brent@itm.UUCP (07/12/83)
A couple of notes on general acoustic principles ...
The two general mechanisms of absorbtion are a) transmission loss
and b) reflection loss. Fiberglass in walls obviously is being
used in transmissive mode. One pass through fiberglass usually
doesn't do too much. (try putting a piece up to your ear) To
be maximally effective, it must pass through several times. The
classic illustration follows:
Set a ringing alarm clock on a table. Put a tin bucket over
it. You will notice that it's still just about as loud. The
bucket is very reverberant, and not very absorptive. Next remove
the bucket and wrap the clock in fiberglass. The sound is attenuated
some, but not much. The sound is only making one pass through the
fiberglass. Now leave the fiberglass on and put the bucket over the
whole works. The sound is greatly reduced. This is because the
bucket makes the sound field reverberant, so the waves pass through
the fiberglass many times.
Fiberglass in walls works best when the walls it is between are
hard, solid, and tight. This provides a very reverberant space
in the wall, so the fiberglass's absorptive properties are multiplied.
This is good only from a few hundred Hertz and up. Bass guitar
is largely transmitted through the studs. Two separate walls built
on offset 24" centers with fiberglass in between usually works even
for low frequencies. Construction as:
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brent (msdc!itm!brent)