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From: bart@videovax.tv.tek.com (Bart Massey)
Newsgroups: comp.dsp
Subject: Re: noise cancellation
Message-ID: <5846@videovax.tv.tek.com>
Date: 11 Jun 90 22:19:45 GMT
References: <595@mercury.iotek.UUCP> <791@anaxagoras.ils.nwu.edu> <170@locke.water.ca.gov>
Reply-To: bart@videovax.tv.tek.com (Bart Massey)
Organization: Tektronix TV Measurement Systems, Beaverton OR
Lines: 24

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)? 

My physics background makes me suspect it works something like this:

In a room where sound is being generated, the generator is an engine which
converts some "higher" form of energy into some "lower" form, while emitting
sound energy.  The sound is eventually absorbed (damped) by the people, walls,
etc., and thus turned into mechanical energy and eventually heat.

Similarly, an active noise canceller is also an engine which converts some
"higher" form of energy into some "lower" form, while emitting sound energy.
Since this sound energy just cancels existing sound energy, there's no net
dissipation of energy into the surroundings.  Effectively, you've set the
two engines working with each other, coupled by the air.  Thus, any heat
emitted comes from the audio source and the noise canceller in this case.

					Bart Massey
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