Difficult Acoustic Topics

Posted: Tue Apr 03, 2007 9:18 pm

Hi,

I hope there's no limit on the number of questions? Smile

I'm new here, and I haven't seen any advice such as breaking them up into separate posts, so here goes...

I have a fairly good background in engineering, music and acoustics. I've read most of F. Alton Everest's books, Rod Gervais's recent book, and most of Ethan Winer's articles and a ton of newsgroup postings. And I've built a studio that turned out well. But I've got some difficult questions that I just can't seem to find the answers to. Can someone help me with these?

1. What is the desired absorption/T60 decay curve?
Mr. Everest goes for completely flat down to 125 Hz, but I don't think he has much bass absorption. Ethan's modules reach lower, but they also absorb quite a bit less at the higher frequencies. I've seen graphs where less decay time at the high frequencies and more decay time at the low frequencies is acceptable. But, Ethan's modules would do the opposite - more decay time at the high frequencies and less decay time at the low frequencies. I have to guess that a room treated with his modules sounds pretty good, so ... ??? Is this not as important as I think it is?

2. What kind of absorption can you get using type 703 rigid fiberglass, below 125 Hz?
For instance, if I use 4" and space it 4" off the wall for a total of 8", what are the absorption coefficients at, say, 62.5 Hz and 31.25 Hz? Or, what about the common advice of putting 2' x 4' 703 over the corners? I can't find any absorption coefficients for these cases. The point here is that I want to save wall space if possible, so I'm wondering if I can just use open-faced modules to cover the whole frequency spectrum, down to say 40Hz. If not, I will have to go with wideband absorbers and low frequency tuned resonators, right? basically doubling my wall space.

3. How do I properly combine wideband absorbers and low frequency tuned resonators?
Assuming I use wideband absorbers and low frequency tuned resonators - I see overlap problems here. Let's say I use 4" of type 703 for wideband absorption. For low frequency I go with 8" deep Helmholtz perforated resonators, tuned to 125 Hz with 4" of type 703 in the cavity to widen the absorption point. From Mr. Everest's books, I get combined absorption coefficients of: 1.2 1.99 1.67 1.38 1.16 1.12 1.09

This looks like way too much absorption in the middle bass region. I can shift the frequency of the resonator down, helping somewhat, but I still have that hump in the middle. Is this good? bad? indifferent?

Now, here is the thing I am really wondering about. Doing the same analysis as before, except with only 1" of type 703 flat on the wall, yields an almost perfectly flat combined response! So this seems like the perfect combination. But I've never ever seen *anyone* recommend putting only 1" of rigid fiberglass in a studio. But the numbers look great. So - what am I missing here?

4. What coefficients do I use for planning purposes, and how do I handle coefficients greater than 1?
For example, Mr. Everest's books all list 4" of type 703 at 0.99. but I have the Owens-Corning Noise Control Guidelines booklet, and it lists quite different coefficients, including some over 1:
0.84 0.1.24 1.24 1.08 1.00 0.97
So, which is it? Smile

5. How do I handle the back wall of a short Control Room?
Due to building restraints, the Control Room is 18' wide but only 10' deep. Maybe some day we'll turn the engineer 90 degrees, but for now we're stuck with it. So the engineer's head is going to be 5' or less from the back wall. This is super close. Most of the advice I've seen says to make the back wall as dead as possible with wideband absorption. But I'd really really like to have more of a Live-End/ Dead-end feel. Could we put Skyline diffusers on the back wall and do okay? If not, what if we mix Skyline diffusers with absorbers, say a checkerboard pattern of 2' x 2' modules? Would this help or hurt? Any experience with this?

6. What is the absorptive effect of the walls? and how do you compensate for the resonant frequency?
Mr. Everest shows a rise in absorption coefficients approaching the lower frequencies, and he also states that maximum absorption occurs at the resonant frequency. But other graphs and Rod's book shows that sound transmission blocking *decreases* with decreasing frequency, and Rod states that at resonance, sound goes directly through the wall. What is the truth? In any case, for now our resonant frequency is fairly high, maybe 50 Hz. What does that mean for absorption in the range from 30Hz to 125 Hz? Would there be any absorption at 62.5 Hz? How do we compensate for the resonance at 50 Hz?

Thanks a ton in advance for any help.

