[catapult]

In this thread , Mark Seaton offered to get into some serious talk about how how to create good bass in a room if somebody would start a new thread. So, here ya go big guy.


Let's start a good free-for-all about how to get the ultimate bass. Anything goes.... room size, room shape, sub type, sub placement, seating position, acoustic treatments, equalization, measurement techniques, whatever.


Myself, I'm interested in the optimum placement of multiple subs. The experts don't all agree. Some say to put them all in a corner and some say to spread them out at, for example, the 1/4 and 1/2 points along the walls.

 

[DarrenW]

Hi Catapult,


I am no expert in audio, however I will take a stab at this soley based upon personal experience without the technical education that some posess in this forum.


Previous Setup

I have battled with this sub issue for about a year now. I have 2 15" powered subs. My room dimensions are 13.5' x 26.5'. I know, not exactly ideal for acoustics. I tried the 2/3 theory, one at 1/3 and the other at 2/3 on opposing walls. For myself, I felt the subeffect was very cancelled. I experimented with turning the subs firing direction in all 4 ways. I was still unsatisfied with the results.


Current Setup

After a lot of experimentation here is where I ended up. I have one sub front firing located below my screen beside my center channel speaker along the 13.5' wall. The other sub is still at the 2/3 dimension just behind the last row of seating firing across the room.


Conclusion:

I am much happier with the current placement of the subs. However, I personally think that the setup of the crossover and volume was the key to the sucess of the current arrangement. I spent hours upon hours testing different crossovers with volume levels. I concluded with a lower volume level than used previously (approx. 25-30%of total volume) along with a crossover point at 60hz, phase at 0.


Once again, these are just my personall thoughts that have no technical support to back them up. I would like to hear a response from some of you technical "Guru's" of audio on my setup.

 

[TV]

I respect the inquire...but the anything goes/no cost issue scenarios just leave such huge gapping holes in any real world situation...it's too time consuming to debate about it. If 10 of us were sitting around a bar one night...it might be fun though


TV

 

[cpc]

I only have one subwoofer, but I would recommend those who have 2 subwoofers to try and locate the subwoofers together if at all possible. I have noticed that in my small room, corner placement sounds poor. The farther from the wall, the better, within reason. About 1/3 to 2/3 along the wall is good.


I have a question. If too many subwoofers in different locations is difficult to tune and tweak, and offers too many standing waves and cancelations, then what about if your mains go to 32hz and you have a subwoofer? Isn't that like having 3 subwoofers in terms of frequencies between 30 hz and 100 or so? When using LARGE and SUB=YES, is this possibly where interference can occur? What about when using SMALL and SUB=YES?


I imagine a good equalizer could really help to tame bass frequencies in most rooms. Could an equalizer be used on the subwoofer pre-out cable between the receiver and subwoofer?

 

[Mark Seaton]

Oh boy, we'll see what I've set myself up for here.

Quote:

Originally posted by cpc I have a question. If too many subwoofers in different locations is difficult to tune and tweak, and offers too many standing waves and cancelations, then what about if your mains go to 32hz and you have a subwoofer? Isn't that like having 3 subwoofers in terms of frequencies between 30 hz and 100 or so? When using LARGE and SUB=YES, is this possibly where interference can occur? What about when using SMALL and SUB=YES?

There is a thread somewhere back in this forum where Brian Florian in particular, and myself included went into depth on this issue of bass management. This is a flat out no-no if you have components properly sized and capable for your room and desires. Combining "LARGE" speaker settings and subwoofers which overlap them is taking up a myriad of compromises which are for another thread.

Quote:

I imagine a good equalizer could really help to tame bass frequencies in most rooms. Could an equalizer be used on the subwoofer pre-out cable between the receiver and subwoofer?

Indeed, PROPER use of a parametric EQ can be quite helpful to improve a system when physical changes can no longer be made to improve the response at the seating locations. Of course we do want to realize that we are really just depriving a resonant system of energy, not entirely solving the problem. Fortunately the manner in which we hear and resonances occur allow this to be a reasonably good sounding solution. Severe room modes are much like when someone tells the doctor "it hurts when I do this"... Of course the doctor's response? "Don't do that!" Our EQ behaves in much the same manner where it is able to greatly reduce the offending input, yet is not directly fixing the problem. This issue in particular is one which would be great for someone to bring up to Greg Miller in the Special Guest Forum this week. Greg is much more the master of this topic.

