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Author Topic: The effectiveness of an air gap.  (Read 4404 times)

jetbase

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The effectiveness of an air gap.
« on: January 22, 2007, 09:33:49 pm »

I have a large-ish booth (or small room) behind my control room, and in thinking about how to improve the sound & versatility of this room. In this context I would like to ask:

What is the effect of an air gap behind absorption in a room? I think it was Ethan who posted somewhere that generally you would use an air gap of the same thickness as the material in front of it (e.g. 4" absoprtion with 4" air gap behind it). How exactly does this improve the absorption? Is it better than double the thickness of absorption or is it just cheaper?

If you had, say, only 4" of space to spare on a wall in a room you wanted to treat using 703 or similar, what do you think would be the difference between the following:

1. 4" of absorption against the wall
2. 3" of absorption + 1" air gap
3. 2" of absorption + 2" air gap

Also, does the air gap have to be sealed (around the sides) to be effective, or could the absorption simply be spaced from the wall?

Thanks,
Glenn
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J.F.Oros

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Re: The effectiveness of an air gap.
« Reply #1 on: January 23, 2007, 04:34:48 am »

Using air space it's just a little less efficient, but alot cheaper  Smile

I don't have real measurement data on this, but you can play with the verry good absorber simulator of Chris Whealy :

http://www.whealy.com/acoustics/Porous.html
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jimmyjazz

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Re: The effectiveness of an air gap.
« Reply #2 on: January 23, 2007, 11:39:09 am »

Within limits, hanging the absorption off the wall puts the absorptive material in a location where the particle velocities are higher; hence, more absorption takes place.  (At a rigid wall, the particle velocity is zero.  With no motion, there is no kinetic energy to convert to heat.)  This effect should mostly be noticed in the low frequencies.  As frequency rises and the wavelengths of the sound waves start to approach the absorber thickness and/or spacing off the wall, you shouldn't detect a difference.  (Note that the wavelength of a 100 Hz tone is roughly 3.4 meters, so we're still talking about small fractions of a wavelength in the bass region.)

I doubt you'd find much measurable difference between the 3 setups you described, except if you're measuring cost.  I do think the 4" absorber mounted on the wall would be better than the other two options, barring impedance-matching issues, but "better" would probably be fairly slight.  I'd like to see some real data to back up my hunch, though.
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jetbase

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Re: The effectiveness of an air gap.
« Reply #3 on: January 24, 2007, 06:09:15 pm »

Thanks for the replies. Sounds like it would be easier to just use the full thickness of absorption.
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LSilva

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Re: The effectiveness of an air gap.
« Reply #4 on: January 25, 2007, 10:18:45 am »

jimmyjazz wrote on Tue, 23 January 2007 11:39

I do think the 4" absorber mounted on the wall would be better than the other two options, barring impedance-matching issues, but "better" would probably be fairly slight.  I'd like to see some real data to back up my hunch, though.


So how about 4" with a gap?

I only ask becuase I currently have some 1" sheets of 703 that are mounted with a gap which I plan to bulk up to 4".

If do 4", should I even bother with the gap?

Thanks,
Lou
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jimmyjazz

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Re: The effectiveness of an air gap.
« Reply #5 on: January 25, 2007, 11:48:33 am »

Sure, I think the gap will help even with 4 layers of 1" 703.  (Make sure the 703 doesn't have a foil facing!)

Again, look at the wavelengths involved.  If the particle velocity is zero at the wall, then it's at a peak 1/4 wavelength away.  Hanging 4" of absorber 4" from the wall would put the middle of the panel 6" from the wall.  The frequency which has its 1/4 wavelength at 6" is ~ 562 Hz.  You'll bump the lab performance up above that frequency, and see less and less augmentation of the absorption as you go lower into the bass frequencies, but it will help.  (At least it should.)

