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Detect sound behind wall/door

Discussion in 'Electronic Design' started by eeh, Sep 25, 2005.

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  1. eeh

    eeh Guest


    Is it possible to detect sound behind concrete wall or door whereas the
    detector is placed outside the room?
  2. You mean like putting a glass against the wall and your ear against the

    No, not possible.

    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email:

    Please visit my GURU's LAIR web site at
  3. Guest

    The correct answer is, "That depends."

    To illustrate, you're on one side of the wall with a glass against the
    wall and your ear against the glass and I'm on the other side of the wall with a
    sledge hammer.

  4. ray13

    ray13 Guest

    It is only not possible if you can prove that it is not possible. No
    proof means it is still possible.

    Remember the US cosulate in Moscow, Russia. The Russians pumped in
    microwaves thru the phone line. And the had a special transponder built
    into the United States Seal that they gave the consulate as a gift. The
    Russians heard everything that was said in the room, that the
    transponder was in.

    Not possible, pshaw.
  5. I read in that eeh <>
    It depends how loud the sound is, obviously. If the microphone is in
    direct and intimate contact with the wall, the sensitivity will be very
    much higher than if it isn't.

  6. Yes.

  7. Microwave through a phone line?
  8. I read in that Michael A. Terrell
    Hollow wires.(;-)
  9. kell

    kell Guest

    Think about this:
    There might be noise outside the room, which would overwhelm any sound
    from inside, no matter how sensitive or powerful an amplifier you use.
    Because the sound coming through the wall from inside the room will be
    extremely weak, any little vibration or noise would totally obliterate
    it. You need to have a very quiet room to place your detector in.

    Through the door would work better, just because the signal from inside
    the room is less attenuated.

    Now get away from my door.
  10. keith

    keith Guest

    Microwaves through twisted pair. Neat!
    I do believe the bugs in question were the precursor to RFID. They only
    responded to (massive, BTW) microwave radiation and were dormant
    otherwise. They did much the same thing to the "new" US Embassy
    in Moscow. You know, the one that was never occupied. Telephone lines,
  11. Mac

    Mac Guest

    Can you be more specific? I imagine that under some circumstances it would
    be possible.

    Are you trying to determine the feasibility of some kind of high-tech
    eavesdropping device?

    I know in the movie _Clear and Present Danger_ some kind of laser
    interferometry microphone was shown being used. Don't know if that works
    in real life, particularly on a concrete wall.

    If it does work, then RF interferometry might work, too.

  12. quietguy

    quietguy Guest

    Or using an electronic stethoscope?

  13. Can you open the door?
  14. BobG

    BobG Guest

    I remember hearing/reading the 'bugs' were resonant cavities in all the
    cast concrete beams courtesy of the local Russian contractors. So all
    the walls were transmitters when a nice beefy beam was pointed at it.
    The resonance was modulated by air pressure across the cavity. Similar
    trick to bounce a laser off a window that is being vibrated by speech
    and demodulate the return. Countermeasure is to play the radio real
    loud in the room.
  15. The US also imported some "cone of silence" rooms that could be installed
    inside the compromised embassy buildings, complete with sound muffling and
    Tempest-quality electronics interfaces.
  16. Ron G

    Ron G Guest

    If the music on the radio is identifiable, then it can be nulled out by
    using the same music to null out the mish-mash (on the recording).
    If it's a radio station, the same should apply.
    The resultant will be the conversation that took place.


    Countermeasure is to play the radio real
  17. Ron G wrote...
    Nope, too much phase shift.
  18. Guest

    Feet of clay... feet of clay...

    I used to consider you my idol, but I think you're simply wrong this
    time. A tracking filter with a bandwidth of a few 10's of Hz derived from the
    interfering audio source should ignore phase shift. The signal may have to be
    run through the filter several times, but the original conversation should
    finally emerge.

    The fact that conversation contains a great deal of redundant
    information makes it possible.

  19. I read in that wrote (in
    The mind boggles!
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