Piezoelectric microphones are devices that detect sound through physical contact. Contact microphones are almost entirely insensitive to air vibrations. They only detect sound waves emanating from solid objects. These microphones are often used by architects and engineers to detect the location of leaks in buildings. They also enjoy wide usage by noise music artists experimenting with sound.
Piezoelectric microphones can be used to amplify sound from acoustic musical instruments, to sense drum hits, for triggering electronic samples, and to record sound in challenging environments, such as underwater under high pressure.
Piezoelectric microphones based on piezo materials are passive and high-impedance, which can make them sound 'tinny' without 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 positively charged while the brass disc is negatively charged. If the silver disc is cracked or scorched, the piezo will no longer function at full sensitivity.