Using loudspeaker port bungs - results in non optimal frequency response? - AVS Forum
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Old 07-21-2014, 02:59 PM - Thread Starter
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Using loudspeaker port bungs - results in non optimal frequency response?

Hi,

Loudspeakers with bass reflex ports are designed to produce a flat or optimal frequency respons with the ports unplugged.

Still some loudspeakers are delivered with port bungs/plugs used to plug the ports e.g. if they are placed close to a wall, to reduce bass resonance. But will using port bungs ever result in an optimal sound? Won't it alter the frequency response in a way not originally intended, since the speakers were designed to produce optimal sound without the use of port bungs?

Would anyone please care to elaborate on this?
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Old 07-21-2014, 03:11 PM
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Many ported speakers and subwoofers are designed to work with all the ports open or some of the ports blocked with included port plugs. Plugging one or more ports will (as you said) change the frequency response of the speaker - it will almost certainly reduce the amount of bass volume but some speakers will actually have a smoother frequency response with one or more of the plugs installed making the cabinet closer to a non-ported acoustic suspension design.

In this age of auto EQ and lots of cheap amplifier power a speaker may actually have a more ideal frequency response when removing the additive port bass (which may give you too much bass at some frequency) and allowing the AVR Auto EQ to add more low bass electronically by boosting the power at those frequencies - this is what they do with modern sealed subwoofers. Max SPL may suffer but you may get smoother in-room frequency response - and since the port plugs are removable you can experiment both ways depending on speaker placement.

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Old 07-21-2014, 03:38 PM
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"Optimal" is relative. The low-frequency response is heavily influenced by the room and of course personal preference. Use of ports (or plugging them) will change the response, which may be better or worse depending upon your room and preference.

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Old 07-22-2014, 02:05 AM - Thread Starter
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Hmm... so does this mean that unless the speakers are in an optimal position, the frequency response will be quite non-linear anyway? So plugging the bass reflex port with port bungs is as likely to improve vs worsen the frequency response linearity?
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Old 07-22-2014, 03:20 AM
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Quote:
Originally Posted by Fjodor2000 View Post

Loudspeakers with bass reflex ports are designed to produce a flat or optimal frequency respons with the ports unplugged.
I'd like to clarify the above to read:

Loudspeakers with bass reflex ports are designed to produce a flat or optimal frequency response in very specific, narrowly-defined and rarely-occurring acoustic environment with the ports unplugged.

Under the covers speakers speakers are designed to work in one of the general kinds of acoustic environment:

(1) Free field - the speaker is suspended in the middle of a very large room.
(2) Half-space - the speaker is placed in the middle of a wall of a very large room.
(3) Quarter-space - the speaker is placed in the middle of a corner where two surfaces of a very large room meet.
(4) Eighth-space - the speaker is placed at the apex of a corner where tree surfaces of a very large room meet.

Some of the better loudspeaker modeling programs provide the above options, but most of the ones designed for consumers and amateurs seem to not do so.

Each one of the above situations is completely different and requires a speaker with a significantly different bass tuning if total system response that is flat is desired.

Furthermore, we are fortunate because every loudspeaker spec sheet specifies which kind of space it is designed for. Well, not so much. In fact I just made this last item up! This is a significant omission.

Furthermore, a speaker that was designed for use in a very large room would be less than completely useful because most listening rooms are small or medium sized compared to the wavelengths of the lowest bass notes that we want reproduced. For example, as music lovers go my needs for bass are modest and I only want flat response down to 20 Hz where a wavelength is about 50 feet, and the largest dimension of my listening room is 22 feet or about half a wavelength. It is not a large room.

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Originally Posted by Fjodor2000 View Post
Still some loudspeakers are delivered with port bungs/plugs used to plug the ports e.g. if they are placed close to a wall, to reduce bass resonance. But will using port bungs ever result in an optimal sound? Won't it alter the frequency response in a way not originally intended, since the speakers were designed to produce optimal sound without the use of port bungs?
The best thing about port bungs is that they will give you some options that you wouldn't have without them. They are just acoustic parametric equalizers and its pretty obvious that compared to an electrical parametric equalizer they have very limited settings. But, they beat a kick in the head!
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Old 07-22-2014, 03:33 AM
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Originally Posted by Fjodor2000 View Post
Hmm... so does this mean that unless the speakers are in an optimal position, the frequency response will be quite non-linear anyway? So plugging the bass reflex port with port bungs is as likely to improve vs worsen the frequency response linearity?
Correct.
You have 2 acoustic amplifiers, the port and the wall (or even better, a corner)
The bung disables the port and (hopefully) compensated for the reflection of the wall.