 

[Mark Seaton]

I'm quoting this because members really need to understand the reality of what we want to discuss.

Quote:

Originally posted by TV I respect the inquire...but the anything goes/no cost issue scenarios just leave such huge gapping holes in any real world situation...it's too time consuming to debate about it. If 10 of us were sitting around a bar one night...it might be fun though TV

I can't even fathom the wild ideas our designer Tom Danley would throw out in such a dream scenario... Realistically though, the question becomes a matter of what can be presently realized, and for those in the position to do so, to realize new solutions. Presently subwoofers all are of limited surface area, typically using a driven cone, yet the real issue is what comes out of the box, not really how the output is created.


The primary thing you will see me focus on is the application. Not many rooms can easily accomidate a 22" x 18" x 37" ContraBass or two , even one would need serious EQ in a very small room, and some rooms would fall apart if it was ever used to it's fullest capability . That said, once we define a space we have to work in, then we can begin to lock in seating locations/areas and then work on from there based on personal desires, placement possibilities, budget, etc.


There are most certainly different methods which can be followed, each with their own benefits and detriments. For example, in some cases, the benefits of corner placement combined with some other tools is a more attractive option than placing multiple subs in the room, yet multiple subs and other tricks can certainly offer their own significant benefits.

 

[catapult]

Okay, here's my fantasy, not that I'd ever get permission to do it. Please don't tell anyone I mentioned this. I can trust you guys, right?


I figure, for the price of a popular major-brand box sub, I could turn my whole ceiling into a DIY infinite baffle subwoofer. A truckload of Shivas or Tempests, mounted about 4' on center, with a mild Linkwitz eq., should do nicely. Call it a plane array, kinda like a 2-D line array, where all the drivers couple and launch a uniform wave front. The goal isn't thunderously loud bass, although that would be a nice side benefit. Rather, I'd want to be able to sit anywhere in the room and hear/feel ruler-flat bass, down to at least 10 Hz, at reference levels, with no peaks or nulls anywhere. I did some napkin calcs and the amplifier power required is surprisingly small. That many drivers, in a series/parallel arrangement, send the efficiency of the system off the chart and any good pro amp should be able to handle it. The sound should be extremely clean as no individual driver would be handling more than a few watts.


Hey, who are those men in the white coats?

 

[Bean]



LOL. You'd prob have to get a license to operate that sucker.



speakers set to small sub=yes and one subwoofer;

means the sub gets all the input from 5 channels in addition to its LFE duties. seems like its asking a lot for one driver to handle.

Can it handle it well, what are the compromises?


It just seems counter to the thinking of multichannel where its preferred to add more channels and therefore more speakers. Does it not apply to bass because of its nondirectional hz or something?

 

[Delicious2]

Quote:

There are most certainly different characteristics to different size rooms and the nature in which bass frequencies are augmented or otherwise. That said, the notion of a room "not being large enough" to support deep bass is pure non-sense. I'll gladly voulanteer my car to show that a 15Hz tone can be reproduced quite authoritatively within a small space. Different considerations certainly come into an ideal design for a large vs. a small space, but in fact, it is EASIER to get deep bass from a small space than a large one, yet it is easier to get flat response in larger rooms. Flat response at the listening position is the most common culprit of such adverse descriptions. Careful placement and/or EQ can certainly tame a sub within a small room to sound quite good.

'Guess I'm still stuck on more basic questions and not yet ready or experienced enough to intelligently discuss trying to achieve excellent low bass in normal listening environs. If it's "pure nonsense" to suggest that a room is not large enough to support deep bass, what about the idea that different size rooms have differing abilities to support full size low bass waves? What may be a given for you experienced bass freaks is new to me. What I am wondering is, do bass frequencies behave differently in rooms not large enough to accomodate their full length than they do in larger room? Is it oversimplifying to say that small rooms produce a different type (and perhaps lesser quality) of low bass than larger rooms because the more they are pressurized with wavelengths larger than their dimensions, the more they behave like the inside of a loudspeaker rather than like the concert hall?


