Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!mips!pacbell.com!pacbell!demo!jgk
From: jgk@demo.COM (Joe Keane)
Newsgroups: comp.dsp
Subject: Re: noise cancellation
Message-ID: <2923@demo.COM>
Date: 12 Jun 90 01:32:30 GMT
References: <595@mercury.iotek.UUCP> <791@anaxagoras.ils.nwu.edu> <170@locke.water.ca.gov>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Object Sciences Corp., Menlo Park, CA
Lines: 16

In article <170@locke.water.ca.gov> rfinch@caldwr.water.ca.gov (Ralph Finch)
writes:
>In an audio noise canceller (sp?), one injects energy into the air.
>Like a truly soundless room, no sound is heard, but now there is
>energy.  What does this energy get converted into, heating the
>air maybe?  In a soundful room, would the energy heat other objects
>(walls)? 

If the sound is really cancelled, there can be no power transmitted through
the air.  Therefore, if one sound source is putting power in, the other has to
be taking it out.  After you add up various losses, it may or may not be
generating electrical power.  It's really more of a sound sink than a source.

Note that to completely cancel the sound everywhere, the sound sources have to
occupy the same point in space.  The previous comment still applies to partial
cancellation, one is probably taking power out.