Piezo Sounders are piezoelectric elements that are encapsulated in a casing and intended to produce an audio signal. Some buzzers have a built-in oscillator (Self driven, buzzer) that sounds with a fixed frequency when energized. Other buzzers (transducers) require a signal to sound. Summers have a resonant frequency where they work best. Outside its resonant range, the sound volume drops drastically. Loose piezo elements can be used both to convert voltage into vibration (sound) or vice versa to convert vibration into voltage.