Mark H

 

[cpc]

Quote:

Originally posted by Bean LOL. You'd prob have to get a license to operate that sucker. speakers set to small sub=yes and one subwoofer; means the sub gets all the input from 5 channels in addition to its LFE duties. seems like its asking a lot for one driver to handle. Can it handle it well, what are the compromises? It just seems counter to the thinking of multichannel where its preferred to add more channels and therefore more speakers. Does it not apply to bass because of its nondirectional hz or something?

Well, the using of multi-channel bass doesn't really fly with me because its like having a bunch of subwoofers and that doesn't seem to work unless the subs are in the same place. Then again, what about using all your home theater speakers set to large if you have no sub? Surely if your speakers are like mine and the main front speakers go to 32hz, center goes to 40hz and your surrounds go to 49 hz, isn't that like having a bunch of subs overlapping down to roughly 40 to 45hz? Surely not below 40-45 hz or so, but will there be problems from 45 to 100hz?


Maybe having too many subwoofers should prove uneven in terms of bass response because wavelengths of 40 hz and below are the ones with lots of cancellations in a multiple speaker/driver situation?


As far as summing 5 channels of bass, my subwoofer can handle lots of bass in terms of depth (sub is flat to 18 hz) and its loud. My subwoofer has 2 12" drivers in a bipolar cabinet design. But if you mean whether the subwoofer can handle all that combined bass, then I see your point, but in the room, ultimately they all combine, and that is where it has to work. I think summing it into mono would result in less phase cancelations that would happen with 5 6 or 7 subs in a room. But then again, my surrounds go to 49hz and my center goes to 40 hz......More on that later.........


My question is about a few things I have touched on. Incidentally, for whatever reason, I tried SUB=YES and FRONT=LARGE and it didn't sound poor. I think when that setup is used, the bass material is split in terms of loudness, between the sub and mains, even though they overlap in frequency. It doesn't sound "bad" though. I'll have to experiment more to see. I did use the SUB=YES and FRONT=SMALL and of course, no complaints there. My subwoofer sounds best when not in the corner. When the subwoofer is in the corner (small 14 x 12 foot room) it sounds like the subwoofer is only capable of reproducing one boomy frequency. I guess thats standing waves or room nodes peaks for you.


But this is ultimately what I would like to know:


Many people say stereo bass is the best. Some people say more than one sub, a sub for each channel, for instance, is best, while others say that you should only use one subwoofer and if you have more than one, then they should be placed as close together as possible. Most subwoofers use a combined (summed) mono bass signal. So, my question is this; If some people insist on "stereo" bass being ideal, and some people say up to 5 subs located apart from one another, next to each mid-high frequency speaker is ideal while others say one subwoofer is best, what is there to say about the bass signal in the room? What I mean is, ultimately, if you use one subwoofer that uses a summed mono bass signal, it is combining the bass electronically. Since bass

 

[Jack Gilvey]

Quote:

I figure, for the price of a popular major-brand box sub, I could turn my whole ceiling into a DIY infinite baffle subwoofer. A truckload of Shivas or Tempests, mounted about 4' on center, with a mild Linkwitz eq., should do nicely.

That sounds like fun. I think "Ultimate" will always come down to IB subs, and having the space to enclose them.


I just took delivery of a Blueprint 1803 . In 14ft.^3 tuned to 18Hz with a 10" port and 1600 watts or so, it should outperform multiples of just about any commercial sub. I'm thinking 3-4 Ariel SW12s, for example. Should be well above true reference levels in-room down to Fb without breaking a sweat. That'll have to be ultimate enough for me at the moment for the ol' HT.

 

[Mark Seaton]

Quote:

Originally posted by Jack G That sounds like fun. I think "Ultimate" will always come down to IB subs, and having the space to enclose them. In 14ft.^3 tuned to 18Hz with a 10" port and 1600 watts or so, it should outperform multiples of just about any commercial sub.

