A contact microphone, otherwise known as a pickup or a piezo, is a form of microphone

Microphone

A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

 designed to sense audio vibrations through solid objects. Unlike normal air microphones, contact mics act as transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

s which pick up vibration

Oscillation

Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

s and convert them into a voltage which can then be made audible. Often used as acoustic

Acoustics

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

 leakage probes, they also enjoy wide usage by noise music

Noise music

Noise music is a term used to describe varieties of avant-garde music and sound art that may use elements such as cacophony, dissonance, atonality, noise, indeterminacy, and repetition in their realization. Noise music can feature distortion, various types of acoustically or electronically...

 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.