I'm not an authority on the subject, but many people have asked me questions, so instead of digging up an old email and updating it, here's a page with some info...
WHAT IS A CONTACT MIC?A contact microphone is a piezo disk. Piezo is a material that produces a voltage when it vibrates, thus produces sound when it picks up physical vibrations. So, it is good for recording sounds that are physical vibrations in an object, as opposed to vibrations in the air, which is what we are accustomed to hearing. A piezo disk can also produce an audible sound when a voltage is applied. It is more commonly used as a buzzer, and sometimes in tweeter speakers. If you ever talk to someone in an electronics store, ask for a piezo buzzer, because they'll likely have no idea what a contact mic is.
Technical aspects: look up "piezo-electric effect" or "Piezo transducers" or start here: http://www.piezo.com/history.html .
BUILDING THEM:Richard Lerman has recorded all sorts of things with contact mics (like spiders, cacti, fences) and has some PDF docs on how to build the mics, as well as a preamp (which i tried to build but doesn't work. my electronics-knowing friend said it's a problem with my voltage supply.). there are some videos of his various setups and installations. http://www.west.asu.edu/rlerman/
Illustrated How-To (replace "Radio-Shack" with your favourite electronics supply place, or "Addison" or "Abra" in Montreal): http://home.earthlink.net/~erinys/contactmic.html
and here's one that has a way to make a contact-mic hydrophone, although i never tried it: http://users.tpg.com.au/users/adsl9dhg/hydro_dunking.htm
Notes:recently i've started using hot glue to seal in the wires after soldering them, covering the whole back surface, making it more waterproof, and less prone to hum when touching it. a friend of mine builds a sort of resonating cup for it.
in the birthday cards that you open and they play a tune, a piezo is used as a speaker. you can find those at the dollar store, but it's cheap one because the metal isn't brass, and i found they don't produce as loud a sound.
SOUND CHARACTERISTICS:
now, piezos come in different diameters, and have different characteristics such as resonant frequency, but i could never correlate or understand anything about what i've unearthed on the web, since they were usually talking about them in terms of buzzers or some industrial applications, not sound. i always kept to the cheaper, low-voltage ones because i thought that one that needs a higher voltage to buzz would produce less sound. but my recent purchase has shown otherwise, they seem to perform quite well. they are a bit of a pain to get out of the case. one can order just the disks in bulk from most electronic parts places.
One problem is that because of impedance, the longer the wire, the more undesirable noise it will pick up. It is best to have the shortest wire possible before going into the mixer or amplifier.
ATTACHING THEM TO STUFF:
the main problem is how to attach them solidly to something. the idea is to have a solid contact, without dampening the sound.
Experiment. Have fun!
-- James Schidlowsky.