That should be quite an impressive sub Jack! Of course I would emphasize the words "just about" with respect to commercial subs, as I can think of 2 off the top of my head .


I believe we've seen that actual quantitative reproduction of low frequencies is quite realizable through various avenues, yet what we really need to address is how to correlate the sub's reproduction to what we get at a given listening position anywhere other than outside. In my next post I'll start to address the questions/issues brought up by Mark H. and cpc. Also realize I don't pretend to know all the answers or the best ones, but the first step toward that understanding is a realization of certain aspects of how sound reacts in a room.

 

[Mark Seaton]

Quote:

Originally posted by Delicious2 'Guess I'm still stuck on more basic questions and not yet ready or experienced enough to intelligently discuss trying to achieve excellent low bass in normal listening environs. If it's "pure nonsense" to suggest that a room is not large enough to support deep bass, what about the idea that different size rooms have differing abilities to support full size low bass waves? What may be a given for you experienced bass freaks is new to me. What I am wondering is, do bass frequencies behave differently in rooms not large enough to accomodate their full length than they do in larger room? Is it oversimplifying to say that small rooms produce a different type (and perhaps lesser quality) of low bass than larger rooms because the more they are pressurized with wavelengths larger than their dimensions, the more they behave like the inside of a loudspeaker rather than like the concert hall? Mark H

Hi Mark,


Thanks for getting right to the topic I was most interested in discussing. Achieving excellent bass in a listening room has more to do with understanding how sound is reacting in the space than just talking about monster sized subwoofers. There are plenty of programs which can be quickly understood to aid someone to design a monster output subwoofer.... but what happens when you put it in your room?


Now, there are certainly many out there with a deeper understanding of the real situation than I, yet the discussions here never get to that point because many levels of confusion regarding what you call "basic questions" are wrongly presumed.


Addressing the confusion about a room's ability to "support full size bass waves" might have been a confusion with a statement I made where small rooms will in fact augment deep bass differently than large rooms. The word "support" will probably bring too many misleading connotations. What we need to identify first is the relation of wavelength to reproduction of sound.


Wavelength of any sound produced is a real value, but really only charaterizes a specific frequency's reproduction in some medium, in this case, air at whatever atmospheric conditions. Now here is the question to answer: Do we actualy hear or percieve wavelength?


NO, our ear detects fluctuations in PRESSURE over time. The mechanism works the same if we are under water or if we are in the thin air of the mountains, yet in both cases, the wavelength of a given frequency is quite different. A microphone works in the same manner, yet in this case it generates an electrical signal related to those pressure changes over time. The output of a microphone only observes the pressure changes at it's specific location. Other sounds or fluctuations in pressure in whatever proximity to the microphone will have no effect if they do not contribute to the pressure changes at the microphone. A good analogy would be to consider a front projection system. Now consider a laser pointer aimed at a right angle to the projector, firing it's laser beam through the pathway of the projector's output to the screen. Presuming the laser beam doesn't reflect back onto the screen, the beam interfeiring with the pathway of the projector does not affect the light observed on the screen. Of course if a light measurement is taken within the laser beam's path you will see the combination of the projector's output AND the laser beam.


Getting back to observing sound, similarly, we are only concerned with the sound and how it combines at the listening position. Of course sound is a function of time, so we are also concerned with WHEN those pressure fluctuations occur. The quantity of wavelength only aids us in predicting the behavior of the moving pressure fluctuations as they encounter boundaries and other non-linearities.


If we had to develop full "wavelengths" to hear sound, heaphones wouldn't be able to even pass voice!


More later...

 

[Jack Gilvey]

Quote:

Of course I would emphasize the words "just about" with respect to commercial subs, as I can think of 2 off the top of my head

Yeah, so can I, hence the inclusion of "just about". I should have added "without taking out a second mortgage".

Quote:

I believe we've seen that actual quantitative reproduction of low frequencies is quite realizable through various avenues, yet what we really need to address is how to correlate the sub's reproduction to what we get at a given listening position anywhere other than outside.

Absolutely. I look forward, as always, to your thoughts.

In a sense, I suppose "drive-in" home theaters (DIHT) would be much more predictable, as ground-plane/anechoic results could be relied upon for real in-use predictions. You'd need a big backyard, though.

 

[Iceman]

Mark Seaton,


Good to see one of the most widely spread myths about bass reproduction exposed. The idea that small rooms cannot "support" low frequencies is just plain wrong. In fact, the opposite is true, just as Mark S noted: small rooms are more efficient in providing low frequency gain than large rooms. I believe the reason for the confusion lies in the fact that the lowest eigenfrequency (resonance frequency) of a room is lower in a large room.


To understand what is going on requires a bit of acoustic background. When the acoustic wavelength is of the same order as the room dimensions, modal or resonance patterns are produced, leading to some frequencies being reinforced and some being attenuated. Since acoustic wavelength is inversely proportional to frequency, the larger the room, the lower the first eigenfrequency. So far so good and there is usually nothing controversial here.


But what happens when the wavelength is significantly longer than the largest dimension of the room? The room enters the so called pressure mode and the room gain asymptotically approaches a whopping +12 dB per each halving of frequency! This is because the room now acts like a gigantic air spring attached to the speaker driver in one end and our ears in the other. The acoustic coupling naturally becomes very efficient. Put differently, the room starts behaving like the inside of a closed box loudspeaker. (This is also the reason why the final roll-off of a closed box speaker is 12 dB/oct.)


Now, what wavelength is "significantly" longer than the room dimensions? The oblique modes have the longest wavelengths and hit a maximum of 2*sqrt(L^2+B^2+H^2) where L,B,H are the room dimensions. When the wavelength of the sound waves are longer than this, the room has started to enter the pressure mode. For a room with dimensions 5*4*2,5 m (17*13*8 ft), this frequency lies around 25 Hz, meaning that the entire true sub-octave (

Since a larger room will have a lower fundamental resonance frequency, there will, on average, be more resonances or modes in a given frequency band than for a smaller room. The so called modal density is higher. This means that the room response will be more even for the larger room due to the individual modes overlapping each other to a higher extent. The acoustical reason for recommending large rooms for good low frequency reproduction stems from this fact rather than the false assumption that small rooms cannot support low frequencies.

 

[Mark Seaton]

Quote:

Originally posted by Iceman Since a larger room will have a lower fundamental resonance frequency, there will, on average, be more resonances or modes in a given frequency band than for a smaller room. The so called modal density is higher. This means that the room response will be more even for the larger room due to the individual modes overlapping each other to a higher extent. The acoustical reason for recommending large rooms for good low frequency reproduction stems from this fact rather than the false assumption that small rooms cannot support low frequencies.

Iceman,


Good to see someone else jumping in. I'm in agreement with all of what your post covers above. For those following along, now remember my point about how wavelength is and isn't significant. The given wavelength determines how bass behaves when it encounters various boundaries, not how we percieve it. In the above example of larger rooms, it is easy to see where corner placement can be useful as it obviously aids in output, but as the dimensions of the room get longer, the fundamental modes of each dimension get lower. Since each mode then has harmonics which are multiples of the original, we see that lower fundamentals will yeild more closely spaced harmonics. As the room gets smaller, those harmonics can leave more significant holes due to their spacing, and if they are related mathmatically, will combine to form large peaks.


Determining what you have to work with then allows us to see the benefits of different solutions.

 

[cpc]

Cool. I'm starting to understand bass in rooms now. Now I understand why my speakers that are rated to 32 hz anechoic (or even my old ones rated to 45hz) are able to reproduce a 20 hz tone in my room of 14 L x 16 W x 7 feet high. Also, If you brought your stereo system/home theater, subwoofers and all outside in your back yard, the bass may not be as good.


Rate the following in order of the smoothest and most accurate bass response (most accurate meaning if its smooth but too low in dB's then thats not accurate and not a good result then is it?):


1) STEREO BASS. Stereo speakers that go to 30-35 hz anechoic. This is how a large amount of music listening is done, and yet, I can't help but notice that its like having 2 subwoofers at 2 different locations in the room and that is often not advisable due to the various room nodes and resonances etc.

