Stereophonic sound
From Biocrawler, the free encyclopedia.
Stereophonic sound is the reproduction of sound, using two independent audio channels, reproduced through a pair of widely separated speaker systems, in such a way as to create a pleasant and natural impression of auditory spatiality.
The word "stereophonic," from stereo-, solid + "phonic", sound, was coined by Western Electric, by analogy with the word "stereoscopic." Western Electric first demonstrated it at an SMPTE meeting in 1937, then to the general public at Carnegie Hall in 1940.
Engineers make a technical distinction between "binaural" and "stereophonic" recording. It is actually binaural recording that is more closely analogous to stereoscopic photography. In binaural recording, a pair of microphones is placed inside an actual model of a human head including external ear and ear canals. Each microphone is located where the eardrum would be.
The recording is then played back through headphones, so that each channel is presented independently, without mixing or crosstalk. Thus, each of the listener's eardrums is driven with a replica of the auditory signal it would have experienced at the recording location. The result is an accurate duplication of the auditory spatiality that would have been heard by the listener placed where the microphones were. Because of the nuisance of wearing headphones, true binaural recordings have remained laboratory and audiophile curiosities.
Stereophonic sound attempts only to produce a pleasing sense of spatiality. When a stereophonic recording is heard through loudspeaker systems rather than headphones, each ear of course hears sound from both speakers. The audio engineer may and often does use more than two microphones, sometimes many more, and may mix them down to two tracks in ways that exaggerate the separation of the instruments to compensate for the mixture that occurs when listening via speakers.
Descriptions of stereophonic sound tend to stress the ability to localize the position of each instrument in space, but in reality the experience of stereophonic sound is not so much directionality, but sound that is dramatically richer and clearer than monophonic (single-channel) reproduction.
From 1940 through 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.
The 1940 Carnegie Hall demonstration
The 1940 Carnegie Hall demonstration used three huge speaker system. 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 reporters. 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."
The motion picture era
The advent of magnetic tape recording made high-fidelity synchronized multichannel recording technically straightforward, though costly. Motion picture theatres could afford the cost, and that is where the real introduction of stereophonic sound to the public occurred, first with Cinerama in 1952. Cinerama was a spectacular wide-screen process fully comparable to today's IMAX. Cinerama practically required a specially built theatre for its presentation. It used six magnetic sound tracks. The system was developed by Hazard Reeves, a pioneer in magnetic recording technology. By all accounts, including accounts by those who have experienced the process in rare recent showings, the sound was as spectacular as the picture and excellent even by modern standards. The movie industry moved quickly to create simpler and cheaper wide-screen systems, such as CinemaScope, which were capable of being retrofitted into existing theatres. Cole Porter memorialized the era in a 1954 song:
- If Zanuck's latest picture were the good old-fashioned kind,
- There'd be no one in front to look at Marilyn's behind.
- If you want to hear applauding hands resound
- You've gotta have glorious Technicolor,
- Breathtaking Cinemascope and
- Stereophonic sound.
In the mid-1950s, companies such as Concertapes 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.