Regards,

Kevin Kaster

Posted: Wed Apr 04, 2007 8:03 am

Kevin,

Kevin_Kaster wrote:

1. What is the desired absorption/T60 decay curve?Is this not as important as I think it is?

I don't feel T60 is very important for small room design. In general, you add enough treatment to take care of any reverberant behavior, which - in a small room - only occurs at high frequencies. Check this studiotips thread for more discussion (particularly the detailed discussion starting on page 2).

If I were forced to give an "ideal" curve for small room RT, it would be relatively flat, but I would allow for some rise in the low bands and some fall in the high bands. The rise would be because LF RT in a small room is dependent on modal decay. Since modal decay is a real challenge to reduce, some rise would, IMO, be acceptable in the lower bands. Percentage-wise, this could be as much as 50% higher relative to the mid bands.

A 10-20% fall in the high bands is also not horrible since sound naturally gets absorbed in this range by furnishings, people, etc. I.e., you're naturally going to be adding more HF absorption to the room. Therefore, a relative fall in RT in the top octave (or two, if you're calculating out to 8 kHz) would be "natural" sounding.

But I have yet to ever use RT as a design tool for small rooms. The above would only be if you absolutely forced me to do it.

Quote:

2. What kind of absorption can you get using type 703 rigid fiberglass, below 125 Hz?

That will entirely depend on the thickness. The coefficients aren't anything you're going to find because the tests generally only go down to 100 Hz. (The tests for absorption were developed in the days when speech intelligibility was of tantamount importance and studios - if they existed at all - used relatively narrowband monaural sources.) In general, the thicker the 703, the more absorption you'll get at the low frequencies. For adequately addressing the 31.5 & 63 Hz bands, you may need more than just thick 703 - which you've surmised below.

Quote:

3. How do I properly combine wideband absorbers and low frequency tuned resonators?
Assuming I use wideband absorbers and low frequency tuned resonators - I see overlap problems here. Let's say I use 4" of type 703 for wideband absorption. For low frequency I go with 8" deep Helmholtz perforated resonators, tuned to 125 Hz with 4" of type 703 in the cavity to widen the absorption point. From Mr. Everest's books, I get combined absorption coefficients of: 1.2 1.99 1.67 1.38 1.16 1.12 1.09

In general, this is probably a pretty good approach. I would forgo worrying about any coefficients as they will neither be accurate nor useful in small room design. But the approach of using thick absorption combined with some Helmholtz devices could work well. The only drawback would be that, to completely address the lowest frequencies in the room, you could wind up needing more Helmholtz devices that can reasonably be placed in your room. In the simplest of terms, I would consider using 2-3 times more of the Helmholtz relative to the amount of 4". Beyond this, things could start to get ridiculous. (If they're not already! Very Happy

)

Quote:

This looks like way too much absorption in the middle bass region. I can shift the frequency of the resonator down, helping somewhat, but I still have that hump in the middle. Is this good? bad? indifferent?

Indifferent. Tune the Helmholtzes where you need to, keeping in mind that they need to reasonably fit in the room.

Quote:

Now, here is the thing I am really wondering about. Doing the same analysis as before, except with only 1" of type 703 flat on the wall, yields an almost perfectly flat combined response! So this seems like the perfect combination. But I've never ever seen *anyone* recommend putting only 1" of rigid fiberglass in a studio. But the numbers look great. So - what am I missing here?

I don't know that you're missing anything. This approach could work well. You just may need more 1" relative the amount of 4" you would have used. (Keeping in mind that I'm not looking at any numbers - just basing this on experience.) One drawback is that you could come up deficient (despite what the numbers say) in the 125-500 Hz range using only 1".

Quote:

4. What coefficients do I use for planning purposes, and how do I handle coefficients greater than 1?
For example, Mr. Everest's books all list 4" of type 703 at 0.99. but I have the Owens-Corning Noise Control Guidelines booklet, and it lists quite different coefficients, including some over 1:
0.84 0.1.24 1.24 1.08 1.00 0.97
So, which is it? Smile

Despite what you may have heard about coefficients representing a percentage of sound absorbed, they are in fact arbitrary numbers - not percentages. How you handle them is a matter of what you're calculating. If you're trying to plug them into the Sabine equation - not advisable for a small room - then you should be able to use them as is. If you're using Eyring - also not advisable for a small room - you will have to round them off to 0.99. IMO, you shouldn't be using them to calculate anything for a small room. For small room design, just use the coefficients to generally select your materials. Want better absorption at 125 Hz? Pick the material with a 1.5 coefficient over one with a 0.7 coefficient. There's really not much more you can do with absorption coefficients with regards to small room acoustical design.

