I read in sci.electronics.design that zipzit wrote
(in ) about 'Ambient
Noise - Volume Adjustment Intercom Circuit', on Sun, 3 Oct 2004:

Is there anyway to use microphone sound density input to generate a
isolated variable resistance output, to work as an automatic volume
control for varying ambient noise levels? Are there other ways to do
this?

Not an isolated one, easily, but a grounded one, quite simply. You can
sue a bipolar or JFET as a variable resistor to form the grounded arm of
a potential divider at the input of an op-amp. The op-amp output feeds a
diode rectifier and the d.c. output of this feeds the base or gate of
the control device. You could use the LM386 instead of an op-amp, I
think.

There are lots of examples of this sort of automatic gain control
circuit on the net. To make it into an ambient noise compensator, you
need to use the rectified, amplified ambient noise from a microphone to
*oppose* a bias voltage or current on a control device that is keeping
the gain low in the absence of the noise-related signal.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk