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Mar30-11, 09:50 AM
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If you shout into a loudspeaker cone, the electrical energy it will produce can drive a distant earphone. With a jet engine a few metres away, the energy level would be a bit higher than that but you could only intercept a small fraction of what the engine is producing.
This article -
gives you some idea of the flow of sound energy.
60dB, which corresponds to 'normal conversation', corresponds to 1 microWatt per square metre.
120dB is around a nearby Jet engine noise level and would correspond to just 1W per square metre.

Allowing for inefficiency in any transducer you were using, this would probably end up as 0.1W of useful electrical energy. As a way of re-using 'lost' noise energy it seems to be pretty poor value, unfortunately.

It is actually amazing what a very good job your ears do, when you think that 0dB corresponds roughlt to the threshold of hearing. A millionth of a normal conversation level can just be detected.