Author Topic: Trying to figure out Noise reduction coefficients, sound attenuation coefficient  (Read 4809 times)

Lenny

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And related stuff.

Does anyone have any good reading material they could point me to on the subject? (it really does not help that there are so many different names for the same thing sometimes).

I understand why the measured sound absorption  coefficients can be more than one, with the refraction at the edge (and it is always rounded down to one in that case).

But.  How is it possible for the coefficient to be one in the first place, the equivalent of an open window, with *no* sound reflected??? There must be some, always.  But that is what it says right in the definition.  What I think they must mean is that it is, for the purposes of measuring the reverberation time, which because it measures the time till the sound energy is reduced to 1/1,000,000 or the original value ( 60 db reduction) then a sabine of 1 must mean the reflected sound wave was reduced by 60 dB.   Is this correct?

Lenny

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Ans even if it is correct, the numbers in this table http://www.bobgolds.com/AbsorptionCoefficients.htm seem to indicate a very high level of soundproofing, on a pound per square foot basis.  And also in terms of the thickness, the second last entry in the first table on the page would be providing 60 db at 125 hertz with only 4 inches and 2 2/3 pounds per square foot.  Or do these measurements assume that the sound wave travels through the material, is reflected, then goes through it again, then is measured? 


oy there are so many questions here, sorry.

Randy S

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Reverberation time (RT) is a measure of the amount of reverberation in a space and equal
to the time required for the level of a steady sound to decay by 60 dB after the sound has
stopped.
The decay rate depends on the amount of sound absorption in a room, the room
geometry, and the frequency of the sound. RT is expressed in seconds.Sound engineers and "ear people" mostly use the usual sound field quantity, they therefore state:
RT60 is the reverberation time, when the sound pressure level is decreased to (−) 60 dB
Acousticians and "people fighting sound" like more the sound energy quantity. They express
this differently:
RT60 is the reverberation time, when the sound intensity level is decreased to (−) 60 dB.
Randy Sieg

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www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040

Randy S

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So the answer would be yes, if you had enough of the material in the room based on NRC value of that material at the frequency your trying to reduce.Now all this sounds fine and dandy however Never use a value higher than .95 NRC even if it claims to be higher....because  coefficients measured by this test method(ASTM 423) 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.So in otherwords you dont always get the results the test data claims...
Especially when you are expecting high reduction at the lower frequencies because you are most likely not accounting for SPL.
Why dont you give me a call and we can dicuss different theory.
Randy S. 760-752-3030 ext 104
Randy Sieg

Super Soundproofing Co
www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040