If you have a mic, record a freqency sweep and you probably will see all kind of non-linearities due to the interaction between speaker and room.
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Old 07-26-2014, 03:41 PM
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Originally Posted by Fjodor2000 View Post
Loudspeakers with bass reflex ports are designed to produce a flat or optimal frequency respons with the ports unplugged.
True, when they're outdoors. Indoors response is as much defined by the room as it is the speakers, so methods must be employed to restore flat response. EQ is one of those methods, blocking ports is another.

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Old 08-02-2014, 09:18 PM
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The goal of damping in ports is to adjust the frequency response for free air vs. 2 surface, or 3 surface location (i.e. wall on floor, corner), because the bass radiation of the speaker is less directional, and therefore the direct sound to the listener may have too much bass if the speaker is in a different location.

This is exacerbated by the fact that 2 surface or 3 surface positioning also loads the system more at low frequencies (lowers the radiation impedance of the air around the speaker at low frequencies) so the speakers also may emit more bass, as well, in the case when they need it less.

Each speaker is different in these regards, however, it's generally wise to use a speaker in the position it was designed for.

JJ

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Old 08-03-2014, 01:36 PM
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Quote:
Originally Posted by Fjodor2000 View Post
Hi,

Loudspeakers with bass reflex ports are designed to produce a flat or optimal frequency respons with the ports unplugged.

Not all ported speaker systems, are tuned to produce 0-ripple. - but your point is taken.

Still some loudspeakers are delivered with port bungs/plugs used to plug the ports e.g. if they are placed close to a wall, to reduce bass resonance. Yes, but it's rare, and included to help mitigate unwanted gains in transfer function, when speaker systems are place near one or more walls. But there are tradeoff's, as always. Increased F3, increase in nominal ripple, tonal smears 1-2 octaves above F3, etc..

But will using port bungs ever result in an optimal sound? Optimal is a theoretical occurrence, in which no electrical ring is produced regardless of input Voltage; however, such is impossible. So optimal is truly a relative term. As a relative term, Yes, something may occur that may be deemed optimal in an application, for the intended audience. However, plugging ported alignment doesn't usually produce electrically optimum results.

Won't it alter the frequency response in a way not originally intended, since the speakers were designed to produce optimal sound without the use of port bungs? It will change every performance parameter, no question, frequency response will change, IM qualities will change, mechanical power handling will increase, etc..

Would anyone please care to elaborate on this?
Please note RED above.
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Old 08-03-2014, 02:03 PM
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Originally Posted by mtn-tech View Post
Many ported speakers and subwoofers are designed to work with all the ports open or some of the ports blocked with included port plugs.

Plugging one or more ports will (as you said) change the frequency response of the speaker - it will almost certainly reduce the amount of bass volume but some speakers will actually have a smoother frequency response with one or more of the plugs installed making the cabinet closer to a non-ported acoustic suspension design.

In this age of auto EQ and lots of cheap amplifier power a speaker may actually have a more ideal frequency response when removing the additive port bass (which may give you too much bass at some frequency) and allowing the AVR Auto EQ to add more low bass electronically by boosting the power at those frequencies - this is what they do with modern sealed subwoofers. Max SPL may suffer but you may get smoother in-room frequency response - and since the port plugs are removable you can experiment both ways depending on speaker placement.
I would like to offer a few clarifications:

1. Plugging a port in a multiport design reduces bass extension, what bass energy that remains is often elevated, due to a positive ripple in response.
2. The occurrence of a positive ripple, is not tantamount to a smoothing of frequency response, but rather the opposite.
3. If several ports are present and even one of them left unplugged, the enclosure is still effecting a 4th order high-pass filter; however, a huge ripple in frequency response is the most likely result.
4. If all of the ports are plugged, the alignment of the enclosure will be effectively be changed from a 4th order reflex type, to a 2nd order acoustically suspended type; however, the volume of the box will be misaligned (too large), resulting in an under dampened condition, which will result in lower power handling and weak bass response, not a smoother base response, but typically an abruptly truncated one.
5. Attempting to over boost a sealed enclosure will not result a superior outcome, to a well suited ported design, it will simply increase the temperature of the voice coil, which in turn will greatly increase the THD and IMD output levels.
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Old 08-03-2014, 03:03 PM - Thread Starter
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Originally Posted by Garidy View Post
I would like to offer a few clarifications:

