A transducer is a device which converts one form of energy to another. Typically, one of these forms is electrical while the other is either mechanical, optical, or thermal. Transducers are usually classified as either input or output devices, depending on the direction in which they transfer energy or information. Input transducers convert some property or effect into an electrical signal, while output transducers start with electricity and generate a mechanical or other effect.
Transducers play such fundamental roles in modern technology that examples of them abound. In virtually every electronic device or instrument, transducers act as translators between electron flow and the physical world. Loudspeakers are perhaps the most well-known transducers, as they are used in nearly every audio system to convert electrical signals into acoustic ones. Like loudspeakers, audio microphones are transducers. Both devices have a small diaphragm which is free to move in either a magnetic or electric field. In speakers, electricity pushes this diaphragm to generate a sound. In microphones, the opposite happens, and sound pushes the diaphragm to generate an electric signal. Another common transducer in audio systems is the photodetector; in a compact disc player this input device combines with a photoemitter to optically sense encoded information on the disc and convert it to music.
A tremendous collection of transducers can be found in the human body. The senses convert complex sights, sounds, smells, and other experiences to electrical signals, which are then sent to the brain for interpretation.