piezoelectricity

The property of certain crystals and ceramics — quartz, and engineered materials such as PZT — of producing an electrical charge when squeezed, and of changing shape when a voltage is applied across them. The two directions have names. The direct effect, pressure to charge, is how a piezoelectric sensor works. The converse effect, voltage to movement, is how a piezoelectric speaker or actuator works.

Bidirectional does not mean interchangeable, and this is the point most often missed. A sensing or energy-harvesting element is wired into a rectifier and a storage circuit. A sound-producing element needs an audio-band signal source and a high-voltage drive amplifier. Those two circuits are opposites. An element built for one cannot perform the other without being rebuilt around it.

Documented real-world example: piezoelectric strips are a standard roadway sensor class listed by the US Federal Highway Administration for vehicle counting and weigh-in-motion, sold commercially by International Road Dynamics and TE Connectivity. The Colorado Department of Transportation operates weigh-in-motion at highway stations including Limon, Trinidad, Monument and Lamar, upgrading through autumn 2026. Where piezoelectric elements are in roads, they are there to read traffic — not to generate power, and not to produce sound.

Related terms: energy harvesting, structure-borne audio, microwave auditory effect

Sources: Traffic Monitoring Guidebook, chapter 4 — FHWA; Piezoelectric sensors — International Road Dynamics; Weigh-in-motion project underway in Limon — CDOT