1. Plugging a port in a multiport design reduces bass extension, what bass energy that remains is often elevated, due to a positive ripple in response.
2. The occurrence of a positive ripple, is not tantamount to a smoothing of frequency response, but rather the opposite.
3. If several ports are present and even one of them left unplugged, the enclosure is still effecting a 4th order high-pass filter; however, a huge ripple in frequency response is the most likely result.
4. If all of the ports are plugged, the alignment of the enclosure will be effectively be changed from a 4th order reflex type, to a 2nd order acoustically suspended type; however, the volume of the box will be misaligned (too large), resulting in an under dampened condition, which will result in lower power handling and weak bass response, not a smoother base response, but typically an abruptly truncated one.
5. Attempting to over boost a sealed enclosure will not result a superior outcome, to a well suited ported design, it will simply increase the temperature of the voice coil, which in turn will greatly increase the THD and IMD output levels.
Based on what you wrote, plugging the port using port bungs does not seem like a good idea. It will alter the sound quite a lot compared to what the speaker originally was intended for, and mostly in a non-desirable way. Is that correct?
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Old 08-03-2014, 03:26 PM
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Quote:
Originally Posted by Fjodor2000 View Post
Based on what you wrote, plugging the port using port bungs does not seem like a good idea. It will alter the sound quite a lot compared to what the speaker originally was intended for, and mostly in a non-desirable way. Is that correct?
Plugging ports may, or may not, improve in-room response. The only way to know whether it's appropriate is by measuring the in-room response in both configurations. Don't over-think it. If it sounds good, it is good.
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Old 08-03-2014, 03:38 PM
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Quote:
Originally Posted by Fjodor2000 View Post
Based on what you wrote, plugging the port using port bungs does not seem like a good idea. It will alter the sound quite a lot compared to what the speaker originally was intended for, and mostly in a non-desirable way. Is that correct?
Take from it what you will; and apply it as best as you're able to your circumstance; as my comments are literal and without bias. Without fully understanding your circumstances, I cannot bias my comments to become meaningful to your application.
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Old 08-04-2014, 03:05 AM
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Originally Posted by Bill Fitzmaurice View Post
Plugging ports may, or may not, improve in-room response. The only way to know whether it's appropriate is by measuring the in-room response in both configurations. Don't over-think it. If it sounds good, it is good.
^^^^ That.

There is no way to know the answer until and unless one would complete rather a tiresome analysis of both your room and the particular speaker in question.

Better, if anything, to try the bungs, measure the results, and then decide.

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Old 08-04-2014, 07:47 AM
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Originally Posted by Fjodor2000 View Post
Based on what you wrote, plugging the port using port bungs does not seem like a good idea. It will alter the sound quite a lot compared to what the speaker originally was intended for, and mostly in a non-desirable way. Is that correct?
No.

The common situation is too complex and variable to make blanket generalizations about.

Listening rooms vary all over the map and the response of a given speaker varies all over the map depending on how you position it in the room.

I see this point made about a dozen times up thread, and I'm scratching my head wondering why it is being fought tooth and nail.

The situation is so complex that even building a sufficiently detailed computer model of your room, which is possible, is probably the wrong thing to do. Too much work.

The expedient thing to do is to rent, buy, or steal a measurement setup and someone who knows how to use it well. The cost of such things is now small compared to even a single mid-priced audio component. The learning curve has been already surmounted by lots of audiophiles right here on AVS.

The above is far from the first time the ideas contained therein has been posted on this thread, even just recently. Failure to grasp it will result in any number of palm plants, head shakes, sieges of head scratching and perhaps a few heads hitting walls. ;-)
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Old 08-04-2014, 01:51 PM
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Originally Posted by arnyk View Post
No.

The common situation is too complex and variable to make blanket generalizations about.

Listening rooms vary all over the map and the response of a given speaker varies all over the map depending on how you position it in the room.

