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#### Dennis Erskine

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In some of my posts about solving reverberation time, boundary effect, and right tricorner problems, I've referred to Sound Absorption Coefficients. Apparently, the audience is listening based upon the e-mail I get with some rather common questions (as people do their homework I guess).

Since these questions keep coming up via e-mail, so I'll answer them for all to see.

The questions are: how can the sound absorption coefficient be greater than 1; what is the NRC number vs the absorption coefficients; and, what does the number actually mean?

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NRC or absorption coefficients at various frequencies can be greater than unity (> 1) as a result of the standardized measurement methodology. In the standard test, an 8' x 9' sample of the material is laid on the reveration room floor and the decay rate is measured. When the coefficient is greater than 1, what it is saying, is the 8'x9' patch of material is acoustically larger than the geometric 72 sq feet usually due to diffraction at the edges of the sample.

The absorption coefficient is a measure of the efficiency of a surface in absorbing sound. If 70% of the incident sound energy is absorbed, the coefficient is said to be .70.

The NRC (as opposed to the coefficients given at the six standard frequencies) is simply the average of the coefficients at 250, 500, 1000, and 2000 Hz to give a single number (in the days before visiCalc?).

The coefficient of a material will vary with the mounting method. Typically what you see is for a Type A mounting (mounted directly to the wall or surface behind the material with no air gaps). Somebody (sorry, I forget who) asked about material mounted several inches from the wall. The E-405 mounting is for absorbing material mounted 16" from a hard surface...so, whoever asked, have the manufacturer give you the absorption coefficients for a Type E-405 mounting.

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D. Erskine

DEsign Cinema Privee
www.DEsignCinema.com

Imagine what you could do, if you could do all you imagine.

#### Don O’Brien

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Thanks for the explanation Dennis. I never quite understood an absorbent coefficient greater than 1.

Don O

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