Quote:

5. How do I handle the back wall of a short Control Room?
Due to building restraints, the Control Room is 18' wide but only 10' deep. Maybe some day we'll turn the engineer 90 degrees, but for now we're stuck with it. So the engineer's head is going to be 5' or less from the back wall. This is super close. Most of the advice I've seen says to make the back wall as dead as possible with wideband absorption. But I'd really really like to have more of a Live-End/ Dead-end feel. Could we put Skyline diffusers on the back wall and do okay? If not, what if we mix Skyline diffusers with absorbers, say a checkerboard pattern of 2' x 2' modules? Would this help or hurt? Any experience with this?

In my experience, diffusers would not be good. The general rule of thumb, particularly with QRDs and PRDs (Skyline is the latter), is that you need about 10' minimum distance between listener and diffuser. This is largely due to the poor near field response of these types of diffuser.

Given your short distance, I would advise to stick with broadband absorption and completely avoid diffusion for the back wall.

Quote:

6. What is the absorptive effect of the walls? and how do you compensate for the resonant frequency?
Mr. Everest shows a rise in absorption coefficients approaching the lower frequencies, and he also states that maximum absorption occurs at the resonant frequency. But other graphs and Rod's book shows that sound transmission blocking *decreases* with decreasing frequency, and Rod states that at resonance, sound goes directly through the wall. What is the truth? In any case, for now our resonant frequency is fairly high, maybe 50 Hz. What does that mean for absorption in the range from 30Hz to 125 Hz? Would there be any absorption at 62.5 Hz? How do we compensate for the resonance at 50 Hz?

Drywall over 2x4s will resonate. The resonance effects can largely be overcome with insulation in the stud cavities. Beyond that, you shouldn't concern yourself too much with "how much" the wall is going to absorb. It's going to absorb some sound in the lowest octaves. This can only help you (unless you're talking about an auditorium Smile

). Avoid the resonance, beef up the walls as much as you need to to combat sound leakage, and then treat the room. The only thing to really keep in mind is that the more mass you add to the wall, the stiffer - in general - it's going to get. Thus, a wall with good isolation properties will keep more sound - including low end - in the room. A well-isolated room could actually sound worse (subjectively) than a room with only a single layer of drywall over insulated studs. It's all up to you how much or how little mass you need on the walls. Which will dictate - to a degree - the amount of low end absorption you will need in the finished room. In practice, this is a minor issue (IMO) - the room is going to need low end control either way. A little more or less is not, IMO, going to matter much at the end of the day.

Posted: Wed Apr 04, 2007 12:16 pm

Kevin,

Quote:

I hope there's no limit on the number of questions? Smile

There is, but it's cumulative over time, and you haven't quite reached the limit yet. Smile

Jeff already gave you great answers, and I'll add only a little.

Quote:

1. What is the desired absorption/T60 decay curve?

Personally I prefer to avoid all resonant (frequency-selective) ringing because it manifests itself as a skewed frequency response. That is, if a room rings more at one frequency than other nearby frequencies, there's more total energy in the room even if the steady state response measures flat. But as Jeff explained, you can't get rid of all modal ringing anyway, so it's more a matter of what you'll tolerate versus how much money you're willing to spend.

Quote:

Ethan's modules would do the opposite - more decay time at the high frequencies and less decay time at the low frequencies. I have to guess that a room treated with his modules sounds pretty good

If you are ever in my neighborhood you are most welcome to stop in and hear for yourself.

Quote:

4. What coefficients do I use for planning purposes, and how do I handle coefficients greater than 1?

Coefficients are derived from sabins, and sabins is what really matters. One problem is the amount of sabins you measure depends entirely on where you put the absorbers when testing! You can see a good example of this in the Product Data page n the RealTraps site. The very same MiniTrap mounted straddling a corner has nearly three times more sabins as when placed a few inches off a wall near the center of the wall.

Personally, my approach is to have as much bass trapping as possible below, say, 300 Hz.

