Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!husc6!mit-eddie!uw-beaver!tektronix!tekcrl!tekirl!jackg
From: jackg@tekirl.TEK.COM (Jack Gjovaag)
Newsgroups: rec.aviation,sci.electronics,rec.audio
Subject: Re: Noise-cancelling microphone
Message-ID: <542@tekirl.TEK.COM>
Date: Fri, 22-May-87 10:00:47 EDT
Article-I.D.: tekirl.542
Posted: Fri May 22 10:00:47 1987
Date-Received: Sat, 23-May-87 18:08:04 EDT
References: <1027@mips.UUCP>
Reply-To: jackg@tekirl.UUCP (Jack Gjovaag)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 33
Xref: mnetor rec.aviation:1656 sci.electronics:728 rec.audio:1704

In article <1027@mips.UUCP> mark@mips.UUCP (Mark G. Johnson) writes:
>I recently read (in Aviation Week & Space Technology) an article about
>a noise-cancellation system for military pilots' helmets.  
> .......stuff deleted.......
> 3.  Can the idea be used "backward" to make a microphone which cancels
>     ambient cockpit noise and only picks up the pilot's voice?  For
>     example, have a 2nd mike that faces _away_ from the pilot, and use
>     this signal as an approximation of the "error" (ambient noise).
>     Or is this already available per Scott Dorsey's recommendation of
>     the "Telex noise-cancelling element"?
Almost all aircraft boom mikes do this already.  The microphone
"listens" through two opposed ports.  One port is closest to the
pilot's mouth and gets most of the voice energy.  Both ports (in theory)
hear ambient noise equally and cancel it.  Listen to the transmissions
from other airplanes.  You never hear engine noise, only the pilot's
voice.

To determine the effectiveness of this, I ran a little experiment
where I made a transmission to a friend in another aircraft while
my outer marker beacon was beeping at full volume.  He could NOT
hear the beacon but could hear my voice (I was speaking quietly)
perfectly.

As an historical note, one early solution to the cockpit noise
problem in combat aircraft was a "throat mike".  It strapped around
the neck with the microphone near the voice box.  It was pretty well
masked from all the engine and airframe noises but it sure did
lousy things as far as speech intelligibility because it left out
a lot of speech sounds that are made in the mouth without the
vocal cords.

  Jack Gjovaag
  Tek Labs