Piezoelectric buzzers and ultrasonic transducers

A piezoelectric buzzer is a small sound-producing device built around a disc of piezoceramic material. The piezoelectric effect means that the material changes shape slightly when a voltage is applied across it, and conversely generates a voltage when it is mechanically stressed. By bonding the ceramic to a thin metal diaphragm and driving it with an alternating voltage, the disc flexes back and forth, pushing the air and producing a tone. Because the effect is direct and efficient, piezoelectric buzzers are compact, low in power consumption and inexpensive, which is why they appear in alarms, timers, appliances and countless small electronic products.

Buzzers come in two broad kinds. A passive buzzer is essentially the piezo element alone and needs an external alternating signal to set its frequency; the controlling circuit decides the pitch. An active buzzer contains a built-in oscillator, so applying a steady supply voltage makes it sound at a fixed tone, simplifying its use. The natural resonance of the disc and diaphragm gives each buzzer a frequency at which it is loudest, and designs are usually tuned so this resonance falls within the audible range.

The same piezoelectric principle is used in ultrasonic transducers, which operate above the range of human hearing. Driven at high frequencies, a piezo element radiates ultrasonic waves, and the reverse effect lets it detect the echoes that return. This pairing of emitter and receiver underlies ultrasonic distance sensors, parking sensors and similar ranging devices, where the time for a pulse to travel out and reflect back reveals the distance to an object.

Frequently asked questions

How does a piezoelectric buzzer make sound?

An alternating voltage flexes a piezoceramic disc bonded to a diaphragm, and the rapid movement pushes the air to create an audible tone.

What is the difference between an active and a passive buzzer?

An active buzzer has its own oscillator and sounds when given a steady voltage, while a passive buzzer needs an external alternating signal that sets its frequency.

How are ultrasonic transducers used for distance sensing?

A piezo element emits an ultrasonic pulse and detects its echo; the round-trip time, combined with the speed of sound, gives the distance to the reflecting object.