Quote:

5. How do I handle the back wall of a short Control Room? ... the engineer's head is going to be 5' or less from the back wall.

I agree with Jeff that broadband absorption (not bass trapping only) makes sense there. I'm surprised Jeff didn't mention that the "10 foot rule" with diffusion depends entirely on the low frequency limit of the diffusor being considered. I learned that from him!

A better "rule" for QRD type diffusors is one foot distance for each inch of well depth. I've used my company's 3-inch deep diffusor/bass trap as close as three feet away and it was definitely much better than a bare wall. Even our six-inch deep QRD is better only a few feet away than a bare wall.

--Ethan

Posted: Wed Apr 04, 2007 4:47 pm

Ethan Winer wrote:

I agree with Jeff that broadband absorption (not bass trapping only) makes sense there. I'm surprised Jeff didn't mention that the "10 foot rule" with diffusion depends entirely on the low frequency limit of the diffusor being considered. I learned that from him!

Actually, after reading through Cox & D'Antonio's book a while back, it's my understanding that the application of diffusers in this sort of application - a relatively close rear wall - is more a function of what diffusers can actually do in the near field as opposed to the frequency limitations - though the latter are certainly important to consider.

In general, QRDs are poor near field performers. Remember that they were originally designed to improve the sound in concert halls - venues where near field performance is not as important. In the near field, "diffusion" isn't really what you get when sound "bounces" off the QRD surface. There are lobing issues that could be worse than if the wall were left flat! Shocked

Quote:

A better "rule" for QRD type diffusors is one foot distance for each inch of well depth. I've used my company's 3-inch deep diffusor/bass trap as close as three feet away and it was definitely much better than a bare wall. Even our six-inch deep QRD is better only a few feet away than a bare wall.

Having only a 3" thick diffuser would reduce the lobing issues I mentioned above for listeners seated at reasonable distances, such as the 3-5 feet mentioned here. Since the cutoff frequency for such a device would be relatively high, it would be of lesser concern relative to the same effects from something larger, like a "Skyline." But the lobing effects could still be audible for thinner diffusive surfaces, particulary for the folks sitting on the couch with the diffusers less than 1 foot behind their ears. Shocked

My main point is that, because of the relatively poor near field performance of QRD (and similar) devices, I would generally prefer broadband absorption on such a close rear wall. A footnote to this involves the relative costs involved. Sitting 5 feet from the rear wall, you would get "all" the performance from an absorber. The same cannot be said of a QRD (or similar) device - it is not being used to its full potential. Ergo, why spend all the extra money? Smile

(Per square foot, diffusion is generally more expensive relative absorption.)

Posted: Wed Apr 04, 2007 8:26 pm

O - kay, I see I really should have posted 6 different questions instead of rolling them all into one. Sorry about that. Confused

Is there a good way to separate these and start over? For now, I've tried to snip things down to the basic points.

Thanks for the reply!

lovecow wrote:

Kevin_Kaster wrote:

1. What is the desired absorption/T60 decay curve? ... Is this not as important as I think it is?

I don't feel T60 is very important for small room design. In general, you add enough treatment to take care of any reverberant behavior, which - in a small room - only occurs at high frequencies. Check this studiotips thread for more discussion (particularly the detailed discussion starting on page 2).

Right, but you also need bass trapping to control the low-frequency resonance - which you mention later. The overall point of all my questions, of course, is "how do I best figure out what type of absorbers to use and where". Sabines aren't perfect but at least it gives me something to calculate.

I guess I'm a little confused by your answer. The overall feel I get is that you aren't very concerned about how flat the decay is. But, if you have a decay time of say 0.3 seconds at 250 Hz, and 0.6 seconds at 2kHz, wouldn't this sound bad?

Thanks for the link, it was interesting.

lovecow wrote:

Kevin_Kaster wrote:

2. What kind of absorption can you get using type 703 rigid fiberglass, below 125 Hz? ...

That will entirely depend on the thickness. ... For adequately addressing the 31.5 & 63 Hz bands, you may need more than just thick 703 - which you've surmised below.

Right - I'm looking to find out if "may need" is actually "do need". I was hoping to find some good data. The best I've seen is the tests Ethan did attached to his faq, but all he measured was partial room results, so that still doesn't answer my question, "do I need more than thick 703".

lovecow wrote:

Kevin_Kaster wrote:

3. How do I properly combine wideband absorbers and low frequency tuned resonators?
Assuming I use wideband absorbers and low frequency tuned resonators - I see overlap problems here....

