Anti-Noise

[From Gary Cziko 930427.1800 GMT]

Bruce Nevin (Tue 930427 12:32:06) says:

I would like to build an analog device that picks up sound
(atmospheric compression waves) coming from a direction D,
provides it as perceptual input to a comparator with reference 0
(zero), and outputs cancelling compression waves from an effector
(speaker) positioned just ahead of the perceptual input device
(directional microphone). The problem is that one must amplify
the very small audio signal from the microphone in order to drive
the speaker. However, as I understand it, reversing the sign in
a conventional audio amplifier is difficult because sine waves at
different frequencies (partials) would arrive at different
phases. Any ideas?

I had a physicist neighbor who went on to work for Ford in Detroit who was
working on something like this to reduce noise in the passager compartment
of cars.

I don't remember any of the details, but I seem to remember that the
silencing effect was localized to a particular location, with other
locations having up to twice as much noise. I think Newton has something
to do with this!--Gary

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