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Sound absorption panel coefficients

slaweks

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Hi,
Over last 2 years I installed quite a few (a few dozens) of 4" sound absorption panels from ATS Acoustics. Most of them of "Bass trap" category with the absorption coefficient advertised as in below image:
ats_bass.png

and others normal, absorbing less lower frequencies.

ats.png


So, first of all, I do not understand how the absorption coefficient can be higher than 1.
Secondly, these absorption coefficient, especially from the "full-range" variety, seem to be well higher than allowed by the 1/4 of wavelength rule.

But they seem to be working, as advertised. This is one of the earliest measurements in my 36*17' room:

ts60_early.png


and this is one the recent ones:

ts60_recent.png




They seem to be reducing reverberations in broad range, up to 60Hz, perhaps.
So, how can this "Physics laws defying" behavior be explained? :)
 

DonH56

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Testing errors or relative comparisons. I have seen a couple of explanations over the years. A quick search turned up this one:

However, you may come across materials with NRC ratings higher than 1.0, which can be confusing. This doesn’t mean that the material can absorb more sound than that arriving at the material; instead, it’s an issue with the testing standards used to determine the NRC rating. The perimeter and thickness of the material being tested will cause an “edge effect” (diffraction), which can result in errors in the calculation used to arrive at the NRC rating, yielding results above 1.0.

Wikipedia says something similar:
Materials can achieve NRC values greater than 1.00. This is a shortcoming of the test procedure and a limitation of how acousticians define a square unit of absorption, and not a characteristic of the material itself.
 
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slaweks

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Testing errors or relative comparisons. I have seen a couple of explanations over the years. A quick search turned up this one:



Wikipedia says something similar:

I would easily accept say 1.02, but 2.04?
 

Cbdb2

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I would easily accept say 1.02, but 2.04?
You would think the people doing the testing would know that a result of 2 is wrong. Makes me think there stupid or liars, either way steer clear.
 
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slaweks

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You would think the people doing the testing would know that a result of 2 is wrong. Makes me think there stupid or liars, either way steer clear.
Sure. But despite of this, the panels seem to be working.
 

Sancus

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So, how can this "Physics laws defying" behavior be explained?
I don't get how this defies physics? Even panels that absorb very little of a frequency will still absorb some if you shove enough of them into a room. Absorption isn't a binary yes/no effect.

That doesn't make it a good way to solve problems at low frequencies.
 
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slaweks

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I don't get how this defies physics? Even panels that absorb very little of a frequency will still absorb some if you shove enough of them into a room. Absorption isn't a binary yes/no effect.

That doesn't make it a good way to solve problems at low frequencies.
Some people believe that absorber needs to be 1/4 of the wavelength to absorb it, clearly it is not true.
 

Sancus

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Some people believe that absorber needs to be 1/4 of the wavelength to absorb it, clearly it is not true.
Well it's not just thickness of the absorber that matters, it's also distance from the room boundary. And it's obviously a gradient of effect, they don't simply turn off when the wavelength reaches a threshold.

So, sure, the idea there's some hard rule is not true, that's correct. I don't think any quality source actually claims there is such a hard rule, though. Toole's book certainly says nothing of the sort.
 
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slaweks

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I don't get how this defies physics? Even panels that absorb very little of a frequency will still absorb some if you shove enough of them into a room. Absorption isn't a binary yes/no effect.

That doesn't make it a good way to solve problems at low frequencies.
I was not after just low frequencies, I wanted drop in reverberations across the full rage, and these 4" panels delivered, of course less in <100Hz range.
 

ozzy9832001

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I've used ATS panels before. Much better company to deal with than some of the others, but they lack selection.

That being said, their 4" 2'x4' are great, but their mounting system is a bit wonky, so I used my own.
 
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