In general, this is probably a pretty good approach. I would forgo worrying about any coefficients as they will neither be accurate nor useful in small room design. But the approach of using thick absorption combined with some Helmholtz devices could work well. The only drawback would be that, to completely address the lowest frequencies in the room, you could wind up needing more Helmholtz devices that can reasonably be placed in your room. In the simplest of terms, I would consider using 2-3 times more of the Helmholtz relative to the amount of 4". Beyond this, things could start to get ridiculous. (If they're not already! Very Happy

)

Aren't they always in a low-budget studio? Smile

So, your reply of "could work well" begs the question - Is there a better way?

Some thoughts: As far as I know, I can only get true low-frequency control by using limp membranes, panel absorbers or tuned resonators. Apparently good limp membrane absorbers are difficult to build, because nobody does. Ethan's faq says that Helmoltz resonators are more narrow-band than panel absorbers (not terribly desirable), but Everest says that panel absorbers are low-efficiency (not terribly desirable either).

lovecow wrote:

Kevin_Kaster wrote:

This looks like way too much absorption in the middle bass region. I can shift the frequency of the resonator down, helping somewhat, but I still have that hump in the middle. Is this good? bad? indifferent?

Indifferent. Tune the Helmholtzes where you need to, keeping in mind that they need to reasonably fit in the room.

I guess this is the same as question one.

lovecow wrote:

Kevin_Kaster wrote:

Now, here is the thing I am really wondering about. Doing the same analysis as before, except with only 1" of type 703 flat on the wall, yields an almost perfectly flat combined response! So this seems like the perfect combination. But I've never ever seen *anyone* recommend putting only 1" of rigid fiberglass in a studio. But the numbers look great. So - what am I missing here?

I don't know that you're missing anything. This approach could work well. You just may need more 1" relative the amount of 4" you would have used. (Keeping in mind that I'm not looking at any numbers - just basing this on experience.) One drawback is that you could come up deficient (despite what the numbers say) in the 125-500 Hz range using only 1".

Okay. That was my thought too, since the numbers aren't exact.

lovecow wrote:

Kevin_Kaster wrote:

4. What coefficients do I use for planning purposes, and how do I handle coefficients greater than 1?
For example, Mr. Everest's books all list 4" of type 703 at 0.99. but I have the Owens-Corning Noise Control Guidelines booklet, and it lists quite different coefficients, including some over 1:
0.84 0.1.24 1.24 1.08 1.00 0.97
So, which is it? Smile

... IMO, you shouldn't be using them to calculate anything for a small room. For small room design, just use the coefficients to generally select your materials. ...

Understood. I guess this comes back to question 1 again. Depending on how this material actually absorbs in my room with my mounting, I could (theoretically at least) wind up with much more or much less absorption in the middle frequencies. That still seems like a bad thing to me, thus question 1.

lovecow wrote:

Kevin_Kaster wrote:

5. How do I handle the back wall of a short Control Room?
... Most of the advice I've seen says to make the back wall as dead as possible with wideband absorption. But I'd really really like to have more of a Live-End/ Dead-end feel. Could we put Skyline diffusers on the back wall and do okay? ...

In my experience, diffusers would not be good. The general rule of thumb, particularly with QRDs and PRDs (Skyline is the latter), is that you need about 10' minimum distance between listener and diffuser. This is largely due to the poor near field response of these types of diffuser.

Given your short distance, I would advise to stick with broadband absorption and completely avoid diffusion for the back wall.

You are such a party-pooper. But, that's what I wanted to know. Thanks.

What about putting them at other places in the room? Obviously the first reflections need to be absorbed. But once that is done, would there be any benefit to having diffusers, say on the rear of the side walls and ceiling? I just don't want it to sound too dead.

lovecow wrote:

Kevin_Kaster wrote:

6. What is the absorptive effect of the walls? and how do you compensate for the resonant frequency?
[ ... Basically I asked if walls absorb or not at resonant frequency]

Drywall over 2x4s will resonate. The resonance effects can largely be overcome with insulation in the stud cavities. Beyond that, you shouldn't concern yourself too much with "how much" the wall is going to absorb. It's going to absorb some sound in the lowest octaves....

