settings and recalibrating Audyssey a few times a week, trying different setups.
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post #4 of 1339 Old 02-20-2018, 10:47 AM
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There is no doubt this comprehensive guide will become an excellent resource for all members of this forum. From my perspective, your contributions to the forum are invaluable.

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Great read. In the end it appears that it all boils down to the individuals expectations and preferences. Your ears are the ultimate judge.
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Quote:
Originally Posted by mthomas47 View Post
This subwoofer guide began on the Audyssey thread as a way to explain Audyssey's calibration process, and to provide guidance with respect to adding bass boosts after calibrations. But, over time, the Guide expanded to encompass discussions of Dolby/THX Reference, how we hear and feel bass frequencies, and how that relates to our bass preferences. The general principles contained in the Guide are believed to be applicable to other methods of HT calibration, and to other systems of automated room EQ.
Thanks for taking the time to do this. Not sure if it's possible but can the section titles link to the appropriate section to help minimize scrolling or can you only link to a single post?
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Originally Posted by gene4ht View Post
There is no doubt this comprehensive guide will become a an excellent resource for all members of this forum. From my perspective, your contributions to the forum are invaluable.
Couldn't agree more. Mike was a major contributor to myself learning and understanding Audyssey a while back. His consistency of non bias helpful posts have made major contribution to this forum.

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^^^

Thank you all very much! I really hope that it will be helpful. If things continue as they have in the past, I will continue to add detail when something interesting comes along.

It probably is possible to create hyperlinks that will enable internal links to different portions of the Guide. Keith Barnes did that with the Audyssey FAQ. But, I think I'm barely computer literate enough to create posts. Edited to add, that while I didn't have a clue, a helpful reader did.

Regards,
Mike

GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.

Last edited by mthomas47; 02-20-2018 at 04:27 PM.
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post #9 of 1339 Old 02-20-2018, 11:11 AM
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Very awesome! Next time you're in town, I'll buy you a beer. Promise.
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Couldn't agree more. Mike was a major contributor to myself learning and understanding Audyssey a while back. His consistency of non bias helpful posts have made major contribution to this forum.
Agreed...not only does Mike willingly share the knowledge he has gained throughout the years but he does so as you suggest in a non biased, non-confrontational, positive, patient, and friendly journalistic style..very rare these days!

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post #11 of 1339 Old 02-20-2018, 11:34 AM
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Awesome write-up, especially the TR section and followup comments.

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Couldn't agree more. Mike was a major contributor to myself learning and understanding Audyssey a while back. His consistency of non bias helpful posts have made major contribution to this forum.
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Very awesome! Next time you're in town, I'll buy you a beer. Promise.
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Agreed...not only does Mike willingly share the knowledge he has gained throughout the years but he does so as you suggest in a non biased, non-confrontational, positive, patient, and friendly journalistic style..very rare these days!
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Awesome write-up, especially the TR section and followup comments.
You guys are too kind!
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GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.

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A mod needs to move this to a sticky.

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A mod needs to move this to a sticky.
Thanks! That's something that Gene had suggested a few days ago, so I created a new thread today with that in mind, and I believe it is under consideration.

GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.
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Thanks to @siuengr I am adding internal links to the Guide. He created the links for me and all I have to do is to copy and paste them. That should make it much easier to navigate through the Guide.

This Guide has always been a community effort as I draw on the knowledge and experience of many others when I write something. Now, if it does become a sticky, we will have Gene @gene4ht to thank for that, and we will have siuengr to thank for being able to use it more easily.
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GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.
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post #16 of 1339 Old 02-20-2018, 03:49 PM
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can too much bass screw up the sound you hear from rest of system? mainly vocals?

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post #17 of 1339 Old 02-20-2018, 04:25 PM
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can too much bass screw up the sound you hear from rest of system? mainly vocals?
Depending upon how high you have the crossover set it can definitely be a problem. I'm particularly sensitive to 'chesty' voices, and running a sub hot with a high crossover is very much an issue for me.
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thanks Jim, I never ever had a problem til I watched bladerunner 2049...that movie is bassy.

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can too much bass screw up the sound you hear from rest of system? mainly vocals?
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Depending upon how high you have the crossover set it can definitely be a problem. I'm particularly sensitive to 'chesty' voices, and running a sub hot with a high crossover is very much an issue for me.
I have noticed the same thing that Jim has. I used to have a center channel which didn't do mid-bass frequencies very well. Voices just sounded a little unnatural to me. But, if I raised the crossover, then my subwoofer boost started to dominate. Ultimately, a better center was the solution for me, so that I could enjoy using an 80Hz crossover.

I also think that overall tonal balance can be compromised with too much sub boost. With some movies, I really enjoy the roller coaster ride that I get with some serious ULF. And, I am usually distracted enough by the on-screen action and special sound effects that I am not too critical about whether music sounds a little bass-heavy. With other movies, and some TV shows, a lot of bass can be a little annoying to me.

