"Joey" wrote in message
...
Suppose someone visited your office or home and tried to make a voice
recording using a hidden recorder.

If they used a older-style dictation machine based on tape then you
could detect the electromagnetic transmissions from the dictation
machine when it was recording.

But how would you detect if someone was secretly recording with an MP3
player that recorded to flash memory?

Is there some transmission which could be detected?
Perhaps some low power ultra high frequency from chip refresh cycles?

What is done is to generate a pattern of sound, only
part of which is audible, and then detect the low level
IF and RF that any sound equipment emits. Then follow
the signal detected to its source.

Luck;
Ken