Stereophonic sound, commonly called stereo, is the reproduction of sound, using two or more independent audio channels, through a symmetrical configuration of loudspeakers, in such a way as to create a pleasant and natural impression of sound heard from various directions, as in natural hearing. It is often contrasted with monophonic (or "monaural", or just mono) sound, where audio is in the form of one channel, often centered in the sound field (analogous to a visual field).

Stereo recordings are used in FM broadcasting and DAB and in several television systems. To record in stereo, sound engineers use various methods, including using two directional microphones, two parallel omnidirectional microphones, or more complex techniques. To remaster monophonic records, various techniques of "pseudo-stereo" or "quasi-stereo" are used to create the impression that the sound was recorded in stereo.

The BBC made radio's first stereo broadcast in December 1925. In the 1930s, Harvey Fletcher of Bell Laboratories investigated techniques for stereophonic recording and reproduction. The first commercial motion picture to be exhibited with stereophonic sound was Walt Disney's Fantasia (1940). By the mid-1950s, multichannel sound was common for big-budget Hollywood motion pictures. In 1953, Remington Records began taping some of its sessions in stereo, including performances by Thor Johnson and the Cincinnati Symphony Orchestra. The US Federal Communications Commission announced stereophonic FM technical standards in April 1961, and licensed regular stereophonic FM radio broadcasting to begin in the United States in 1961. In 1984, Multichannel television sound was adopted by the FCC as the U.S. standard for stereo television transmission.

Description

The word "stereophonic"  derived from Greek stereos = "solid" and phōnē = "sound"  was coined by Western Electric, by analogy with the word "stereoscopic". In popular usage, stereo usually means 2-channel sound recording and sound reproduction using data for more than one speaker simultaneously. In technical usage, stereo or stereophony means sound recording and sound reproduction that uses stereographic projection to encode the relative positions of objects and events recorded. A stereo system can include any number of channels, such as the surround sound 5.1- and 6.1-channel systems used on high-end film and television productions. However, in common use it refers to systems with only two channels. The electronic device for playing back stereo sound is often referred to as "a stereo".

During two-channel stereo recording, two microphones are placed in strategically chosen locations relative to the sound source, with both recording simultaneously. The two recorded channels will be similar, but each will have distinct time-of-arrival and sound-pressure-level information. During playback, the listener's brain uses those subtle differences in timing and sound-level to triangulate the positions of the recorded objects. Stereo recordings often cannot be played on monaural systems without a significant loss of fidelity. Since each microphone records each wavefront at a slightly different time, the wavefronts are out of phase; as a result, constructive and destructive interference can occur, if both tracks are played back on the same speaker. This phenomenon is known as phase cancellation.