2) MONO BASS. Stereo Speakers crossed over @ 80-100hz or so to a subwoofer that goes to 16 hz anechoic.

3) Same as #2 but with overlap in frequency reproduction, as in FRONT=LARGE and SUB=YES. I know its not good, but elaborate.

4) Same as #2 but with more than one subwoofer, say 2 to 5 subs? How many is too many? Is 2 subs together fine, but 2 or 3subs apart not ok? What about using your home theater with all speakers set to LARGE and SUB=NO. If your speakers go to 32hz for the front, 40hz for the center and 49 hz for the surrounds, isn't that like 5 subwoofers for most frequencies between 40 and 100 hz???


I'm confused by the fact that people insist that STEREO BASS exists and that it is better and more desirable than MONO BASS and yet many say that reproducing BASS at 2 different points in the same room is not desirable. Which do you believe correct or which do you prefer?

 

[Delicious2]

In the original thread I said.

Quote:

Browsing the Nov/Dec Listener mag yesterday I skimmed a comparison of the Linn Sizmik, Spendor Sub3, REL Stadium III and HSU VTF-2. The reviewer (can't remember his name) preferred the Linn with his homemade Lowther horns. I was impressed by the reviewers knowledge re: his room dimensions only easily supporting bass down to about 33Hz. He critisized the HSU for "overhang" as I recall.

Mark S. in the same message said:

Quote:

There are most certainly different characteristics to different size rooms and the nature in which bass frequencies are augmented or otherwise. That said, the notion of a room "not being large enough" to support deep bass is pure non-sense. I'll gladly voulanteer my car to show that a 15Hz tone can be reproduced quite authoritatively within a small space. Different considerations certainly come into an ideal design for a large vs. a small space, but in fact, it is EASIER to get deep bass from a small space than a large one, yet it is easier to get flat response in larger rooms. Flat response at the listening position is the most common culprit of such adverse descriptions. Careful placement and/or EQ can certainly tame a sub within a small room to sound quite good.

In this new thread, Iceman added an interesting illustration:

Quote:

But what happens when the wavelength is significantly longer than the largest dimension of the room? The room enters the so called pressure mode and the room gain asymptotically approaches a whopping +12 dB per each halving of frequency! This is because the room now acts like a gigantic air spring attached to the speaker driver in one end and our ears in the other. The acoustic coupling naturally becomes very efficient. Put differently, the room starts behaving like the inside of a closed box loudspeaker.

I never claimed or thought that a given room size could not support low bass. Lets leave that mistaken notion aside.

What I am starting to understand thanks to Mark S. and Iceman is that different size rooms have different frequencies below which they begin to go into a "pressure mode" . That is, they begin to behave like the inside of a closed box loudspeaker. And that frequencies that pressurize a room in this way are amplified not attenuated. So, as has already been said, it is actually easier to get really low sub-sonic bass in smaller rooms.

With that understood, what seems intuitive to me, however, is that the quality of that low bass in our small listening rooms is not the same as that in concert venues. I'm talking quality now, not quantity. I seems to me that you could reposition your subs wherever you want, carefully measure every bass frequency, even equalize them within +- 1db, but, the quality will never be the same as low bass in large concert hall environments because in that "pressure mode" you have to deal with the lack of fast transient response of the sub/room interaction. Low bass notes simply can't start and stop as fast in that pressure mode.

I wonder if this phenomenon is what led the Listener reviewer to critisize the HSU sub for "overhang". Lack of transient quickness is probably not a characteristic of the HSU relative to those other subs. It seems more likely to me that the HSU appeared to have this characteristic compared to the others because it interacted with the revewer's room differently in this "pressure mode".

If my speculations have any truth, it leads to complications both in reviewing and comparing subs, and new questions about how to get the best low bass in a given room. How do we control for these sub/room interactions in the "pressure mode" beyond placement and equalization? How do we control for changes in quickness, responsiveness, transient speed in a given room? How can we make our small rooms behave or sound more like concert halls when it comes to low bass?