I see this point made about a dozen times up thread, and I'm scratching my head wondering why it is being fought tooth and nail.

The situation is so complex that even building a sufficiently detailed computer model of your room, which is possible, is probably the wrong thing to do. Too much work.

The expedient thing to do is to rent, buy, or steal a measurement setup and someone who knows how to use it well. The cost of such things is now small compared to even a single mid-priced audio component. The learning curve has been already surmounted by lots of audiophiles right here on AVS.

The above is far from the first time the ideas contained therein has been posted on this thread, even just recently. Failure to grasp it will result in any number of palm plants, head shakes, sieges of head scratching and perhaps a few heads hitting walls. ;-)
If you can get a decent omni mic and a preamp/ADC that will talk to your computer, go look here: http://www.aes.org/sections/pnw/pnwrecaps/2012/jj_jan/

There are scripts and a video with instructions that will allow you to do your own measurement of your own room. I run this all on a lame laptop.

You might want to change the plotting system to show low-frequencies more clearly. That shouldn't be hard. If somebody wants to pretty up the plotting, feel free, by the way. I just use the click/scale functionality myself.

Octave, audacity, and the scripts are all free to anyone who wants them, so the only thing you need is an omni microphone with suitably reasonable frequency response. Nowdays you can buy one of those for under USD 100.

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Old 08-04-2014, 02:04 PM - Thread Starter
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Based on the comments above, I'm surprised there is so much focus on getting loudspeakers to provide a straight frequency response (by manufacturers and in reviews). Because room characteristics will screw it up anyway... with or without port bungs...

Last edited by Fjodor2000; 08-04-2014 at 02:08 PM.
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Old 08-04-2014, 03:39 PM
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Originally Posted by Fjodor2000 View Post
Based on the comments above, I'm surprised there is so much focus on getting loudspeakers to provide a straight frequency response (by manufacturers and in reviews). Because room characteristics will screw it up anyway... with or without port bungs...
Quite right. Perfectly flat response is beneficial if your room has perfectly flat response as well. Guess what? If it's not an anechoic chamber, it doesn't. Flat response is useful above 200Hz or so, where the direct radiation from the speaker is the main component of what's heard, but below 200Hz, where the room dominates the result, flat anechoic response can actually be counter-productive.

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Old 08-04-2014, 04:08 PM
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Originally Posted by Fjodor2000 View Post
Based on the comments above, I'm surprised there is so much focus on getting loudspeakers to provide a straight frequency response (by manufacturers and in reviews). Because room characteristics will screw it up anyway... with or without port bungs...
Well, what one needs to do is to make the acoustic system (speaker, room, furniture, etc) work together.

Changing the low frequency response of the speaker can be legitimate, or can be terrible. It all depends.

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Old 08-04-2014, 04:12 PM
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Originally Posted by Bill Fitzmaurice View Post
Quite right. Perfectly flat response is beneficial if your room has perfectly flat response as well. Guess what? If it's not an anechoic chamber, it doesn't. Flat response is useful above 200Hz or so, where the direct radiation from the speaker is the main component of what's heard, but below 200Hz, where the room dominates the result, flat anechoic response can actually be counter-productive.

Flat response about 200Hz can also be a big problem, because in fact room contribution falls off with frequency, and you can wind up, depending on the window you measure with, a system that is way, way too bright at high frequencies.

What you want to measure above about 500Hz is the DIRECT COMPONENT of the speaker, as determined not by the room, but by the ear, i.e. you need to use a frequency-varying window with different time lengths at different frequencies.

If that includes a room bounce, it does. If it doesn't, it doesn't, but if it does, you have other problems.

At 200Hz, you still want to look at direct, but you want to actively consider room bounces, because you can't be rid of them. Still a window no longer than 1/30th of a second is appropriate.

Below that, the room is primary, and you need to look at a much wider window, maybe up to .125 seconds at 90Hz, and up to a second at 20Hz.

But then you also want to know the room power response, as opposed to the pressure response at one point, below about 100 to 200Hz anyhow.

I'll let Amir translate that for you since he volunteered the other day.

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Old 08-05-2014, 02:13 AM
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Originally Posted by Fjodor2000 View Post
Based on the comments above, I'm surprised there is so much focus on getting loudspeakers to provide a straight frequency response (by manufacturers and in reviews). Because room characteristics will screw it up anyway... with or without port bungs...
Hold that thought.