So when Rod Gervais states that the low string on a 5-string bass guitar will cut through the walls like a hot knife through butter (what a great quote), does he mean that the wall will absorb a maximum at this point? Or will I get no absorption at this point?

Thanks again,

Kevin

Posted: Wed Apr 04, 2007 8:50 pm

Ethan, thanks for your reply!

Ethan Winer wrote:

Kevin,

Kevin_Kaster wrote:

I hope there's no limit on the number of questions? Smile

There is, but it's cumulative over time, and you haven't quite reached the limit yet. Smile

I shall endeavor to conduct myself within the rules of this great and respected forum. Cool

Ethan Winer wrote:

Kevin_Kaster wrote:

1. What is the desired absorption/T60 decay curve?

Personally I prefer to avoid all resonant (frequency-selective) ringing because it manifests itself as a skewed frequency response. That is, if a room rings more at one frequency than other nearby frequencies, there's more total energy in the room even if the steady state response measures flat. But as Jeff explained, you can't get rid of all modal ringing anyway, so it's more a matter of what you'll tolerate versus how much money you're willing to spend.

Hmm. Maybe I need to understand the difference between "flat decay time" and "modal ringing". If I absorb sound equally well at all frequencies and achieve a nice short decay time of 0.3 mS (this is for a rock music studio), will I still have modal ringing problems? If so, and I add a bunch of low-frequency absorbers such that the bass decay time goes down to 0.1 mS, won't that sound strange?

Ethan Winer wrote:

Kevin_Kaster wrote:

Ethan's modules would do the opposite - more decay time at the high frequencies and less decay time at the low frequencies. I have to guess that a room treated with his modules sounds pretty good

If you are ever in my neighborhood you are most welcome to stop in and hear for yourself.

I would love to do that, thanks.

Ethan Winer wrote:

Kevin_Kaster wrote:

4. What coefficients do I use for planning purposes, and how do I handle coefficients greater than 1?

Coefficients are derived from sabins, and sabins is what really matters. One problem is the amount of sabins you measure depends entirely on where you put the absorbers when testing! You can see a good example of this in the Product Data page n the RealTraps site. The very same MiniTrap mounted straddling a corner has nearly three times more sabins as when placed a few inches off a wall near the center of the wall.

Personally, my approach is to have as much bass trapping as possible below, say, 300 Hz.

This is back to my question 1, I think - don't you want a flat decay time across all frequencies? I'm starting to think the general opinion is that you can never have too much bass trapping. Now, I'm pretty sure that if I stuff a room full of Auralex it will be *way* too dead in the high frequencies. Does something different happen at the low frequencies?

Ethan Winer wrote:

Kevin_Kaster wrote:

5. How do I handle the back wall of a short Control Room? ... the engineer's head is going to be 5' or less from the back wall.

I agree with Jeff that broadband absorption (not bass trapping only) makes sense there. I'm surprised Jeff didn't mention that the "10 foot rule" with diffusion depends entirely on the low frequency limit of the diffusor being considered. I learned that from him!

A better "rule" for QRD type diffusors is one foot distance for each inch of well depth. I've used my company's 3-inch deep diffusor/bass trap as close as three feet away and it was definitely much better than a bare wall. Even our six-inch deep QRD is better only a few feet away than a bare wall.

--Ethan

Hmm. Okay, but - was it better than plain absorption?

Thanks again,

Kevin

Posted: Wed Apr 04, 2007 11:25 pm

Kevin_Kaster wrote:

"how do I best figure out what type of absorbers to use and where". Sabines aren't perfect but at least it gives me something to calculate.

The answer to this - and it may answer some of the stuff below as well - is "bass traps" in corners and some well-selected broadband absorption for the walls, with possibly some diffusion thrown in to "ice the cake." I often fall back on percentage coverage of walls and ceiling when trying to answer your question. Generally, a minimum of about 20-25% coverage begins to make a reasonable difference. Anymore coverage than about 75% starts to go into "overkill." Somewhere in between will be what you like. Smile

