transducer, device that accepts an input of energy in one form and produces an output of energy in some other form, with a known, fixed relationship between the input and output. One widely used class of transducers consists of devices that produce an electric output signal, e.g., microphones and photoelectric cells. Other widely used transducers accept an electric input, e.g., loudspeakers, light bulbs, and solenoids. The term transducer is sometimes applied to devices producing an output in the same form as their input, e.g., transformers and filters.