Let's back up to see the big picture.

Since the room and the speakers have such profound effects on at-the-ear frequency response, why do we worry about any component in the chain being flat? Why worry about music players being flat? Why worry about amps being flat?

The answer is that our lives are very much simplified if there are generally agreed-upon goals for the tools we work with. Flat response is the simplest possible goal to express for audio gear frequency response. Flat response does not get us into a lot of trouble that we could be in, if highly non flat response were acceptable.

Audio gear in general used to be a whole lot less flat. Go back to the 1950s and earlier. The flatter audio gear got, the easier it was to put together a good sounding system without a lot of thought or diddling. By the 1990s most electronic components were so flat that making them any flatter was not going to make an audible difference. This particular phase started as soon as the digital revolution chased LP players away. LP players and tape machines were the last class of electronic components that were difficult or impossible to make flat or keep flat.

This leaves speakers and rooms which arguably need to be designed to work together, but obviously are not yet sold together.

There are actually 4 kinds of speakers as far as bass response goes, maybe 5.

There is the eighth-space speaker that is designed to be put near floors or ceilings but only in corners.

There is the quarter-space speaker that is designed to be put near floors and ceilings or in corners away from floors and ceilings.

There is the half-space speaker that is designed to be put in the middle of a wall or our on the floor away from the walls.

There is the full space speaker that is designed to be suspended in free space.

There is the speaker that has such limited bass extension that where you put it pretty much doesn't matter. ;-)

And of course people fudge things, and there are all these speakers which fit in-between the places I just mentioned. (the current real world).

It is theoretically and to some degree practically possible to use a specialized equalizer (a specialized form of parametric that I don't think actually exists on the market) to adapt a speaker from where it was designed to be put to where it is actually put.

A general purpose parametric can come very close but you need to also need to be able to do acoustical measurements or have very well experienced ears to come close.
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Old 08-05-2014, 02:38 AM
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Originally Posted by arnyk View Post

... There are actually 4 kinds of speakers as far as bass response goes, maybe 5...


How (if designed for the specific placements) are they different?

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Old 08-05-2014, 03:01 AM
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How (if designed for the specific placements) are they different?
Their anechoic frequency response below a few 100 Hz, which is usually due to the tuning of the woofer, is different.

The toughest speaker to actually build is the full-space speaker. I don't know of any commercial products that are that way. There are some claims. The full-space speaker's bass dynamic range needs are great and if one were implmented I would expect to see electronic bass boost.

Note that 1/8 space speakers were more often built commercially long ago, and is probably most heavily represented among legacy speakers that were also called "Corner Horns".

Can a bung change a quarter-space speaker into a half-space speaker or vice-versa? It can go some distance, but probably not the whole way.
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Old 08-05-2014, 08:44 AM
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Originally Posted by arnyk View Post
Their anechoic frequency response below a few 100 Hz, which is usually due to the tuning of the woofer, is different.
That's not exactly true. There are some speakers, such as the KHorn, which realize a very different response curve in 1/8th space (corner loaded) than against a wall or away from the wall. That's because they're horn loaded, and a horn gives very different results with different degrees of boundary loading. The same does not apply to direct radiators.
Direct radiators do realize gain from boundary loading, but that gain is quite linear below the baffle step frequency, so while they will go louder they won't go lower. What does cause a speaker to go lower is cabin gain, also known as pressure vessel gain.
Cabin gain will extend the useful low frequency response of a speaker because it's not linear with respect to frequency. How much cabin gain is realized varies with the size of the room; the smaller the room the higher the gain. The effect of cabin gain is easily seen in auto sound, where very small subs can go very deep and loud due to the small size of the 'room'. A look at the SPL charts here, which show the same subwoofer in and out of a car, reveals why in the auto sound genre it's critical to consider the effect of room size in the design phase:
http://www.luxusni-elektronika.cz/pr...9fe274f633a095

Cabin gain in a typical room doesn't show as much effect as in a car, but it still can be significant. It's what allows sealed subs that have a nominal f3 of 40Hz to work well to 20Hz, and what can cause a ported sub that's tuned too high to be boomy. Lowering the tuning of a ported sub or even sealing it to make it work better in a smaller room is one of the valid uses for port plugs.

