Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!seismo!beno!black
From: black@beno.CSS.GOV (Mike Black)
Newsgroups: comp.dsp
Subject: Re: noise cancellation
Message-ID: <48884@seismo.CSS.GOV>
Date: 12 Jun 90 22:47:06 GMT
References: <595@mercury.iotek.UUCP> <791@anaxagoras.ils.nwu.edu> <170@locke.water.ca.gov> <2923@demo.COM>
Sender: usenet@seismo.CSS.GOV
Organization: Center for Seismic Studies, Arlington, VA
Lines: 38

In article <2923@demo.COM> jgk@osc.COM (Joe Keane) writes:
>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.

I beg to differ, but just because the sound is 'cancelled' doesn't mean that
there is no power being transmitted.  A sound wave in a gas has accociated 
with it an excess pressure, an excess density, a particle displacement, and
a particle velocity.  It is the oscillation of these that produce the effect
we know as sound.  If you provide an opposite phase oscillator (typically
by simply inverting the signal close to the ear of the listener), you will
make these items constant instead of oscillating.  In effect, you will
create a constant increase in pressure, density, displacement, and velocity.
You still have the energy deposited by both transmitters, but without the
oscillations a human won't hear it.  An atmospheric pressure gauge might
(if it could detect the one-millionth atmospheric pressure change).  
You could also take this to the extreme and eventually cause quite a bit
of pain to someone (probably from heat before pressure) and they'd never
hear a thing.

--
-------------------------------------------------------------------------------
: usenet: black@beno.CSS.GOV   :  land line: 407-494-5853  : I want a computer:
: real home: Melbourne, FL     :  home line: 407-242-8619  : that does it all!:
-------------------------------------------------------------------------------