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Old 24th July 2019
  #11
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Quote:
Originally Posted by Soundman2020 View Post
Simple simulation models only take into account generic parameters. They don't take into account the specifics of individual products. Thus, they produce incorrect results for specialized products that have been specifically designed for improved absorption at low frequencies. There's more to the how an absorber works, than just pure Gas Flow Resistivity! There's things like porosity, tortuosity, viscous length, thermal length, etc. The type of fiber used, the type of binder used, and the manufacturing process all have an effect on the acoustic properties that is beyond the simple measurement of gas flow resistivity. (You are also using the wrong flow resistivity for OC.703: it's considerably higher than what you used.)

The model you used does not take those additional factors into account. It ONLY considers the gas flow resistivty, and thus produces generic predictions based on that alone. You need a more complex model that considers all the other factors as well, to come up with a valid answer. I'm not saying your model is not good: I do use that one myself when I just need a quick ball-park prediction for something, but then I can refine the prediction with more accurate simulations if I need to. I don't rely on just simple, basic models when I need higher accuracy.


- Stuart -
I am not going to comment on the data. I showed you a link to the same manufacturer pointing to 703, not any other product, a porous absorber.

Anyways, regarding your last words: you cannot escape physics. The flow resistivity is the effective predominant parameter for a porous absorber. No technology will have the potential to increase performance down to such low frequencies, and based on what? On a strong membrane effect? Even FRK is only a light paper. What effect shall be responsible for such a performance increase? I don't believe it, there is no hidden magic, and if there was, it would have been reported by others in other books and papers.

If 4 inch fiberglass was so effective, you could place it against the front wall to suppress comb filtering of the front wall, and place the speaker somewhere 85 cm away from the front wall killing the reflection at 100 Hz. Does it work? Any source that this works? No, because the data is based on hall room measurements, and apparently doesn't apply, your 4 inch fiber glass will fail.

So lets start over and taker a look at hall room measurements.

First of all, I notice that in the US using sabins, makes things unnecessary complicated because I have to look for the size of the absorber as a test unit in order to derive a relative absorption area comparable to an absorption coefficient.

Lets first look how to define the absorption coefficient according to ASTM C432
(https://www.astm.org/Standards/C423.htm)

I dont wanna pay 50$, but this link gives a hint:

https://www.acousticalsurfaces.com/e...AS_SA1448C.pdf

So they divide the effective area of perfect absorption by the area of the test unit to obtain the coefficient.

1.)

For GIK monster bass trap https://static.gikacoustics.com/wp-c...-Datasheet.pdf), the peak absorption is

25 sabins at 80 Hz, most probable for a test unit of size (the make it a little unclear) 24.25 inch x 48.5 inch = 8.1 square feet.

Thats a factor or 25/8.1 = 3.1

2.)

For a RealTraps Mega trap, "the best bass trap ever made" (https://realtraps.com/p_megatraps.htm), we have

an area of 4 feet x 34 inch = 11.3 square feet for the test unit in the reverberation chamber and 28 sabins peak absorption resulting in a factor of

28/11.3 = 2.5

3.)

For a plate absorber based on a German patent made by Fraunhofer (https://patents.google.com/patent/DE10213107A1/de) we have a factor of 2.7.

They mention that the placed the absorbers in the corners.

So it seems that companies and others place their test absorbers in the corners in the reverb chambers in order to maximize low frequency performance, and this boosts the effectiveness by a factor of around 2.5 to 3, probably depending in frequency and the particular room, being modal at that frequency.

I assume that the mentioned absorbers would have an absorption coefficient approaching unity if measured with a tube (normal incidence, direct measurement of reflected wave).

If the room boosts by a factor of 2.5, the real absortion coefficient would then by around 1.

On the other hand, if some company publishes data with equivalent area around 1, this would mean that dividing by the boost factor of the room gives a real absorption of around 0.4, just as the model dictates.

But this only(!) applies to data at "low frequencies" (125 Hz), not high frequencies (anything above and including 500 Hz) where the room is NOT modal and does not have such a boost in the corners.

So the model cannot be that wrong. There is no magic, it is just the way things are measured, and as the standard says (https://www.astm.org/Standards/C423.htm):

5.4 The coefficients measured by this test method should be used with caution because not only are the areas encountered in practical usage usually larger than the test specimen, but also the sound field is rarely diffuse. In the laboratory, measurements must be made under reproducible conditions, but in practical usage the conditions that determine the effective absorption are often unpredictable. Regardless of the differences and the necessity for judgment, coefficients measured by this test method have been used successfully by architects and consultants in the acoustical design of architectural spaces.


No more, no less. Again: you will, most probably be not able to use 4 inch fiber to kill a reflection at 125 Hz even if data will tell you otherwise. So a standard text book should point this out right away, otherwise it is just misleading. And what I therefore believe is that there is no hidden performance boost in the manufacturing of glass fiber to appear so "powerful".