Quiet-List 1997

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Re: Technology and Coping

On Sun, 15 Jun 1997 23:26:24 -0400 (EDT), 
Sorrento95@aol.com  <Sorrento95@aol.com> wrote:

>In a message dated 97-06-15 23:17:51 EDT, you write:
><< I'd offer the caveat that "noise cancellation" is not necessarily...
>    Will someone explain this to me? 
>    How can noise be "cancelled"?
   I first came across the technique as a means of suppressing 
audience noise from concert hall recordings.  You record one track 
with a microphone aimed directly at the audience, away from the 
stage.  Later, in the studio, you reverse polarity on the audience 
track so that the waveforms are an *exact* inversion of what they 
were originally.  When you mix that track against those of the stage
performance, the audience sounds are virtually eliminated by the 
*cancellation* effect of combining the inverted signal with its exact 
opposite.  The positive and negative amplitudes of each sound wave 
net to zero.

   I believe all "active noise cancellation" technologies employ 
basically the same principle.  One of the reprints available from 
Right to Quiet is an article from the October 1991 issue of 
Engineering Digest describing a system devloped by Lotus which 
achieves a 20dB reduction in automobile interior noise.  The article 
says the system would add only about $200 to the cost of the car and 
would be commercially available by 1992.  I don't know of any U.S. 
cars using the system, but I expect some are by now.

David Staudacher - quiet@igc.org 
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