Particularly if I am just listening to music played by acoustic instruments, where I know what the music should really sound like, I find that too much bass can be very distracting and unappealing. So, I think that this is one of those issues that depends on circumstances, and on the individual listener, but I definitely don't want to have the same amount of bass for everything I listen to.

Regards,
Mike

GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.
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post #20 of 1339 Old 02-20-2018, 05:18 PM
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This subwoofer guide began on the Audyssey thread as a way to explain Audyssey's calibration process, and to provide guidance with respect to adding bass boosts after calibrations. But, over time, the Guide expanded to encompass discussions of Dolby/THX Reference, how we hear and feel bass frequencies, and how that relates to our bass preferences. The general principles contained in the Guide are believed to be applicable to other methods of HT calibration, and to other systems of automated room EQ.

This thread should be in the "Sticky"


Hopefully, a Moderator can move-it there!
A must Read! And a very good supplement to the Audyssey guide and FAQ.


It help me in my set-up, even after doing a calibration, for doing some tweaks that made my bass better sounding.
I put a link of your Guide, in many of my responses to other Members.


Ray
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With other movies, and some TV shows, a lot of bass can be a little annoying to me.

Particularly if I am just listening to music played by acoustic instruments, where I know what the music should really sound like, I find that too much bass can be very distracting and unappealing. So, I think that this is one of those issues that depends on circumstances, and on the individual listener, but I definitely don't want to have the same amount of bass for everything I listen to.
True dat.

One size - trim level in this case - does not fit all, at least for me anyway. It's not uncommon that I drop the subwoofer level on the AVR by as much as 2dB because the source material is just overpowering otherwise. It becomes distracting if I don't. I would prefer not to fiddle so much with the output, but sound quality and fidelity is the first priority so I find myself having to do that very thing with regularity.
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post #22 of 1339 Old 02-20-2018, 06:03 PM
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Amazing article! Very helpful. Thanks for taking the time to do this.
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This thread should be in the "Sticky"


Hopefully, a Moderator can move-it there!
A must Read! And a very good supplement to the Audyssey guide and FAQ.


It help me in my set-up, even after doing a calibration, for doing some tweaks that made my bass better sounding.
I put a link of your Guide, in many of my responses to other Members.


Ray
Thank you very much, Ray! I hope that will happen.

I am pleased if you and others want to link the Guide for others to refer to. That should be a little easier to do now that it's a separate thread with its own title. This is also the place where I will be making any future additions, or clarifications in the Guide. I won't edit the original post in the Audyssey thread again.

Regards,
Mike
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GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.
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post #24 of 1339 Old 02-21-2018, 06:36 AM
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Thank you very much, Ray! I hope that will happen.

I am pleased if you and others want to link the Guide for others to refer to. That should be a little easier to do now that it's a separate thread with its own title. This is also the place where I will be making any future additions, or clarifications in the Guide. I won't edit the original post in the Audyssey thread again.

Regards,
Mike

Thanks for the heads-up.
Already add this thread in my favorite, for easy reference


Ray
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After reading through some of the calibration info and refreshing my memory i decided to purchase the mic stand as it seems like stacking couch cushions to get all eight audyssey positions is the WRONG way lol. I really didn't think it would have made much difference but hey for $40 its worth a try and if post calibration is better then it will be a cheap investment for improved results. Without measuring equipment hopefully my old damaged ears will hear the difference.

Also just recently i started using all eight mic positions vs just three before. I noticed better overall sound/detail/and separation, but also noticed the overall volume is not as loud including bass. L/R set at -3/C at -5/ side surrounds -3,-5/rear surrounds -5,-4/ subs -6 at 6db's hot. I'm finding i need about two extra DB's on the MV to equal the old setting's volume.

It's quite amazing how minor mic position adjustments can change the overall sound. There is so much good info to read and absorb.

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Hi Mike @mthomas47 ,

An incredible body of work here Sir. This is now the "Reference" language of Guides of any kind here on AVS. You are to be Commended for your work Authoring, Researching, Editing and Organizing this massive, complex, and yet easy to understand Sub Guide!

So many will benefit from this work! Your writing skills are strikingly impressive and unmatched. You demonstrate a "selflessness" that I can only strive to emulate, but never duplicate.

Major Kudos Mike!!!!

Mods: Count me as another Vote for Sticky Worthiness.

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Hi Mike @mthomas47 ,

An incredible body of work here Sir. This is now the "Reference" language of Guides of any kind here on AVS. You are to be Commended for your work Authoring, Researching, Editing and Organizing this massive, complex, and yet easy to understand Sub Guide!

