Help me understand this acoustic report

[Revolite]

I am considering some room treatment mainly for the purpose of reducing reverb. This product has provided a testing report and sound absorption coefficients but I don't understand enough to know if they are good or ok. Can someone please explain or point me in the right direction to learn what this all means?

Are similar MDF cladding solutions with a polyester backing likely to have similar performance? I have seen a range of pricing for this sort of thing.

 

[MarsianC#]

200 mm air gap makes no sense for this kind of panel since they are usually glued directly to the wall.

Helmholtz Absorber Calculator

As an example, I have simulated a panel with and without rear air gap. Felt has a rather high flow resistivity, guesstimated the rest. 21 mm is really thin!

 

[Revolite]

I saw this table that makes me think an a=0.7 should be good. But are you saying that the original testing method is artificially inflating the number?

Acoustic Treatments – Acoustical Coefficients
Cellulose Fibers (1″)	0.75
Polyurethane Foam	0.3
Semi Rigid Fiberglass (1″)	0.75
Cork Tiles	0.7
Acoustic Ceiling Tiles	0.5

 

[kemmler3D]

I am considering some room treatment mainly for the purpose of reducing reverb. This product has provided a testing report and sound absorption coefficients but I don't understand enough to know if they are good or ok. Can someone please explain or point me in the right direction to learn what this all means?

Are similar MDF cladding solutions with a polyester backing likely to have similar performance? I have seen a range of pricing for this sort of thing.

The way they tested this is not super realistic as @MarsianC# mentions.

If we assume it works the same way in-room, (doubtful) you still have a peak of absorption at 400hz with even-ish but not very strong absorption up to 5khz, beyond which they don't seem to test. I think in a real room it's going to absorb a bit up to 5khz (probably beyond) and basically not much below 500hz.

Depending on the room and application I might not buy this treatment.

What room is this for, what are the current problems, what's your budget, etc?

 

[Revolite]

It's just a study sized room in a normal house, not a studio or anything fancy. 2 walls are mainly glass and 2 walls are mainly plaster with hardwood floors.

 

[Keith_W]

I saw this table that makes me think an a=0.7 should be good. But are you saying that the original testing method is artificially inflating the number?

0.5

Yes that is exactly what he is saying. A coefficient of absorption ranges from 0 (100% reflection) to 1 (100% absorption).

 

[HiMu]

Yes that is exactly what he is saying. A coefficient of absorption ranges from 0 (100% reflection) to 1 (100% absorption).

Absorption coefficient is not a percentage. Think of it through this analogy: If you have 1l or gal of water in a jug and pour it out based on AbsCo value. With 1.9 You would pour out 190% of the existing water.
Screenshot from GIK acoustics.

 

[Rick Sykora]

It's just a study sized room in a normal house, not a studio or anything fancy. 2 walls are mainly glass and 2 walls are mainly plaster with hardwood floors.

That room sounds like quite a challenge! Would suggest modeling with REW's room simulator. It will help better characterize your biggest issues and then applying room treatments more effectively. Some of the best advice I have ever received is to ensure you understand what (the problem really is) before determining how (to solve it). In this case (as in others), likely may need more than one solution too.

Is there sound system involved or just trying to make the study itself less reflective?

 

[Flaesh]

I would suggest taking the microphone and whistling. Sorry for the tediousness, repetition and monotony .

 

[NTK]

Absorption coefficient is not a percentage. Think of it through this analogy: If you have 1l or gal of water in a jug and pour it out based on AbsCo value. With 1.9 You would pour out 190% of the existing water.
Screenshot from GIK acoustics.
View attachment 399257

See this for an explanation of why (edge diffractions enlarge the effective area of the absorber under test) measured absorption coefficient can be >1.

https://sengpielaudio.com/AbsorptionsgradGroesserEins.pdf

Also, other effects can also be in play: https://www.acoustics.asn.au/conference_proceedings/AAS2012/papers/p55.pdf

 

[HiMu]

That would still mean that if AbsCo 1 = 100%, the tested sample absorbs more acoustic energy that is being radiated to the test environment.

The following is presented as educational. This is based on
the latest research in the field of acoustics.

A common misunderstand of absorption coefficients is that
they are a percentage. This is false. It was never devised as a
percentage but rather a multiplier in a formula; coefficient - a
numerical or constant quantity placed before and multiplying
the variable in an algebraic expression.
The absorption coefficient of 1.0 equals the attenuation of 10
dB at the frequency tested. For example:
0.999 = -9 dB
0.99 = -8 dB
0.9 = -6 dB
0.8 = -4.81 dB
0.7 = -3.68 dB
0.6 = -2.71 dB
0.5 = -1.88 dB
0.4 = -1.18 dB
0.3 = -0.61 dB
0.2 = -0.21 dB
0.1 = -0.1 dB

Source: https://jhbrandt.net/wp-content/uploads/2022/11/Absorption_Coefficient_Constant.pdf

Ever since 1929, when the “absorption coefficient”
was proposed by Paul Sabine, there have been
numerous papers (132) written about the problems
with this “constant”.

Source: Ron Sauro“Absorption Coefficient”: Dead or Alive? (Results of the “ASTM C423 ILS#898 (Inter Laboratory Study)

NWAA Labs -- Complete High-speed Speaker and Material Testing

Spoiler

I'm currently building diffraction absorbers / geometric scattering panels in my workshop.

 

[amirm]

As noted, the main take away from this report is that the absorber has peak efficiency around 400 Hz. And continues to have good absorption above that. Going below 400 Hz, it also has decent absorption to be "broadband." The uneven response is typical of many absorbers and hence the reason you have to be careful in wall papering the whole room with it.

As also noted, just run REW and measure Topt. If it is well above 0.5 seconds in the range above 300 Hz and is an average room, then you should add some absorption in the form of furnishings (e.g. thick carpet) and/or absorber panels.

 

[Rick Sykora]

I saw this table that makes me think an a=0.7 should be good. But are you saying that the original testing method is artificially inflating the number?

Acoustic Treatments – Acoustical Coefficients
Cellulose Fibers (1″)	0.75
Polyurethane Foam	0.3
Semi Rigid Fiberglass (1″)	0.75
Cork Tiles	0.7
Acoustic Ceiling Tiles	0.5

Yes, an NRC of .7 is pretty good, but worth knowing conditions matters too. Here is a more useful look at Acousta-Blue as an example..