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8th August 2019
#1
Here for the gear
Questions about the properties of air

Hello all,

Love this forum. I've been lurking here for years. Several of you have written books. I have bought and read most of them. Thanks for all the amazing information. If these questions were answered there, I apologize for missing it.

Two main questions:

1) If material cost was no issue, would 4" of OC703 or similar directly mounted to the wall absorb more that 2" spaced 2" off the wall. Or does the ability for the sound to move freely for 4" before hitting the backside of the 2" somehow increase effectivity?

2) When calculating the depth of a cavity to be used as an air spring, is it actually volume of air or actual depth? i.e. a cavity stretched across a corner - at the deepest point it could measure say 16" to the middle of the resonator face, but as you move towards the edges the depth decreases. Are we after an average (volume) or the actual depth of the deepest point?

Thanks team. I've wondered these things for a while and figured it was time to get some expert thoughts.

Bob
8th August 2019
#2
Lives for gear

Quote:
Originally Posted by BobJencks

Two main questions:

1) If material cost was no issue, would 4" of OC703 or similar directly mounted to the wall absorb more that 2" spaced 2" off the wall. Or does the ability for the sound to move freely for 4" before hitting the backside of the 2" somehow increase effectivity?

2) When calculating the depth of a cavity to be used as an air spring, is it actually volume of air or actual depth? i.e. a cavity stretched across a corner - at the deepest point it could measure say 16" to the middle of the resonator face, but as you move towards the edges the depth decreases. Are we after an average (volume) or the actual depth of the deepest point?
1. With the proper material, the same. Perhaps not clear to some readers the sound travels through the 4" also to the backside. BBC has used absorbers with gap to absorbent ratios as high as 5:1. Galaxy studios has up to 2:1.

2. If it is a membrane absorber then the cavity volume.

If porous material then the depth at the height of the triangle. Many people have problems thinking of corner as reverse horn, focusing the sound into the corner.

Enjoy!
9th August 2019
#3
Here for the gear
Thank you

1) I'm not sure I understand how the two conditions could be the same - with one traveling through 4" of absorption and the other through 8" of absorption - but this seems to be the generally understood theory. Even if it's not understood by me!

2) This makes sense in both the equation and in theory. My confusion is in one publication (I think it was Rod's?) or another there was a helmholts (slat) resonator built into a corner mentioned as a broadband absorber. I was under the impression they were fairly specific in frequency (therefor not broadband), so I considered if the gradual depth change from each individual slat(er... slot) to the back wall as it followed the slope caused it to absorb a wider q. It now seems silly as the mass of air as a whole acts together. That's why I thought I'd ask.

I appreciate you taking the time to answer.

Bob
9th August 2019
#4
Lives for gear

Bob, I get the feeling that avare may have misunderstood your first question, at least he understood it different to me.

4” of OC703 up against the wall will outperform 2” of OC703 placed 2” away from the wall.

2” of OC703 placed 2” away from the wall will outperform 2” of OC703 placed against the wall.

Last edited by Starlight; 9th August 2019 at 07:19 PM.. Reason: Clarity
9th August 2019
#5
Lives for gear

Quote:
Originally Posted by Starlight
Bob, I get the feeling that avare may have misunderstood your first question, at least he understood it different to me.

4” of OC703 up against the wall will outperform 2” of OC703 placed 2” away from the wall.
You are trying to explain your thoughts by twisting my answer. I started with "With the proper material." If you halve the thickness the proper material changes. The old 703 had a GFR of 16,000 Rayls/m. 4" thick would have a resistance 1,600 Rayls. The proper 2" thick material would have a GFR of 32,000 Rayls/m to match the old 703.
9th August 2019
#6
Lives for gear

Sorry, Andre, I did not mean to twist your answer, rather I tried to answer Bob’s question in simpler terms as his reply to you sounded like he was unsure.
17th August 2019
#7
Lives for gear

Quote:
Originally Posted by BobJencks
1) I'm not sure I understand how the two conditions could be the same - with one traveling through 4" of absorption and the other through 8" of absorption - but this seems to be the generally understood theory. Even if it's not understood by me!
I'm not sure if this is what you are referring to, but I did a graph a while back to show the predicted difference for several similar situations, all of them being 6" deep. There are various combinations of layers of porous absorber with various combinations of air gap, but all of them total 6" thick. The graph is below. As you can see, the frequency peak is the same in all cases: that changes is how effective the absorber is, plus some "wiggly stuff" due to resonances in the air cavities. But the basic overall behavior is the same. If you make the total depth thicker, the all of those curves move over the the let (lower frequencies). If you reduce the total depth, all of the curves slide over to the right.

Once again, I'm not sure if that was your question, but hopefully it's useful anyway!

Quote:
2) ... My confusion is in one publication (I think it was Rod's?) or another there was a helmholts (slat) resonator built into a corner mentioned as a broadband absorber. I was under the impression they were fairly specific in frequency (therefor not broadband), so I considered if the gradual depth change from each individual slat(er... slot) to the back wall as it followed the slope caused it to absorb a wider q. It now seems silly as the mass of air as a whole acts together. That's why I thought I'd ask.
That's a tougher one! And as with most other answers in acoustics, the answer here is the same: "Sort of maybe yes no it all depends"...

Not very helpful!

Yes, slot walls can be tuned. But they can either act mostly as a bunch of individually tuned devices, each slot tuned to one frequency, or they can act as a broadband absorber, where the entire wall works as one, with a sort-of center frequency and a broad but lower Q. One of the factors involved in determining that, is the "percentage open area". In other words, how much of the total front surface of the device is air gaps, expressed as a percentage. If the total open area is low, then the device acts more like a series of individual
Helmholtz resonators. If the percentage is high, then it acts more like a single device tuned broadband. And if the percentage is very high, then it doesn't resonate at all, and just mostly absorbs lows while mostly reflecting highs.

I don't recall off-hand the percentages where one effect becomes dominant. I'll see if I can find that. But as far as I recall, above about 20% open area it basically acts like a low pass filter, where the slat width is mostly what affects the center frequency. And if it less than something like 5%, it acts like individual resonators.

I may be wrong on some of the above: I don't have all of my reference material accessible right now, to check on that. Hopefully I'm not too far off!

Once again, I'm not sure if that was your question, but maybe it's useful anyway...

- Stuart -
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Last edited by Soundman2020; 17th August 2019 at 08:35 AM..
17th August 2019
#8
Lives for gear

Quote:
Originally Posted by Soundman2020
The graph is below.
Did you forget to attach a graph, Stuart?
17th August 2019
#9
Lives for gear

Quote:
Originally Posted by Starlight
Did you forget to attach a graph, Stuart?
Darn! Yes I did... Fixed it now! Thanks for letting me know...

- Stuart -
17th August 2019
#10
Gear Addict

From 3rd edition of "Acoustic Absorbers and Diffusers" by Cox & D'Antonio:

"The absorbers exploit mechanisms that have been understood for more than a century. One problem with this type of construction is getting the perforated sheet with the correct hole size and open area. Standard perforated board, such as peg board, has too small an open area, and most perforated metals have too large an open area. Consequently, the perforated sheet often has to be specifically constructed for acoustic purposes, which makes it more expensive. However, computer numerical control (CNC) fabrication offers an unlimited choice of design parameters." and:"The hole spacing should be large compared to the hole diameter."

Above roughly 20% open area, the perforated panel becomes more and more transparent to "higher frequencies".
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