On Tue, 10 Oct 2006 21:36:48 -0500, "Ken Maltby"
wrote:


Acoustic energy then, what the microphone/audio detection
picks up. (It will cover more frequencies that humans can
hear.)

The pattern is on at a particular time then off, it varies in
certain ways that will enable further analysis of any:
IF and RF?
Intermediate Frequencies (IF) or Radio Frequencies (RF)
given off by a device reacting to the pattern of sound.
If there is any IF or RF detected that corresponds/matches
the on off times of the pattern of sound, you know there is
a device responding to the sound in the room.

A more sophisticated analysis of the detected response
to the pattern, can provide a great deal of information
about the device detected. That would be beyond the
scope of your question, and your security clearance, as
well.


I'm not so sure this technique will work with an IC that has
constant current, continual encoding of even silence as a
typical MP3 player/recorder is likely to use. There might
be a theoretical difference but one far more difficult to
measure than even anything at all from the device.