A contact microphone, otherwise known as a pickup or a piezo, is a form of microphone designed to transmit audio vibrations through solid objects. Unlike normal air microphones, contact mics act as transducers which pick up vibrations and convert them into a voltage which can then be made audible. Often used as acoustic leakage probes, they also enjoy wide usage by noise music artists experimenting with sound. Plain contact microphones are passive and high-impedance and this can cause them to sound ‘tinny’ unless used with a matching preamp.

wiki: http://en.wikipedia.org/wiki/Contact_microphone

google: http://www.google.nl/search?hl=nl&um=1&q=contact%20microphone&ndsp=18&ie=UTF-8&sa=N&tab=iw