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>I may be mistaken but I think the piezoelectric effect produces a >charge, not a voltage, to be precise. well, the charge is measured as a voltage... as soon as a current is drawn, though, the charge and thus the voltage changes... thats a problem for DC use and low frequencies. Its exactly the oposite of the coil (magnetic pickup) which only produces a current. To make a flat amp for it is more dificult, since the coil has an internal resistance and capacitance, too, so its not as ideal as the piezo. >They have ideally infinite DC impedance, like a capacitor. True >charge amps are hard to build but are used, instead of voltage >preamps, for piezoelectric accelerometers in the acoustics and >vibration instrumentation. amazing... I had understood that the charge amps are needed in cases where you want to measure a constant pressure, like a balance for example: you short the piezo for tara and then put the weight on it and measure the charge which has to stay constant. >But I've had fine results just plugging a bare disc into a guitar >amp, and sticking the disc between bridge and body as Matthias >suggests. The amp I use is simple: a 2n2 cap to the positive opamp input a 2M2 resistor to ground. A guitar amp input is similar to this, but sometimes has another cap to deal with the coil and usually does not cut the low end, so you can drive it into clipping simply by puting your hand on the bridge, for example. >Another conceptually interesting feature of piezo material is its >symmetry-- if you apply an external charge to the material, it >moves! (a very tiny bit). In my copious spare time I hope to develop >an e-bow/Sustainiac like device that would work on non-metallic >objects, using this principle. (Don't hold your breath.) I made this work for very high notes. For the medium and bass range of the guitar, the piezo movement is by far to small. >Most piezoelectric material is also pyroelectric, meaning it >generates an electrical response to heat. Perhaps some of the recent >threads on "hot" looping can take note of this to good effect. :-) > >Does anyone really know how C-ducers work? I believe they are >capacitive, because the foil inside the pickup thingy is not Kynar >(a piezoelectric mylar) or anything like it. In fact I have made >pickups with aluminum foil and scotch tape, which work amazingly >well into a C-ducer preamp but don't make a peep plugged into >anything else. Right, thats because you have to apply a voltage to it, I think, similar to a condenser microphone. > >Here's a link to some cool piezo sensor products: >http://www.msiusa.com/piezo > >I bought this company's design kit from Digikey. They now have >online ordering from their site, and I notice there are now some $25 >hydrophone products listed too. The piezo cable looks really >interesting. > >Bernie Krause recommended to me hydrophones from Underwater >Acoustics in Vancouver BC. Barbara there was very helpful and sold >me one of the very nice units they use for eco-tourist whale >listening. Bernie also echoed the comments of many film sound >designers, that a real world recording rarely sounds like we imagine >it, especially with something so subtle as water. You often need to >create the desired "nature" sound with multiple tracks and more in >the studio. > >I couldn't find a URL for that company, but have the info somewhere >on paper if anyone's interested. I did just find a link to another >very nice looking professional hydrophone product: > >http://www.dolphinear.com/pro/index.htm > >If anyone has the $300 to blow on one of these, let us all know how >well it works!!! > >My all time favorite underwater recordings use the tried-and-true >but rather risky Neumann-with-a-condom technique. The serpent fight >scene in the live action Jungle Book movie features sounds I >recorded in this manner in a hot tub with my wife. I should loop >those tracks and call it Freudian Field Day... > >-Alex S. > > >At 4:57 PM -0300 10/3/02, Matthias Grob wrote: >>>The piezo mic is two thin layers of metal, with a layer >>>of ceramic between. Typically 1 to 2 centimeters in diameter, >>>and less than a millimeter thick. When the element flexes, >>>a tiny amount of electricity is generated. >> >>they react on pressure in the first place. In a lighter, you press >>on them and the resulting voltage is so hight that a spark results. >> >>>A pre-amp is >>>needed. >> >>the impedance depends on size. Its always bigger for low >>frequencies, so a small element for full range needs about 2Megohm >>input impedance. If you have a big element and/or dont need the low >>frequencies, it can be much lower. >> >>>As far as I know, they work best on large, flat surfaces >>>that resonate with the sound. >> >>between an activator and the base they work even better. >> >>>The prime example is the front of an accoustic guitar. >> >>better example is the bridge, between string and body. You can get >>several volts there. >> >>>A flute probably won't work. >> >>yes, air movement is not enough, it takes a vibrating hard >>material. The tube of a flute vibrates but maybe not enough. >> >>> >>>However, what is so cool about them is that they are cheap >>>if you make your own, which is pretty easy. Buy the bare >>>element, or disassemble a Radio Shack buzzer, attach the >>>wires, and you've only spent a couple bucks. Stick it to your >>>instrument and see what happens. Try different spots, and >>>wire two together for fun. I plan to embed a couple >>>inside a hollow body kalimba I'm building. >>> >>> >>>I originally found via Google some long posts from an >>>guitar discussion group on the topic, but much instruction >>>isn't needed because it is so easy. >>> >>>Yours in rhythm, >>>Steve >> >> >> ---> http://Matthias.Grob.org -- ---> http://Matthias.Grob.org