Bill Fitzmaurice Loudspeaker Design

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Old 08-05-2014, 09:04 AM
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Originally Posted by jj_0001 View Post
I'll let Amir translate that for you since he volunteered the other day.
I did? I don't recall but if I said that I can translate your English to my English, it must have been a moment of insanity! I will try below.

Quote:
Flat response about 200Hz can also be a big problem, because in fact room contribution falls off with frequency, and you can wind up, depending on the window you measure with, a system that is way, way too bright at high frequencies.
JJ, I assume you mean "above 200 Hz" as opposed to "about 200 Hz." If so, indeed the room contributions drops off substantially as you get above the transition frequencies of 200 to 300 Hz. You can see this (indirectly) from this measurement of the same room in different places in the room from my article on bass frequency optimization:



As the graph indicates, above transition frequencies the speaker response dominates. The room has little effect there. So if you are not happy with that response, it is an issue with your speaker, not the room.

The second part of JJ's comment is that if you have a flat response from low to high frequencies, subjectively your system will sound bright, i.e. too much high frequencies. So you want a "target response curve" that is sloping down like this implemented by JBL Synthesis ARCOS automatic EQ system (this is for our theater):



Now, do we know what the talent heard when he/she approved the content? No. Maybe they heard the bright version and that is what they liked. Maybe not. Audio is "broken" in that we have no reference as to what is right. We have to go by what sounds good and research shows that the above or some variation of it is. I can provide references if there is interest.

Unfortunately automatic EQ in consumer AVRs rarely if ever lets you select the target response. A crude way around that is to boost the level of your sub (assuming you have one) a few dbs post calibration.

Quote:
What you want to measure above about 500Hz is the DIRECT COMPONENT of the speaker, as determined not by the room, but by the ear, i.e. you need to use a frequency-varying window with different time lengths at different frequencies.
I hope this part is self-explanatory. If not, JJ is saying to vary the time window over which you measure the frequency response to match what we hear, devoid of the contributions from reflections. Alas, no standard measurement system does this.

I use an alternate method which is to apply increasingly higher amount of averaging as frequencies go up. This is because the frequency selectivity/bandwidth of the ear goes up as frequencies go up. I explain this in my article on perception of room reflections with this graph related to this topic:



This means that if you are measuring bass frequencies, you want to have high precision down to 1 Hz detail so that you can find the room contributions/modes. But if you are trying to determine what is going on say, above 1 Khz, you want to use averaging as to better match our perception there.

If you are using REW for example, you want to use 1/24 or 1/12 smoothing for bass frequencies but go down to 1/6 or even 1/3 for higher frequencies.

Quote:
At 200Hz, you still want to look at direct, but you want to actively consider room bounces, because you can't be rid of them. Still a window no longer than 1/30th of a second is appropriate.
200 Hz is in the transition frequency meaning you have not gotten rid of room modes but they are starting to overlap good bit as to randomize the response. So you want to look at the peaks and valleys but don't go nuts analyzing them at high detail.

Quote:
Below that, the room is primary, and you need to look at a much wider window, maybe up to .125 seconds at 90Hz, and up to a second at 20Hz.
As I showed in the first graph, the room determines the response, not the speaker below transition frequencies. You can put any speaker in your room and the response more or less will be the same. Your measurements need to be accurate down to 1 Hz or even lower. JJ is using the time version of the same metric which goes up, as frequency selectivity narrows.

Quote:
But then you also want to know the room power response, as opposed to the pressure response at one point, below about 100 to 200Hz anyhow.
Same thing I explained above .

Amir
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Old 08-05-2014, 03:18 PM
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Well, one other detail, (using appropriate windows) you don't need a target curve, because you get it automatically with the right length windows.

The shorter window still captures most of the speaker energy because the room is not as much involved (at least with most speakers), but only the appropriate amount at lower frequencies. (Note, the "transition" region is really much wider at the high-frequency end than most people allow, but fortunately without any painfully early reflections it usually doesn't matter, thank you cochlear compression effects.)

There are a couple of papers on this kind of room correction, of course, one from Microsoft itself, and one from another company, delivered at one of the London AES conventions sometime later. I'd say if I remembered, sorry. 2011 I think.

James D. (jj) Johnston
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