Posted by more acoustics 101, Mark in Ohio on August 18, 2000 at 22:26:55:
In Reply to: Re: Kleenex and Spit posted by Marty Neilan on August 18, 2000 at 09:23:50:
Rick & Marty,
These devices are useful for controlling room reverberation. They are wholly inneffective at controlling sound transmission, which is Rick's first concern here.
Sound Transmission and Interior Acoustics are seperate issues. Acoustic Foam is only intended for the latter.
Manufacturers of these products provide NRC (noise reduction coefficient) ratings, which are useful to acousticians when calculating room reverberation and other elements of 'room sound'.
HOWEVER, The spec to look for in this case is STC (Sound Transmission Coefficient) which represents the transmission loss that one can expect this product to provide, indicated in A-weighted decibels. You will notice that in Sonex and any other foam absorber's literature the STC rating will be curiously absent. A material with an STC rating of "STC 40" will provide roughly 40 dbA of attenuation ON AVERAGE in and of itself. A 90 db Honk would be 50 db on the other side of a partition rated STC 40. However, sound can go right around said material through any unseen leaks or lessser materials.
At bass frequencies, corner absorbers have an STC rating of nearly 0. Not enough mass or size to stop a bass wave. Sound behaves like a fluid, and will go right around or through relatively small & lightweight objects.
The tests for STC and NRC are only made from 250-2000 Hertz, so buyer beware! (250 Hz is about middle C, above bass clef staff)
I don't mean to 'dis' acoustic foam products, which do have an application. Just not for controlling sound transmission. Now when we get this noise leak solved, then we can talk interior acoustics (reverb, tonal balance, etc).
Read the books I cited in my post yesterday. These books are aimed at intelligent entry level students of the science of sound. They are well-illustrated and practical books.
Remember that the tuba is a totally acoustic device too. If we could take shortcuts in tuba design and construction, everyone would own the BAT of their dreams which they made in their own garage for $85. There are no shortcuts for this level of acoustic problems.
~-0^0-~ Mark in Ohio