So many will benefit from this work! Your writing skills are strikingly impressive and unmatched. You demonstrate a "selflessness" that I can only strive to emulate, but never duplicate.

Major Kudos Mike!!!!

Mods: Count me as another Vote for Sticky Worthiness.
Wow, Adam! Thank you very much! I don't know that I deserve all of that, but I'm not going to ask you to delete it.

Seriously, its nice to have a friend offer me that kind of support, and I appreciate it.


* Something that I want to say to everybody is that I did start the thread primarily so that a moderator could make it into a sticky if he wants to. But, after writing my first post and then copying the Guide into the second post, I went back and edited the first one. It occurred to me that we have a perfectly good thread here where people can talk about any calibration, room EQ, subwoofer, bass-related subjects they like. And, it may be nice to do that sometimes away from owner's threads. So, I hope that anyone will feel comfortable having conversations with, and getting help from, all of us on this thread.

Regards,
Mike

GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.
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post #28 of 1339 Old 02-22-2018, 01:55 PM
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Wow, Adam! Thank you very much! I don't know that I deserve all of that, but I'm not going to ask you to delete it.

Seriously, its nice to have a friend offer me that kind of support, and I appreciate it.


* Something that I want to say to everybody is that I did start the thread primarily so that a moderator could make it into a sticky if he wants to. But, after writing my first post and then copying the Guide into the second post, I went back and edited the first one. It occurred to me that we have a perfectly good thread here where people can talk about any calibration, room EQ, subwoofer, bass-related subjects they like. And, it may be nice to do that sometimes away from owner's threads. So, I hope that anyone will feel comfortable having conversations with, and getting help from, all of us on this thread.

Regards,
Mike
In the many years I've been posting in these threads, there have always been the occasional topic meanderings to other related topics. I personally do not find it uncomfortable/objectionable and find that it more often than not contributes to better overall understanding. Moreover, your concern for others is exactly why so many kudos are coming your way!

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post #29 of 1339 Old 02-22-2018, 02:37 PM - Thread Starter
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In the many years I've been posting in these threads, there have always been the occasional topic meanderings to other related topics. I personally do not find it uncomfortable/objectionable and find that it more often than not contributes to better overall understanding. Moreover, your concern for others is exactly why so many kudos are coming your way!
Thank you, Gene!

I agree with you about discussions just going where they go, and I like that too. We all learn from that, in my opinion. Hopefully, people will find this a comfortable place to have some of those discussions, away from the conflicts, or excessive enthusiasm, that sometimes manifest on various types of owner's threads.
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GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES

* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.
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post #30 of 1339 Old 02-22-2018, 04:33 PM
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Quote:
Originally Posted by Adamg (Ret-Navy) View Post
Hi Mike @mthomas47,

An incredible body of work here Sir. This is now the "Reference" language of Guides of any kind here on AVS. You are to be Commended for your work Authoring, Researching, Editing and Organizing this massive, complex, and yet easy to understand Sub Guide!

So many will benefit from this work! Your writing skills are strikingly impressive and unmatched. You demonstrate a "selflessness" that I can only strive to emulate, but never duplicate.

Major Kudos Mike!!!!

Mods: Count me as another Vote for Sticky Worthiness.

And Kudo to You Sir, for such a nice reply!

Quote:
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Wow, Adam! Thank you very much! I don't know that I deserve all of that, but I'm not going to ask you to delete it.

Seriously, its nice to have a friend offer me that kind of support, and I appreciate it.


* Something that I want to say to everybody is that I did start the thread primarily so that a moderator could make it into a sticky if he wants to. But, after writing my first post and then copying the Guide into the second post, I went back and edited the first one. It occurred to me that we have a perfectly good thread here where people can talk about any calibration, room EQ, subwoofer, bass-related subjects they like. And, it may be nice to do that sometimes away from owner's threads. So, I hope that anyone will feel comfortable having conversations with, and getting help from, all of us on this thread.

Regards,
Mike

First, yes you do, a tremendous amount of work and information's you have provided

Quote:
Originally Posted by gene4ht View Post
In the many years I've been posting in these threads, there have always been the occasional topic meanderings to other related topics. I personally do not find it uncomfortable/objectionable and find that it more often than not contributes to better overall understanding. Moreover, your concern for others is exactly why so many kudos are coming your way!
Exactly


Quote:
Originally Posted by mthomas47 View Post
Thank you, Gene!

I agree with you about discussions just going where they go, and I like that too. We all learn from that, in my opinion. Hopefully, people will find this a comfortable place to have some of those discussions, away from the conflicts, or excessive enthusiasm, that sometimes manifest on various types of owner's threads.

I think it started already
Understanding all pieces of the puzzle, on how they can react with each other, can help novice or someone with lots of experience, and you did in my case by changing one little setting!


Ray

Last edited by darthray; 02-22-2018 at 04:42 PM.
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