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Sinusoidal
#1
18th June 2013
18th June 2013
#1
Gear interested

Joined: Sep 2008
Location: Melbourne
Posts: 5

Calculating mean absorbtion coefficient

Gd Day,
I have absorbtion coefficients of a material available to me at thickness of 25mm , 50 mm and 100 mm respectively.
Is there a way to calculate the absorbtion co-efficient of the same material at 150mm using the data available ?

Cheers
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John White
#2
18th June 2013
18th June 2013
#2

Joined: Feb 2011
Posts: 487

Quote:
Originally Posted by Sinusoidal
Gd Day,
I have absorbtion coefficients of a material available to me at thickness of 25mm , 50 mm and 100 mm respectively.
Is there a way to calculate the absorbtion co-efficient of the same material at 150mm using the data available ?
You may be able to extrapolate using the data. Approximately- the absorption will drop an octave for each doubling of thickness. Gas flow resistance will not be relative between thicknesses, however due to binding material, etc., so for a given material a smaller thickness would typically have a greater GFR.

According to the charts on Bob Gold's page we see some confirmation:
Using OC at 3pcf or 48kg/m3 we see an absorption coefficient of .11 for 1" at 125Hz.
At 2" we have .17 so not quite.
At 4" it's .84 so not linear in the least at this frequency.
At 3" it's .53 and at 6" 1.19. There you go.

The Porous Absorber Calculator will show similar trends.

Would you post it here for us to see how this works out compared to theory?
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Sinusoidal
#3
19th June 2013
19th June 2013
#3
Gear interested

Joined: Sep 2008
Location: Melbourne
Posts: 5

Quote:
Originally Posted by John White
You may be able to extrapolate using the data. Approximately- the absorption will drop an octave for each doubling of thickness. Gas flow resistance will not be relative between thicknesses, however due to binding material, etc., so for a given material a smaller thickness would typically have a greater GFR.

According to the charts on Bob Gold's page we see some confirmation:
Using OC at 3pcf or 48kg/m3 we see an absorption coefficient of .11 for 1" at 125Hz.
At 2" we have .17 so not quite.
At 4" it's .84 so not linear in the least at this frequency.
At 3" it's .53 and at 6" 1.19. There you go.

The Porous Absorber Calculator will show similar trends.

Would you post it here for us to see how this works out compared to theory?
thanks John, the porous absorber calculator is quite resourceful however i dont quite understand what u ment by absorbtion dropping by an octave ...
id be happy to post my extrapolation from the data i have already or even prepare a model for extrapolating data if i can find a mathematical pattern as this help people building panels/traps over 200-250mm thick
John White
#4
19th June 2013
19th June 2013
#4

Joined: Feb 2011
Posts: 487

Quote:
Originally Posted by Sinusoidal
what u ment by absorbtion dropping by an octave
Yeah that's probably a stretch, but fairly close. As a matter of fact if we look at just the absorption coefficients of most any of the products in those charts, we'll see that from 125Hz, the absorption doubles per octave pretty accurately up to the point of full absorption.

But again when dealing with thickness, it's just not linear as evident in both the PAC and the charts.