From ???@0x00000F49 Mon Feb 25 12:03:41 2002 Path: pitt.edu!newsflash.concordia.ca!sunqbc.risq.qc.ca!nf3.bellglobal.com!wn1feed!worldnet.att.net!135.173.83.71!wnfilter1!worldnet-localpost!bgtnsc05-news.ops.worldnet.att.net.POSTED!not-for-mail Message-ID: <3C7A6DF7.20404@worldnet.att.net> From: Angelo Campanella Reply-To: a.campanella@worldnet.att.net Organization: Campanella Associates & ACCULAB User-Agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:0.9.4) Gecko/20011019 Netscape6/6.2 X-Accept-Language: en-us MIME-Version: 1.0 Newsgroups: alt.sci.physics.acoustics Subject: Re: stupid question References: <1014634514.275207@seven.kulnet.kuleuven.ac.be> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Lines: 58 Date: Mon, 25 Feb 2002 17:02:24 GMT NNTP-Posting-Host: 12.87.146.201 X-Complaints-To: abuse@worldnet.att.net X-Trace: bgtnsc05-news.ops.worldnet.att.net 1014656544 12.87.146.201 (Mon, 25 Feb 2002 17:02:24 GMT) NNTP-Posting-Date: Mon, 25 Feb 2002 17:02:24 GMT Xref: pitt.edu alt.sci.physics.acoustics:24054 Status: N Wim wrote: > I'm pretty sure that with double walls it is best to put as much mass as > possible on the inside (near the sound source). > But the others don't agree.... > Who's right? 1- Double walls work this way. a- Students should study electrical engineering lore to understand what is meant by a "two-pole filter", since that is what the double wall is, in effect. There is a resonance frequency because the two surfaces act in concert with the trapped air (acting as a spring). b- The resonance frequency, Fr, is calculated from F=170/SQRT(Wd), where W is the VIRTUAL area mass of the two surfaces [W=W1*W2/(W1+W2)] in pounds per square foot (5/8" drywall=2PSF), and d is the panel separation in inches. c- The short story is that at the resonant frequency there is NO attenuation; rather more likely a 10 dB amplification. d- Below this frequency, the two walls act as one, the sum of the area masses. "Mass law" noise reduction applies. This can be quite small for the materials computer users will allow in their offices. e- Above the resonant frequency, the noise reduction increases dramatically, reaching in theory 40 dB per decade. I use a conservative 30 dB per decade as a rule of thumb. As for the interior surface, I think it makes very little difference. Inside, the most effective action is to cover the inside surface with sound absorber (e.g. 1" fiberglass duct liner board) to prevent a reverberant buildup of noise therein. The most effective computer CPU Box noise control method I can think of is a thermostatically controlled fan, preferable a variable speed drive (VSD), where it turns very slowly (silent) and speeds up only if it gets too hot inside the Box. The next most effective is that the fan should be BURIED somewhere deep inside the CPU Box, it's air blast directed at the most critical components. The makeup air and the the warmed air exhaust are to be conducted through sound absorbing ducts. This requires intelligent packaging Box design by the computer industry, which it never has done until MOST recently for laptops to a small degree. Most early PC packaging design vis-a-vis noise has been an after- thought ("Gee, the Box runs hot! No place for a fan in the Box! Rx=Add a flat fan pack on the outside of the CPU Box" (where everybody can hear it!)) Angelo campanella --------- www.CampanellaAcoustics.com --------- ------ a.campanella@worldnet.att.net ------ "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh.