Low Frequency Absorption Methods in Small Room?

Posted: Fri Apr 06, 2007 5:13 pm

Hello All,

[This was split off a previous post, see "Difficult Acoustic Topics" at http://www.recording.org/ftopict-41527.html.]

Low frequency absorption in a small room is giving me a headache. Confused

I understand most of the methods of low-frequency absorption. What I don't know, for DIY, is how to chose which method, how much to add, and if it's a tuned system of some sort, what frequency to tune it to.

The usual tried-and-true advice is to first treat the first reflection points with pure wideband absorption, to achieve an NRZ. Got it so far, this makes sense. And, I have a pretty good feel for how this is going to affect decay times at various frequencies. But wait - note that right off the bat, this skews your decay times - short at high frequencies, long at low frequencies.

The next advice is usually to keep adding wideband absorption but in the corners, thus extending absorption down to lower frequencies, and having a higher effect on room modes (since all room modes terminate in the corners). The general rule of thumb is around 30% of the surface area in the room. Looking at it another way, that translates to enough absorption to achieve roughly 0.2 - 0.6 mS decay times, as per Everest and using the Sabine equation. Okay, I understand this reasoning, but now I'm starting to lose my feel for how this affects decay times. How low does this extend absorption, and by how much? I can't find any data on this. And the really low frequencies still aren't getting handled, I don't think.

Inserted in here might be some advice to try some good mathematical diffusors on the rear wall and/or ceiling. See the original post for ongoing conversation about that. This may help the sound, but it only muddies the waters for mid and high frequency decay times. And, this doesn't help the low frequency at all.

The last advice is usually to add tuned low-frequency absorbers of some type for the very low end. Now I'm really lost. I can calculate the frequency of the absorbers, but:
-What frequency should they be calculated to? First I have to know how low the wideband absorption in the corners is going. I can't find any information on that.
-What is the width and coefficients of absorption? I can't find much on this. Everest has some results from Russia, but only for perforated-face Helmholtz resonators. For slat-type Helmoltz resonators, closed-panel resonators (pressure resonator), and limp membrane resonators, I can't find anything. I've heard that Helmholtz resonators have a lower Q than the others (using insulation in the cavity), but I've also heard the opposite. All from very reputable sources.
-What is the efficiency of these absorbers? I can't find much on this either. I've heard that one has a wider Q than the other, and vice-versa. I've also heard that closed-panel resonators are very un-efficient, but I've seen other data that indicates they aren't.
-I've already put wideband absorbers over the corners. But oops -that's where the really low frequency absorbers should go! In fact, at least the Helmholtz resonators are stated to be mostly uneffective at low frequencies if they are on the walls.

So here is my dilemma: If I go with the tried-and-true advice (NRZ and wideband absorbers in the corners):
-I have a skewed decay rate at low frequencies
-I don't know at what frequency that skew starts at
-I don't have enough information to fix it with tuned low-frequency absorbers
-None of it matters anyway because I have no place to put them

Hopefully you can see the dilemma here, it seems like a huge problem to me. So ... how are you supposed to go about make a great-sounding small room?

Am I making this harder than it is??? How do the pros do it?

(Oh wait - Pros - SMALL rooms - those don't go together, do they Smile

)

Thanks in advance,

Kevin

Posted: Wed Apr 18, 2007 12:14 pm

this is confusing , my understanding was that small room don't have reverberation in the low frequencies...just cancellation... so how can be the low frequencies reverberation time too long?

Probably the high frequencies are too short that's all..?

Posted: Wed Apr 18, 2007 12:50 pm

You wrote:
So here is my dilemma: If I go with the tried-and-true advice (NRZ and wideband absorbers in the corners):
-I have a skewed decay rate at low frequencies
-I don't know at what frequency that skew starts at
-I don't have enough information to fix it with tuned low-frequency absorbers
-None of it matters anyway because I have no place to put them

I'll make a couple suggestions, but I'm a newbie here so beware.

First, I highly recommend that you pick up a copy of Rod Gervais book "Home Recording Studio". It's an excellent book and the answers to many of your questions are in there.

