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From: cslug2@dalcsug.UUCP
Newsgroups: sci.electronics
Subject: Re: Volume based on ambient noise
Message-ID: <90@dalcsug.UUCP>
Date: Thu, 28-May-87 23:08:28 EDT
Article-I.D.: dalcsug.90
Posted: Thu May 28 23:08:28 1987
Date-Received: Sat, 30-May-87 02:46:55 EDT
References: <6412@reed.UUCP>
Organization: Dalhousie University, Halifax, N.S., Canada
Lines: 28

In article <6412@reed.UUCP>, omsi@reed.UUCP (OMSI) writes:
> 
> I have an application where it would be useful to boost the volume through
> a speaker based on the level of background noise in the area.  The obvious
> problem is how to cancel the sound from the speaker so the noise level can
> be measured.  Any ideas?
> 	Thanx,  Bob Ankeney   tektronix!reed!omsi

ELECTOR magazine (UK publication) had a motorcycle intercom a while back
using that idea. They placed a mike at the front of the helmet and another
in the hollow space below the cheekbone. By feeding the two mike signals
to an OpAmp as common mode signals, the side mike was algebraically
subtracted from the front mike. The output could then be fed to another
OpAmp or audio amp for boost and distribution. Don't forget to return
an attenuated signal in real time to the user's earphone so they can
judge voice volumn (like a phone instead of like a CB radio).

See:

front mike [----------|\   LM741
		     +| \
		      |  \_____ 'clean' ouput
		      |  /
		     -| /
side mike [-----------|/

Both mikes to common ground. Some feedback will be required or clipping will
result. Grab a textbook.