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Detecting Ultrasound

Discussion in 'Electronic Design' started by [email protected], Mar 10, 2007.

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  1. Guest

    Hi chaps,

    I suspect a neighbour of a friend of mine is using an ultrasonic bird-
    scarer to frighten off his pets. The man concerned won´t admit to it,
    but there are times when his dog and two cats just seem to get
    suddenly very distressed and hypermanic for no apparent reason. I`d
    like to at least eliminate this possibility before considering any
    others. So the question is, what´s the simplest way to detect
    ultrasound? My web research leads me to believe the area of interest
    is between 20 and 30khz. Most common bird scarers warble between these
    two limits which are of course above the range of human hearing. I´ve
    acquired an ultrasonic transducer that transmits on 41khz. If I couple
    this up to a wien-bridge oscillator trimmed to the same frequency, I
    figure I ought to be able to hear a warble if indeed this guy is using
    a birdscarer, because the difference between 41khz and 20khz-30khz
    will be audible to me. Is this feasible to "air mix" the two
    frequencies in this simple way and hear a result, or is something more
    complicated required?
  2. On 10 Mar 2007 05:18:34 -0800, in may work

  3. chuck

    chuck Guest


    The mixing process requires a non-linear device, which, for your
    purposes, I suspect the air is not. I've often considered, but never
    attempted, a similar mixing process using human ears (connected and
    intact, of course), since ears are quite non-linear. Ears won't work as
    mixers in your case since, at least for most adults, they are
    insensitive to ultrasonic frequencies (as well as to intelligent
    political analysis, it seems).

    The suggested bat detector is far more promising.

  4. PeterD

    PeterD Guest

    Why worry about it... His yard, his pets, his life...


    Take a microphone with a frequency response > 30Khz, and an amplifer.
    Monitor the amp's output with a scope. <bg>
  5. Suspend a thin shaving razor blade between two pieces of dental floss and put
    your ear close to the blade. Watch the razor vibrate.
  6. Google 'bat detector'.
  7. clifto

    clifto Guest

    That's pretty cool, MacGuyver. Does the razor blade do anything?
  8. Scott

    Scott Guest

    It cuts off your ear if you get too close ;)
  9. Guest

    The obvious "detection" would be oscilloscope observation
    of the amplified microphone signal. That's been mentioned.

    Some commercial ultrasound detectors simply heterodyne
    the ultrasonic range down to audible frequencies...good if
    your hearing goes on up to the high end of human response.
    Expensive as portable devices but easily genned up on the
    average home workbench.

    There are a couple of claims of outdoor advertising via sound
    through using high-power ultrasound generators in pairs, one
    modulated in amplitude the other unmodulated. The air acts
    as the non-linear "mixer" and the claim is that such beams
    of ultrasound can be focussed on particular locations. One
    such company is located in San Diego, California, if memory

    73, Len AF6AY
  10. John Smith I

    John Smith I Guest

    Be better if you could use it as an electric razor ...

  11. Chris Jones

    Chris Jones Guest

    To get the air to be non-linear enough to mix the two frequencies, you would
    need very high sound pressure levels that could potentially be hazardous
    (or at least much more of a nuisance that whatever your neighbour is
    doing). I would suggest getting a wideband microphone (a very small
    electret might do, and would be less high-Q than a typical ultrasonic
    transducer), and then attach a preamplifier and an electronic mixer with an
    adjustable local oscillator (e.g. make a bridge from switches using a 4066
    or FST3125 that alternately inverts or doesn't invert the signal). The
    output of the mixer can be fed to an audio amplifier and headphones. You
    can then test this receiver if you buy or borrow one of those cheap "pest
    annoyer" things. Once you know that your receiver works, you could make a
    parabolic reflector (e.g. spin cast a dish on an old record player from
    plaster of paris in a bin lid), so that you can search for sources of

  12. clifto

    clifto Guest

    Replace the dental floss with Litz wire and attach ends to 110 VAC.

    Wait. Maybe not. Considering how many stations you can pick up with a
    razor-blade radio, that would probably interfere with vital communications
    all throughout the spectrum. What's the resonant frequency of a whisker?
  13. Guest

    You need your detector to be low Q since you don't know the offending
    frequency. Some of these fancy sound cards do 196kHz sampling. A
    piezo microphone with amp into the sound card might do the trick.

    Also, as other have suggest, the bat detector.
  14. John Smith I

    John Smith I Guest

    Do you really want to hear it, or have a "field strength indicator"/locater?

    Why not just a mike capable of "hearing" the ultrasonic freqs in
    question--feeding an opamp and meter?

    Pointing the mike around should lead you into the correct direction and
    following the meter reading should lead you to the source(s).

    Possibly can substitute a light or even a led in series with a pot and
    use a visual indication of strength ...

  15. John Smith I

    John Smith I Guest

    Come to think of it, wouldn't take much more to square up the sine wave
    out of the opam and drop a cmos decade-divider onto that output of the
    opamp and feed an ear phone with the dividers output--30,000 becomes
    3,000 hz--easily "hear-able!"

  16. jasen

    jasen Guest

    only if theres no other sound present in the mic signal.

    and it will give no indication of signal amplitude.
  17. Doug Smith

    Doug Smith Guest

    I *think* what the OP is worried about is that his friends' pets are being
    scared off by his freinds' neighbor.

    While I hate to discourage anyone from building something electronic, I do
    have to ask: what will one do if they learn that a bird-scarer *is* in
    use? Best of my knowledge, they aren't illegal.

    If a bird-scarer works on dogs, then I find the details quite interesting.
    May have to work up a mobile version. Loose dogs allowed to roam a
    neighborhood are a serious safety issue for cyclists. (and I wonder if
    that's why the neighbor in question is trying to scare them off?)

  18. They don't make the blue blades anymore.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida

  19. Yes, but the blueing was what made a detector.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  20. Rich Grise

    Rich Grise Guest

    I don't know how your sense organs are arranged, but I know that if I
    dangle something next to my ear, it's terribly hard to watch it
    simultaneously. ;-)

    What does that have to do with dangling the blade from dental floss? Or,
    for that matter, what does dangling a blade from dental floss even DO?

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