Second, it seems that you are trying to completely determine how your room is going to sound without ever being in the room. As I understand it, no one can tell exectly what frequencies will need to be addressed without actually making sounds in the room & then taking detailed measurements using sophisiticated software analysis tools.

Rod addresses this in his book pretty thoroughly.

I hope this helps.
Jim Watson

Posted: Wed Apr 18, 2007 12:52 pm

You wrote:
So here is my dilemma: If I go with the tried-and-true advice (NRZ and wideband absorbers in the corners):
-I have a skewed decay rate at low frequencies
-I don't know at what frequency that skew starts at
-I don't have enough information to fix it with tuned low-frequency absorbers
-None of it matters anyway because I have no place to put them

I'll make a couple suggestions, but I'm a newbie here so beware.

First, I highly recommend that you pick up a copy of Rod Gervais book "Home Recording Studio". It's an excellent book and the answers to many of your questions are in there.

Second, it seems that you are trying to completely determine how your room is going to sound without ever being in the room. As I understand it, no one can tell exectly what frequencies will need to be addressed without actually making sounds in the room & then taking detailed measurements using sophisiticated software analysis tools.

Rod addresses this in his book pretty thoroughly.

I hope this helps.
Jim Watson

Posted: Sun Apr 22, 2007 8:00 pm

Thanks for the replies,

I do have Rod's book. It helped fill in the gaps from the F. Alton Everest books. You're almost right, I do want to be able to determine how the room is going to sound before finishing it, but I don't need it to be perfect. I'm sure that a pro acoustical designer doesn't just throw stuff in the room and then measures it to see what happened - you know they do some calculations and have a pretty good idea of probably 90% of what is required.

But more important, I want to be able to pick the correct acoustical treatment so I don't have to redo it.

I'm sure the tried-and-true works. But I'd like to understand, in a way I can put some figures to, how it works. Everything below 125 Hz is somewhat a black hole, although I've found a few resources for open-face absorption panels:

http://www.gikacoustics.com/absorption.html
http://www.readyacoustics.com/index.php?go=acoustics-advice.acoustic-data
http://www.realtraps.com/data.htm
www.bbc.co.uk/rd/pubs/reports/1992-10.pdf
http://forum.studiotips.com/viewtopic.php?t=536
http://www.readyacoustics.com/documents/Playing-with-baffles.pdf

All of these graphs show very similar low-frequency curves, although some go lower or peak more depending on various factors.

An interesting thing I think I've noticed, is the difference in thickness/airgap, between on-the-wall applications and in-the-corner applications. For on-the-wall application, increasing the thickness/air-gap extends the low-frequency responsebut doesn't change the shape of the curve. For in-the-corner applications, increasing the thickness/air-gap increases the low-frequency absorption but doesn't seem to have a direct effect on lowering the effective frequency. That's a very sketchy conclusion drawn from just observing some of these curves, in particular the BBC data (wall-mount) and Super-Chunks data (corner-mount).

Kevin

Posted: Thu Jul 26, 2007 9:41 am

Same confuse with me..................
Gurus, help us.................

Posted: Fri Aug 17, 2007 8:45 pm

Covering the entire cieling with sound absorbing cotton can
make a dramatic improvement in a small room.

Posted: Fri Aug 17, 2007 10:42 pm

Kevin,

if you have my book and you still hae the delimina - then that tells me you never went and examined the toolbox you have access to online........
the sabin-RT60 calculator does just exactly what you're looking to do.........

PUt in your room size - along with the absorption values of your walls - ceiling floor,

and you'll see what the bare room calculates to as well as your goal for the room....... this based on the ITU values for small rooms.....

Then you will be eady to begin playing with what you want for treatments - and by entering the absorbtion at the various frequencies you can see exactly how it will affect your room in theory.........

And yes - it's just theory - the only way you can tell for sure in the real world is through physical testing........

Sincerely,

Rod

ps: you should be able to get mmost of all the data you need at Bob Golds website

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