Often times, starting with the "traps" followed by treating early reflection points with some broadband absorption will get you most of the way there. The rest, typically, is "tweak." Some diffusers here, a few additional panels there. Like making a good pot of chili. A good pot of chili doesn't follow a recipe. It uses some guidelines - beans, tomatoes, meat (possibly), onions, spices - but the actual quantities are going to be the chef's call...and it could take some trial and error to get it where the chef wants it to be. I put a very precise blend of hot sauces and spices in my chili; a combination that I've spent the last few years perfecting. (It's still not "there," but much closer... Wink

) It's not going to be to everyone's taste, but (a) it is recognizably chili and (b) it is culinary ambrosia, IMHO. Very Happy

Quote:

I guess I'm a little confused by your answer. The overall feel I get is that you aren't very concerned about how flat the decay is. But, if you have a decay time of say 0.3 seconds at 250 Hz, and 0.6 seconds at 2kHz, wouldn't this sound bad?

Yes, that would sound bad. But it is highly unlikely that you would have a small room with those characteristics. Which is why these "rules of thumb" tend to work.

Quote:

Right - I'm looking to find out if "may need" is actually "do need". I was hoping to find some good data. The best I've seen is the tests Ethan did attached to his faq, but all he measured was partial room results, so that still doesn't answer my question, "do I need more than thick 703".

My answer was not intended to evade. I honestly don't know. Can you make the room sound good using only 703? Yes. Is that what you should do? I cannot be sure. But, it sounds like you don't want the room too dead. Based on that, I would advise to use some more dense materials - possibly one of the myriad products utilizing 705 with some sort of fabric covering. Something around 4" thick should pretty well cover it. In fact, applied well - in corners and at first reflection points - it could be all you need. And you needn't worry about going overboard if you follow the loose percentage "rules" above. That, and using flat panel absorbers means less off-axis absorption. Which results in a well-controlled room that doesn't sound "too dead."

Quote:

So, your reply of "could work well" begs the question - Is there a better way?

Some thoughts: As far as I know, I can only get true low-frequency control by using limp membranes, panel absorbers or tuned resonators. Apparently good limp membrane absorbers are difficult to build, because nobody does. Ethan's faq says that Helmoltz resonators are more narrow-band than panel absorbers (not terribly desirable), but Everest says that panel absorbers are low-efficiency (not terribly desirable either).

I think you can get pretty decent results any number of ways. The "could work well" was not a speculation. Please understand that my answers are always tempered - acoustics is art as much as science. If I were to tell you that so-and-so's membrane trap is 5.312% more effective at 35.8 Hz, what difference would it make? The ultimate judge of how the room sounds is you. It "could work well" because I cannot be in your head judging the outcome. Hence, my answer to your other question: There could be "a better way"! Smile

I'm not trying to be a d1ck. (But I'm sure it's working... Confused

) I just wouldn't want to lead you astray with a purely scientific reply. Yes, in the strict sense of Helmholtz devices combined with broadband absorbers, the results will be fantastic...on paper. Confused

Quote:

Understood. I guess this comes back to question 1 again. Depending on how this material actually absorbs in my room with my mounting, I could (theoretically at least) wind up with much more or much less absorption in the middle frequencies. That still seems like a bad thing to me, thus question 1.

Yes. You're grasping this stuff just fine. It could be a bad thing. (The midband over-absorption, that is - your grasping things is not a bad thing!) The only way to tell for sure is to choose products using the best information available and apply them until the room is where you want it to be. Eventually, picking ingredients has to lead to cooking has to lead to taste-testing has to lead to consuming the chili. At the final stage, whether the chef used canned or fresh peppers won't really matter - the chili is either going to taste good, or not. Unless the chef really bombed, some will think good, some not. Smile

Quote:

What about putting (diffusers) at other places in the room? Obviously the first reflections need to be absorbed. But once that is done, would there be any benefit to having diffusers, say on the rear of the side walls and ceiling? I just don't want it to sound too dead.

Well, the first reflections shouldn't automatically be absorbed, but it is generally considered the best approach for your type of application. Besides that, I think I sort of answered this above - placing diffusers around other parts of the room would be "tweak." In your case, it would be tweak applied towards the correct goal, that being a well-controlled, non-dead room.

Quote:

So when Rod Gervais states that the low string on a 5-string bass guitar will cut through the walls like a hot knife through butter (what a great quote), does he mean that the wall will absorb a maximum at this point? Or will I get no absorption at this point?

