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Detuning/widening bandwidth of piezo transducer?

Discussion in 'Electronic Basics' started by Lars Torben Wilson, Oct 27, 2005.

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  1. Hi all,

    I recently posted about a bat detector (read ultrasonic pitch
    downshifter) I'm building from the following schematic:

    http://home.kabelfoon.nl/~bertrik/bat/simphet.htm

    My goal is to build it without the indicated coils. Bob Eldred
    showed me the light on getting rid of one of the coils, reminding
    me that the 47mH coil and its cap were simply a passive LPF, which
    I then replaced with an active Sallen Key LPF (TL072 and some
    resistors/caps).

    Now my detector works quite well. I can easily hear several
    harmonics from my TV, my pocket chain clinking sounds REALLY
    weird, etc. But I want to see if I can improve the bandwidth
    of the piezo UST.

    Bertik (author of the schematic) has a good page on how to
    do this with caps, resistors, and a 5-8mH coil, but before
    I get into trying to make the coil, I was wondering if there
    are any active/non-coil ways to do it.


    Any answers/tips/links appreciated,

    Torben

    P.S. I've googled/teoma'd/mootered for this a million times,
    but either the answer's not out there or I didn't
    recognize it for what it was.
     

  2. I think you can mechanically broaden the response of the piezo and
    also increase its sensitivity by putting an exponential horn on it.
     
  3. mike

    mike Guest

    Agree. Fixing it mechanically is likely better than trying to patch
    up a high Q response after the fact.
    First thing I'd try is to run the piezo into the virtual ground of an
    op-amp. But the intrinsic series impedance of the transducer may make
    this unfruitful. May have to resort to mechanical damping. Glue
    the piezo to a piece of foam rubber and load it with a horn as
    described above.


    I have a commercial ultrasonic leak detector. Can't see in the hole too
    well, but it looks more like a plastic dome on a voice coil than a piezo
    transducer.
    The transmitter and receiver transducers "look" to be the same, but
    that's not necessarily a valid conclusion. I never found any data on
    the setup, so don't know what the claimed bandwidth is.

    Might be interesting to plug on a capacitor microphone element and see
    what that does. Most of the microphone specs I've seen are still going
    strong at the 20 KHz. end of the graph.
    mike

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  4. OK, that's two votes for the horn, so it's probably not a bad idea at
    all. However, I have both a space limitation and a horn-manufacturing
    limitation, not to mention a limitation in my understanding of acoustics
    which I figure would help me work out the size needed for best
    performance from ~30kHz to ~100kHz. Quicky paper horns (pretty much
    linear) seem to help with the sensitivity, though.
    I'll give that a shot on my breadboard. I still have to check the actual
    response of the transducer; I've been assuming so far that it has a
    fairly tight spike around 40kHz, but I've also read that some work
    just fine with no detuning at all. Given that I'm already fairly
    happy with the sound, what I'm trying for now is to solidify my
    understanding of the detuning. I'm currently under the (probably
    wrong) impression that it's mostly phase games. Am I close?
    Hm...that sounds neat. I'll see if I can find one.


    Thanks for the tips,

    Torben
     
  5. mike

    mike Guest

    Lars Torben Wilson wrote:
    snip
    Good place for microphones is an old cellphone, cordless phone, cassette
    tape recorder...
    mike


    --
    Wanted, Serial cable for Dell Axim X5 PDA.
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Bunch of stuff For Sale and Wanted at the link below.
    MAKE THE OBVIOUS CHANGES TO THE LINK
    ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
     
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