Audio transducers and buzzers are used in electronic equipment, toys and various warning devices as sounding components. They can be either electromagnetic or piezoelectric in operation. Electromagnetic transducers generate either a single continuous sound or intermittent tones and are available with either self-contained or external drive circuitry. Piezoelectric transducers include a metal plate that vibrates with the expansion or contraction of a piezoceramic element. Some audio transducers can transmit signals, some can receive signals, and some can both transmit and receive signals.