A piezo disk exploits the reversible crystal-electricity effect to work as a contact microphone or a driver

The piezoelectric effect is reversible: mechanical stress on a piezo crystal generates a voltage (striking it can generate enough to light a flashlight bulb), and conversely sending current into it makes it physically twitch. A single cheap piezo disk — the beeper salvaged from phones and appliances — can therefore serve as both a contact microphone and an exciter/driver, paralleling the speaker/microphone symmetry. As a contact mic wired to shielded cable and an amplifier, it picks up sound conducted through solid objects (structural vibration) rather than airborne sound, letting you amplify very quiet vibrations in guitars, drums, springs, ice, or any physical structure — often catching vibration before the air-coupled sound reaches a room mic. The ceramic (crystal) side is the signal conductor; the brass side is ground. As a driver, a backwards output transformer steps up voltage to maximise vibration. A key constraint: piezo disks have uneven frequency response and very high output impedance, so they require a high-impedance amplifier input. Coating with Plasti-Dip waterproofs them into hydrophones.

Examples

Tape a piezo disk to an acoustic guitar body to pick up structural vibration before the room mic hears it; clamp one to a Slinky, spring, or sparkler. Freeze one in ice and record the clicks and whistles as it thaws. Build a cheap plate reverb: wire one piezo as a driver (via a stepped-up transformer) to a thin metal sheet or pie tin and a second piezo as a pickup on the same sheet, feeding an amplifier. Coat with Plasti-Dip for underwater recording.

Assessment

Explain the difference in what a contact mic and an air mic pick up, and why piezo contact mics require a high-impedance input. Describe how to build a plate-reverb substitute from two piezo disks, a transformer, an amplifier, and a thin metal sheet — state which disk serves which role and why, invoking the reversibility of the piezoelectric effect.