Author Topic: absorption coefficients  (Read 3389 times)

Lee

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absorption coefficients
« on: August 28, 2001, 11:02:11 PM »
My understanding of absorption coefficients is that they are based on a "percentage system" i.e. an absorption coefficient of 0.5 represents 50% of the sound energy being absorbed at that particular bandwidth, an absorption coefficient of 0.75 represents 75% of the energy absorbed, and an absorption coefficient of 1 representing 100% of the energy absorbed. My question is: why do some acoustical treatment devices, sonex etc, have absorption coefficients that go above 1, some as high as 1.2 (indicating 120% of the energy absorbed?)Thanks for your time.

Jeff D. Szymanski

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Re: absorption coefficients
« Reply #1 on: February 14, 2002, 03:51:00 PM »
: My understanding of absorption coefficients is that they are based on a "percentage system" i.e. an absorption coefficient of 0.5 represents 50% of the sound energy being absorbed at that particular bandwidth, an absorption coefficient of 0.75 represents 75% of the energy absorbed, and an absorption coefficient of 1 representing 100% of the energy absorbed. My question is: why do some acoustical treatment devices, sonex etc, have absorption coefficients that go above 1, some as high as 1.2 (indicating 120% of the energy absorbed?)Thanks for your time.
Actually, this is a common miconception that even I believed until very recently. In actual fact, the absorption coefficients and hence the "NRC" of a material represents nothing more than a relative absorptive effectiveness in a certain frequency band. Absorption materials are measured (usually) in accordance with ASTM Standard C423. Nowhere in that standard is it stated that the absorption coefficients represent a "percentage" of sound absorbed. The formula used to calculate the absorption coefficients is based on Sabine's orginial RT60 equation. This equation also has no mention of a "percentage" of sound absorbed. (For a more complete treatment of this, see "The Sabines at Riverbank" by John Kopec - published by the ASA.)
An unfortunate result of this misconception is that many consultants will treat coefficients greater than 1.0 as "0.99". If they are using Sabine equations to predict the RT in a space, there would be no harm in using numbers greater than 1.0. This is simply the reverse application of the methods used to determine the absorption coefficients in the first place.
Finally, I believe one of the reasons that absorption coefficients have been misperceived as "percentages" is because if certain assumptions are made, absorption coefficients are used to calculate sound levels before and after absorption is added. Since this is ultimately an application of the wave equation, using "percentages" greater than 100% doesn't make mathematical sense. Hence numbers greater than 1.0 are rounded off to 0.99. I am still on the fence on this one. Since the same method is NOT used to determine the absorption coefficients to begin with, I do not believe you can use absorption coefficients on the "back end" to calculate this. The coefficients would have to be derived using a method other than Sabine's in order to have numbers that would work in this fashion. (And these numbers WOULD always be less than 1.0.)
This may seem complex, but if you reference Chapter 4 of the latest (3rd) edition of the "Handbook for Sound Engineers" which I cowrote with Doug Jones, you can get some idea of the comparison between a measurement of absorption in terms of Sabine versus a measurement of absorption in terms of differences in sound intensity (and hence SPL). This treatment of absorption coefficients is not as thorough as a paper Doug and I plan on preparing for AES very soon.
If you have more questions about this, I can be contacted directly at consulting@auralex.com.
Best regards,
Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.