He's very much talking about transmission. Very Happy

This should not be confused with absorption. Generally, both happen when sound hits a wall. Some goes through, some bounces back, some gets sucked up. Resonating walls also tend to be good LF absorbers and also tend to allow plenty of sound pass through them. (Confused yet?)

The important thing to remember is the usefulness of fuzz in the cavities. The LF absorption stays, but the annoying "ah-oom" from the wall goes bye-bye.

I hope I am not making things worse for you!

Posted: Thu Apr 05, 2007 11:40 am

Jeff (Mr. Cow?),

> In general, QRDs are poor near field performers. <

I have to say that the listening tests I've done recently show my 6-inch deep QRD diffusors to be not as bad close up as I'd been led to believe by you and others. Twisted Evil

I haven't yet compared absorption versus diffusion close up, and I plan to. But when the choice is between diffusion and a sheet rock wall from a foot away, the diffusor wins hands down IMO.

> In the near field, "diffusion" isn't really what you get when sound "bounces" off the QRD surface. <

Yes, but you do get a reduction in comb filtering, and that's the part that seems most damaging to me in a small room where all the surfaces are nearby.

I recall a fabulous post you made somewhere about a year ago, where you explained in great detail why a QRD diffusor avoids comb filtering better than a curved poly. I have searched for that post many times because I wanted to read it again. Do you happen to recall where it is?

> But the lobing effects could still be audible for thinner diffusive surfaces, particulary for the folks sitting on the couch with the diffusers less than 1 foot behind their ears. Shocked

<

Agreed. This may be an unsolvable problem. Who do you optimize the control room for, the engineer who's making the mix decisions or the producer who's footing the bill?

> because of the relatively poor near field performance of QRD (and similar) devices, I would generally prefer broadband absorption on such a close rear wall. <

Agreed, and I explain to potential customers almost daily that as much as I'd love to sell them a diffusor, they'll do better with absorption behind their couch directly in front of a wall. Also, good diffusion costs a lot more than good absorption.

--Ethan

Posted: Thu Apr 05, 2007 1:27 pm

Ethan Winer wrote:

> In general, QRDs are poor near field performers. <

I have to say that the listening tests I've done recently show my 6-inch deep QRD diffusors to be not as bad close up as I'd been led to believe by you and others. Twisted Evil

I haven't yet compared absorption versus diffusion close up, and I plan to. But when the choice is between diffusion and a sheet rock wall from a foot away, the diffusor wins hands down IMO.

> In the near field, "diffusion" isn't really what you get when sound "bounces" off the QRD surface. <

Yes, but you do get a reduction in comb filtering, and that's the part that seems most damaging to me in a small room where all the surfaces are nearby.

Yes. Compared to a flat wall, a QRD will generally provide a better overall sound. Looking back, my posts may have been a bit misleading in this regard. (It was pretty late for my last post. I should know better! Confused

) The "lobing" effects shouldn't be as bad as a flat wall or a poly. They could be, especially close up, but there are other factors to consider. I probably should temper my language on this a bit since QRDs would be a far wiser choice than, say, angled pieces of plywood. Shocked

Of course, based on what I've learned recently, I have a strong feeling that the audible benefits from QRDs in the near field actually have more to do with the absorption they provide than because of any diffusive effects. In general, the science of it is still young. There simply hasn't been enough work done in correlating measurements, things like "scattering coefficients," and other objective measures to what the audible effects are.

Quote:

I recall a fabulous post you made somewhere about a year ago, where you explained in great detail why a QRD diffusor avoids comb filtering better than a curved poly. I have searched for that post many times because I wanted to read it again. Do you happen to recall where it is?

I don't, but it was a good post, if I recall correctly. Having heard some truly awful acoustical artefacts from things like polys and pyramidal "diffusers," I can conclude that proper diffusers are a much better way to go.

The bottom line for me is, as we've both mentioned, that cost and performance simply don't compare well between broadband absorbers and QRDs for applications like Kevin's. When faced with situations like Kevin's, I have almost always applied broadband absorption to the rear wall.

Quote:

> But the lobing effects could still be audible for thinner diffusive surfaces, particulary for the folks sitting on the couch with the diffusers less than 1 foot behind their ears. Shocked

<

Agreed. This may be an unsolvable problem. Who do you optimize the control room for, the engineer who's making the mix decisions or the producer who's footing the bill?

All the more reason to lean towards absorption. Smile