"Joey" wrote in message
...
"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?

On 10 Oct 2006, Ken Maltby wrote:

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


Generate a pattern of sound part of which is inaudible?

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.

Luck;
Ken