Piezoelectric speaker

A piezoelectric speaker (also known as a piezo bender due to its mode of operation, and sometimes colloquially called a "piezo", buzzer, crystal loudspeaker or beep speaker) is a loudspeaker that uses the piezoelectric effect for generating sound.

Piezoelectric speakers have several advantages over conventional loudspeakers: they are resistant to overloads that would normally destroy most high frequency drivers, and they can be used without a crossover due to their electrical properties.

This is why they are generally used in applications where volume and high pitch are more important than sound quality.

Piezoelectric speakers can have extended high frequency output, and this is useful in some specialized circumstances; for instance, sonar applications in which piezoelectric variants are used as both output devices (generating underwater sound) and as input devices (acting as the sensing components of underwater microphones).

They have advantages in these applications, not the least of which is simple and solid state construction that resists seawater better than a ribbon or cone based device would.

A piezoelectric buzzer. The white ceramic piezoelectric material can be seen fixed to a metal diaphragm.
When fixed to a metallic diaphragm and excited with an alternating voltage, the diameter of the disc varies by a small amount, this causes dishing of the diaphragm which gives a much louder sound.