A headphone is one or two earpieces with head support that deliver sound energy directly into one or both ears of the wearer. Where there are two earpieces, the phones can be either monaural or binaural. Monaural headphones accept electrical signals from an audio amplifier, convert the signals into acoustic energy, and present the same acoustic information to both ears. Binaural phones accept separate signals from each of two amplifier channels, convert the signals into acoustic energy, and usually present three-dimensional sound. Because the acoustic signal is directly coupled into the ear, little power is used and little sound energy is radiated into the surroundings. Thus, the sound from the headphone usually is inaudible to persons near the wearer.

The mechanism that converts the incoming electrical signals into acoustic energy is called a transducer. It makes use of a current-carrying coil that moves a diaphragm back and forth, or it makes use of a special crystalline material that deforms under an applied electric field and moves a diaphragm back and forth. The transducer most often is located in the earpiece, but it sometimes is located remotely in a separate enclosure. In the first case, electrical signals flowing to the earpiece are carried by wires; in the second case, sound waves traveling to the earpiece are sent through a hollow acoustic tube.

Headphones of various designs and quality are widely used in personal radios, high-fidelity sound reproducers, computer-assisted instructional systems, and other communications systems.

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