Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!linus!philabs!cmcl2!floyd!whuxlb!pyuxll!eisx!npoiv!npois!hogpc!houxm!ihnp4!we13!otuxa!ll1!sb1!sb6!emory!msdc!itm!brent From: brent@itm.UUCP Newsgroups: net.misc Subject: dead walls Message-ID: <1021@itm.UUCP> Date: Tue, 12-Jul-83 12:56:12 EDT Article-I.D.: itm.1021 Posted: Tue Jul 12 12:56:12 1983 Date-Received: Thu, 14-Jul-83 05:09:36 EDT Lines: 31 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: ______________________________________________ || || || || (top view) ________||____________||_____________||________ brent (msdc!itm!brent)