History
1881
Clément Ader demonstrated the first two-channel audio system in Paris in 1881, with a series of telephone transmitters connected from the stage of the Paris Opera to a suite of rooms at the Paris Electrical Exhibition, where listeners could hear a live transmission of performances through receivers for each ear. Scientific American reported,
Every one who has been fortunate enough to hear the telephones at the Palais de l'Industrie has remarked that, in listening with both ears at the two telephones, the sound takes a special character of relief and localization which a single receiver cannot produce. . . . This phenomenon is very curious, it approximates to the theory of binauriclar auduition, and has never been applied, we believe, before to produce this remarkable illusion to which may almost be given the name of auditive perspective.
This two-channel telephonic process was commercialized in France from 1890 to 1932 as the Théâtrophone, and in England from 1895 to 1925 as the Electrophone. Both were services available by coin-operated receivers at hotels and cafés, or by subscription to private homes.

1930s
In the 1930s, Harvey Fletcher of Bell Laboratories investigated techniques for stereophonic recording and reproduction. One of the techniques investigated was the 'Wall of Sound,' which used an enormous array of microphones hung in a line across the front of an orchestra. Up to eighty microphones were used, and each fed a corresponding loudspeaker, placed in an identical position, in a separate listening room.
Several stereophonic test recordings, using two microphones connected to two styli cutting two separate grooves on the same wax disc, were made with Leopold Stokowski and the Philadelphia Orchestra at Philadelphia's Academy of Music in March 1932. The first, made on March 12, 1932 of Scriabin's Prometheus: Poem of Fire, is the earliest surviving stereo recording.
Bell Laboratories gave a demonstration of three-channel stereophonic sound on April 27, 1933 with a live transmission of the Philadelphia Orchestra from Philadelphia to Constitution Hall in Washington, D.C. Leopold Stokowski, normally the orchestra's conductor, was present in Constitution Hall to control the sound mix.
A stereo recording method, using two channels and coincident microphone techniques, was developed by Alan Blumlein at EMI in 1931 and patented in 1933. A stereo disc, using the two walls of the groove at right angles to carry the two channels, was cut at EMI in 1933, twenty-five years before that method became the standard for stereo phonograph discs.

1940 to 1970
From 1940 to 1970, the progress of stereophonic sound was paced by the technical difficulties of recording and reproducing two (or more) channels in synchronization, and by the economic and marketing issues of introducing new audio media and equipment. To a rough approximation, a stereo system cost twice as much as a monophonic system. Actually, in the 1950s that was an accurate approximation, since a stereo system had to be assembled by buying two preamplifiers, two amplifiers, and two speaker system. It was not clear whether consumers would think the sound was so much better as to be worth twice the price.
In the early 1950s, companies such as Concertapes and Victor began releasing stereophonic recordings on two-track prerecorded reel-to-reel magnetic tape. Serious audiophiles, the sort of people who would later be called "early adopters", bought them, and stereophonic sound came to at least some living rooms. Stereo recording became widespread in the music business by the fall of 1957, superseding monaural (single-channel) recording.The small record company Audio Fidelity released the first commercial stereophonic disc in 1958.
The 1940 Carnegie Hall demonstration
The Carnegie Hall demonstration by Bell Laboratories on April 9 and 10, 1940, used three huge speaker systems. Synchronization was achieved by making the recordings in the form of three motion-picture soundtracks recorded on a single piece of film. Because of dynamic range limitations, volume compression was used, with a fourth track being used to regulate volume expansion. The Dolby noise reduction system of the 1970s was a far more sophisticated version of a basically similar technique. The volume compression and expansion were not fully automatic, but were designed to allow manual studio "enhancement", i.e. the artistic adjustment of overall volume and the relative volume of each track.
The recordings had been made by the Philadelphia Orchestra, conducted by Leopold Stokowski, who was always interested in sound reproduction technology. Stokowski personally participated in the "enhancement" of the sound.
The speakers used generated 1500 watts of acoustic power, producing sound levels of up to 100 decibels, and the demonstration held the audience "spellbound and a little terrified", according to one report. Sergei Rachmaninoff, who was present at the demonstration, commented that it was "marvellous" but "somehow unmusical because of the loudness". "Take that 'Pictures at an Exhibition'", he said. "I didn't know what it was until they got well into the piece. Too much 'enhancing', too much Stokowski."

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