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A contact microphone, otherwise known as a pickup or a piezo, is a form of microphone designed to sense audio vibrations through solid objects. Unlike normal air microphones, contact mics are almost completely insensitive to air vibrations but transduce only structure-borne sound. 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.
The most commonly available contact microphone element is made of a thin piezoelectric ceramic round glued to a thin brass or alloy metal disc. This center disc is positive while the brass disc is negative. If this silver disc is cracked or scorched, the piezo will no longer function at full sensitivity.
More recently, flexible PVDF fluoropolymer piezo film has been developed and is available in tabs, strips, cables and even large sheets. Piezo cable is often used buried under roads as traffic sensors.