Military equipment: microphone eavesdropping - (DIY)

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Military equipment: microphone eavesdropping - (DIY).
 

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For this purpose all the magic would be in the DSP post processing. It's not all that much of a stretch to get down to the theoretical 1.7 nV per root Hz noise floor of the bipolar transistor. So the magic has got to be digging the signal you want to hear out of background noise (that the mic will pick up). Those algorithms are probably mega-classified an not easily DIY. A low noise preamp is the EASY part.
 
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