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From: dralle@lll-lcc.aRpA (Sir Matthew G. Dralle)
Newsgroups: rec.aviation,sci.electronics,rec.audio
Subject: Re: Noise-cancelling microphone
Message-ID: <867@lll-lcc.aRpA>
Date: Wed, 20-May-87 14:53:09 EDT
Article-I.D.: lll-lcc.867
Posted: Wed May 20 14:53:09 1987
Date-Received: Sat, 23-May-87 07:22:07 EDT
References: <1027@mips.UUCP> <689@cod.UUCP>
Organization: Lawrence Livermore Labs, Livermore Ca
Lines: 23
Keywords: noise microphones
Summary: No Big Deal...
Xref: mnetor rec.aviation:1604 sci.electronics:717 rec.audio:1669

>  Noise cancelling microphones have been around for a long time. I use one
> regularly in light planes. I have been flying since 77 or so and they were
> in use then.

   The idea behind the noise cancelling mic is really no big deal.  In 
aircraft handheld mics and in the headphone style boom mics there are two 
"channels" to allow sound to enter.  One channel in in the front where you
would talk into, and the other is in the BACK of the mic.  What happens is 
noise enters from the front *AND* the from the back.  The two signals then
subtract in effect because both are trying to vibrate the element together,
but from different sides.  When you talk into the mic, your sound is applied to
one side of the mic only, and has no opposite vibration to subtract from it.

   Obviously, this system does not work %100, so the mic element is frequency
bandwidth is limited to the vocal spectrum only (aprx 200-5000Hz).  With the
combination of the two cancelling systems, the little jobbers are fairly
good at noise cancellation -- at least *I'M* imppressed! :-)


						Matt Dralle

						Mild Mannered Tech By Day...
					     ...Pilot By Weekend.