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Surpizing results from Piezoelectric coin speaker.

Discussion in 'Electronic Design' started by The Flavored Coffee Guy, Nov 29, 2006.

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  1. Have you ever made a wine glass or crystal sing by wetting your finger?
    Would you believe that when you hit the resonant frequency of a tuning
    fork, wine glass, or even a plain old drinking glass that is made of
    glass, a very small signal, not even audiable, at the resonant
    frequency of the glass can be very loud. But, there is a trick too it.
    Either the very center of the piezoelectric coin speaker has to be in
    contact with the rim of the glass, or the very outer most edge. When
    you pull the speaker away, it is quite. But, when that thing has
    contact, it is ear piercing.

    It almost makes me think that if piezoelectric elements were used, and
    formed into bells, that you could use the mechanical resonant frequency
    of the object, using mathcad, or some other form of simulation software
    find the standing wave in the shaped piezoelectric ceramic, and use
    it's resonant frequency to generate more power than you were using.
  2. Eeyore

    Eeyore Guest


    Where's your perpetual motion machine ?

  3. Genome

    Genome Guest

    Errrrr...... you have just responded to The Flavored Coffee Guy.

  4. Bob Eld

    Bob Eld Guest

    Of course it does NOT generate more power than you are using. What it does
    is concentrate the available energy into a single frequency and emit the
    energy with much greater effiiciency because of the large area of the glass
    vs. the piezo element alone.
  5. Eeyore

    Eeyore Guest

    Is that a 'bad thing' ?

  6. hope TFCG doesn't mate with that Flying Skybuck dude, could end up
    with an extra Startrek episode

  7. Genome

    Genome Guest

    Only if you try to take the piss out of him......

  8. J.A. Legris

    J.A. Legris Guest

    Of course you'll never get more energy than you put in (unless you set
    the thing on fire), but the same principle is used in those piezo
    beepers that are so loud - the cavity in the plastic housing resonates
    with the active element.
  9. When you are dealing with a mechanical resonant vibration, it's like
    applying 5lbs of force to a swing with some-one on it. Each time they
    return, the altitude is a little higher, and the energy of interia is
    summed up. The action of the pendulum, only looses energy as a product
    of wind resistance. A bell, however, is quite different. It's the
    combination of the elasticity of the metal, and the elasticity of air.
    That's why when you fill a glass with a uncompressable fluid like
    water, the crystal will ring at a higher frequency. If you filled with
    lanoline, the resonant frequency would drop because the speed of sound
    is so much slower in lanoline. It is concievable to move that all down
    an octive.

    Now, if your material were piezoelectric, which of most is not really
    solid, nor hard, but if you had something that wasn't full of holes
    like a sponge, you could make a bell, and your first AC connection
    would be 90 degrees from the the piezoelectric transducer. The second
    would be + 45 degrees. A standing wave in a circular bell, produces a
    oval that is oscillating at 90 degrees from zero. But, with a
    piezoelectric material, you need a compression zone, and a
    decompression zone to produce power.

    I have just done this experiment today, and it was too easy. I took
    the coin speaker from an old alarm clock, and used my signal generator,
    which has a peak to peak output of 2 volts, and the impedance of the
    output is 50 ohms. I couldn't turn up the power high enough to but
    barely hear that thing at it's own resonant frequency. But, finding
    the resonant frequency of a glass that was half full, made a very loud
    ear piercing resonant sound. You can talk, but you can't **** with the
    real world results.

    I don't have the materials to research a machined bell made of
    peizoelectric ceramic, nor do I have access to that material. I have
    little tiny bits of the substance that is lodged in my cigarette
    lighter about the size of a flint. That tiny little object is probably
    resonant to frequency in the megahertz, if not nearly gigahertz. It is
    too small to evaluate such a set of conditions. The bell would have to
    be made of that piezoelectric material. Some materials make crappy
    tuning forks, and crappy bells. That could be true, but there are a
    range materials that fall into same catagory, that are not used in
    making these lighters. Without access, I can't say, and didn't say it
    was producing more power than it was using, I only held it in question,
    because the sound energy out is very obviously more than the sound
    energy in.
  10. If you looked at the base of a brass tuning fork. If there were a seat
    for a piece of piezoelectric material from a lighter right at the base,
    and you could electrically isolate it ridgedly with glass on one end,
    to place a piece of wire, and have set screw making the whole tuning
    fork ground, then a measurement could be made in respect to the energy
    in vs energy out. But, you would have to have some pressure applied to
    the crystal constantly, and that pressure would have to vary with the
    mechanical oscillation of the tuning fork. The majority of the mass
    would be working for you, and right at the fulcrum of the event, your
    piece of piezoelectric material would be working. Your set screw and
    mounts would be right at the base of the fork where the metal started
    off into two directions.
  11. Then you could use a step down transformer, determin phase, and finally
    see if the audio transformer didn't make the tuning fork continue to
  12. Eeyore

    Eeyore Guest

    You're an idiot.
  13. John Fields

    John Fields Guest

    Yes, but the _power_ of the acoustic output will be less than the
    electrical power input to the system.

    An experiment you can do which will be equivalent to what you're
    experiencing would be to connect an inductor and a capacitor in
    series and drive them at their resonant frequency from a voltage
    source. If you do, you'll find that the voltage at the junction of
    the L and the C is many times higher than the voltage driving the

    However, if you measure the power going into the circuit from the
    generator and the power going into a load from the LC junction,
    you'll find that the power going into the load will be less than the
    power going into the circuit from the generator.
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