Mark H

 

[JMC]

Mark H.,


You made a point that I think deserves repeating, especially when one is looking for a good sub:

Quote:

If my speculations have any truth, it leads to complications both in reviewing and comparing subs, and new questions about how to get the best low bass in a given room...How do we control for changes in quickness, responsiveness, transient speed in a given room?

I quote you here because I think a lot of magazines mislead buyers by claiming that "X" sub is the fastest, or that this sub has a trace of overhang while the other does not. Because of the room interactions that have been discussed in this post, I think it is extremely difficult to make those kinds of observations with authority. For example, The Perfect Vision gave the $4,200 Linn 5150 sub a solid rating, but the reviewer, Alan Taffel, found that the unit had a somewhat fat sound with a bit of overhang. While Richard Hardesty, or "Dr. Boom" as he is sometimes called, found that the Linn has absolutely no trace of overhang. And although Taffel admitted that room interactions might have caused the Linn to sound a tad slow, the final verdict of the magazine, as demonstrated by their speaker buyer's guide, was that the sub was a little fat and lacking in timbral accuracy.


Now I'm sure that both of these reviewers are probably outstanding listeners. And of course we all hear differently, but I have to think that room interactions played a major role in these two contradictory evaluations of the same sub. So as others have pointed out, it seems that the only way to truly evaluate a sub is to hear it in your room, after much experimentation with placement and calibration.


Jim

 

[Mark Seaton]

Quote:

Originally posted by Delicious2 "his room dimensions only easily supporting bass down to about 33Hz." I never claimed or thought that a given room size could not support low bass. Lets leave that mistaken notion aside.

While it is a mistaken notion, some have been lead down this path of reasoning, and I wanted to be sure the situation was understood. It looks like we've moved past this, so on we go...

Quote:

What I am starting to understand thanks to Mark S. and Iceman is that different size rooms have different frequencies below which they begin to go into a "pressure mode" . That is, they begin to behave like the inside of a closed box loudspeaker.

This is an analogy I want to make and objection or ammendment to. This ideally closed room being discussed is only like a sealed box loudspeaker in that sound pressure on one side of the driver is contained within the space. UNLIKE a closed loudspeaker, it does not alter how the subwoofer operates, which would result in a change in the impedance curve of the subwoofer. This only occurs when the space in front of the woofer, in this case the entire room, approaches the internal volume of the subwoofer. In my own car this has a slight affect, since the cabin volume is probably less than 3 times the trunk's volume. For the record, if the front and rear volume are identical, the sub will behave as an infinite baffle where the air-spring no-longer dominates the alignment.


While "pressure mode" is a reasonably accurate term, we need to recognize what is going on, and why.

Quote:

And that frequencies that pressurize a room in this way are amplified not attenuated. So, as has already been said, it is actually easier to get really low sub-sonic bass in smaller rooms.

True, yet we need to realize that the woofer is not producing any more acoustic power into this closed room. What we find is that in fact the woofer is producing the same acoustic power, but into a smaller space! Therefore, the INTENSITY is increased much in the same way a horn loudspeaker works to confine the acoustic output at higher frequencies. A horn lense works to confine the radiation of a drive unit to focus and confine it's output which thereby makes the drive unit produce a higher intensity (observed as higher SPL) within the space confined by the horn lense.


The same concept allows corner loading of a subwoofer to generally have 6dB more output above what would be measured outdoors, in a ground plane situation. A theoretical "anechoic" environment infers no boundaries to confine a loudspeaker's radiation. In mathmatical co-ordinate terms, this is considered 4Pi space, or full-space. A good explanation of this concept by John Murphy can be found here at True Audio's website, where he also offers some good graphic aids to the explanation.