One caveat:  these simple calcs are for normal incidence (perpindicular) only.  There is one school of thought that oblique angles of incidence "use more" of the absorbing material, and that this helps explain what seems to be excessively good performance at lower frequencies.  In other words, the wavelength calculations are overly pessimistic.
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franman

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Re: The effectiveness of an air gap.
« Reply #6 on: January 28, 2007, 03:30:28 pm »

I'm down with everything that Jimmy has already explained.. one other point to consider: The air gap can sometimes cause a little comb filtering in the frequency response.. Lars is really hot about this these days and we're gonna try some experiments to try and documents it (some day!!)..

Generally the air gap will increase LF bandwidth on your absorber but the angle of incidence discussion is very valid.. How much sound is hitting the porous absorption at a right angle?? Not that much so your effective depth is typically much deeper than the nominal thickness of the material being used...

Food for thought!
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jimmyjazz

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Re: The effectiveness of an air gap.
« Reply #7 on: January 29, 2007, 09:04:19 pm »

Fran, that's interesting . . . any thoughts on the mechanism which causes the comb filtering?
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Ethan Winer

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Re: The effectiveness of an air gap.
« Reply #8 on: January 30, 2007, 06:50:55 pm »

jimmyjazz wrote on Mon, 29 January 2007 21:04

Fran, that's interesting . . . any thoughts on the mechanism which causes the comb filtering?


Jimmy, see the section Optimizing the Air Gap in my Acoustics FAQ for a detailed explanation:

http://www.ethanwiner.com/acoustics.html

--Ethan

jetbase

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Re: The effectiveness of an air gap.
« Reply #9 on: January 30, 2007, 07:24:18 pm »

Ethan, I just read the section you linked to. In relation to sealed membrane traps, I understand the reason they must be sealed. Does this, however, mean that porous traps are less efficient, or are they a different beast entirely?

Incidentally, I am in the process I treating my control room in a more permanent manner. At first reflection points on the side walls I am putting 2" of Tontine Acoustisorb & 1" of 703, spaced out 1" from the wall (making the most of materials I already have). I intend to cover it with fabric to make it look nice. I'll hopefully finish it off tonight & see what kind of a difference it makes.
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jwhynot: "There's a difference between thinking or acting dogmatically and drawing from experience."


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Ethan Winer

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Re: The effectiveness of an air gap.
« Reply #10 on: February 03, 2007, 02:27:12 pm »

Glenn,

> In relation to sealed membrane traps, I understand the reason they must be sealed. Does this, however, mean that porous traps are less efficient, or are they a different beast entirely? <

Wood panel traps and fiberglass-only traps are not only different, they work on opposite principles. Wood panel traps can be made more effective at very low frequencies without having to be three feet thick, but smaller rooms usually benefit more from broadband fiberglass-based traps.

--Ethan

franman

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Re: The effectiveness of an air gap.
« Reply #11 on: February 04, 2007, 01:01:15 pm »

I'm gonna ask Lars to explain his feelings on the air gap and comb filtering... it's his soap box, not mine!!

Lars??
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L_Tofastrud

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Re: The effectiveness of an air gap.
« Reply #12 on: February 05, 2007, 05:48:28 pm »

The "comb filter" effect is easily illustrated in the absorber simulator spreadsheet mentioned in an earlier posting.

With a 4" gap and 4" glass-fiber there is not much of an effect to see but it becomes quite dramatic with thinner absorbers and larger distances.

Excellent spreadsheet.  Smile

Lars Tofastrud
Senior Acoustician - FM Design Ltd
www.fmdesign.com
Director - Griffin Audio USA LLC
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J.F.Oros

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Re: The effectiveness of an air gap.
« Reply #13 on: February 05, 2007, 08:18:34 pm »

L_Tofastrud wrote on Tue, 06 February 2007 00:48

Excellent spreadsheet.  Smile

Yes indeed, it's a verry useful tool. At least for visualising the way absorbtion varies on different devices and parameters.

I think Chris made it using the formulae published by Cox and D'Antonio in "Acoustic Diffusers and Absorbers"
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jimmyjazz

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Re: The effectiveness of an air gap.
« Reply #14 on: February 06, 2007, 10:50:54 am »

I'm sorry, but what spreadsheet are you referring to?  Can you post a link?
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