Now we get back to my point about sound being pressure variations over time, and not truely wavelengths, which only serve to help us predict how these pressure waves interact and react with boundaries and obstructions. I'm guessing someone is asking why intensity increases as frequency goes lower when the room size/volume is staying the same? How is the space getting smaller? Answer:


Acoustically, apparent size of a space or object varies with frequency and is directly related to wavelength (See it is of use after all ). What we find is that when 2 sound sources, or a boundary and a sound source are within 1/4 to 1/3rd of a wavelength apart they become "acoustically coupled." What this means is that the 2 sound sources behave as one, and the boundary no longer creates reflections, but rather confines the space the sound source is working in/on. If you start to really think about the implications of this phenomenon, you will begin to get a sense of why moving your speakers around a room have such an effect on the bass response. Furthermore, you will observe that at some low frequency, all 6 boundaries of a rectangular room will be "acoustically coupled" to a subwoofer, and will therefore confine the acoustic space, which results in the very low frequency gain observed in rooms.


Tom Nousaine's own measurements follow this very closely where his 7000 cu.ft. room begins to show gains below ~16Hz. While at 1/3rd to 1/4 wavelength we get full coupling, we find that this is not an immediate, all or nothing effect. In fact, we find that the gain can be approximated to start at 1/2 wavelength of the room's longest dimension. If you calculate the longest dimension of Tom Nousaine's 7000 cu.ft. space as a cube, you get ~33 ft. which corresponds to the 1/2 wavelength of ~17Hz. Considering I know T.N.'s room is not a cube, the diagonal would be a little longer, and perfectly explains the 16Hz hinge point for gain. His previous, smaller room also follows this prediction reasonably well. We do need to realize that this is not an exact prediction as it assumes true acoustic boundaries, and other than concrete walled basements, most rooms "leak" quite a bit at low frequencies. Just ask your neighboors!


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With that understood, what seems intuitive to me, however, is that the quality of that low bass in our small listening rooms is not the same as that in concert venues. I'm talking quality now, not quantity. I seems to me that you could reposition your subs wherever you want, carefully measure every bass frequency, even equalize them within +- 1db, but, the quality will never be the same as low bass in large concert hall environments because in that "pressure mode" you have to deal with the lack of fast transient response of the sub/room interaction. Low bass notes simply can't start and stop as fast in that pressure mode.

Actually, in this "pressure mode" resonaces and modal problems don't have room to build up, so the only issue is how much energy is contained within the room, and how much leaks out. Once you measure this, and get a flat response within the space, you can get the same quality bass as is heard at an outdoor concert performance. In fact, it is above this frequency range where all the concern and headaches come in, as we now have to be concerned with what happens to the radiated energy as it reflects or is absorbed as it encounters different boundaries and obstructions in the room, and finally what percentage and WHEN the reflected energy gets back to the listener. Looking at the efforts taken by consultants like Russ Herschelman, Tony Grimani, and Keith Yates can make your head spin and eyes gloss over... all in the interest of controlling the room's affect on sound produced in the room.


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I wonder if this phenomenon is what led the Listener reviewer to critisize the HSU sub for "overhang". Lack of transient quickness is probably not a characteristic of the HSU relative to those other subs. It seems more likely to me that the HSU appeared to have this characteristic compared to the others because it interacted with the revewer's room differently in this "pressure mode". If my speculations have any truth, it leads to complications both in reviewing and comparing subs, and new questions about how to get the best low bass in a given room. How do we control for these sub/room interactions in the "pressure mode" beyond placement and equalization? How do we control for changes in quickness, responsiveness, transient speed in a given room? How can we make our small rooms behave or sound more like concert halls when it comes to low bass?

Placement and equalization are in fact our best methods to improve the bass reproduction in most rooms.


The perception of "overhang" is most commonly caused from peaks in response from either a small room or much more commonly due to the excitation of modes causing peaks in the response. Low distortion at all output levels and reasonably flat response still appear to be key to getting accurate and articulate bass reproduction. Many small subwoofers which claim very low frequency extension employ limiters which limit deep bass output to set levels, and many other "musical" subwoofers simply don't have much deep bass output. These designs skate around the problem by not energizing a room at lower frequencies, or by limiting the engergy at low frequencies with their limiters. While this does greatly reduce the chance of causing modal peaks in the response, it also completely fails to reproduce the lower octaves, and has led to the great deal of mis-information and understandable, yet incorrect conclusions which are so prevalent in the industry.