Space...the Final Frontier SIDEBAR 2
Interestingly, Alan Blumlein's original 1931 patent application on stereophonic reproduction was quite specific about a two-channel system's ability to reproduce two dimensions---width and depth---to the front of the listener. This might be expected from the nomenclature: the word "stereophonic" is derived from the Greek word for "solid." To quote from the patent application, as reprinted in the 1987 AES Anthology, Stereophonic Techniques (footnote 1) compiled and edited by John Eargle:
"The fundamental object of the invention is to provide a sound recording, reproducing and/or transmission system whereby there is conveyed to the listener a realistic impression that the intelligence is being communicated to him over two acoustic paths in the same manner as he experiences in listening to everyday acoustic intercourse and this object embraces also the idea of conveying to the listener a true directional impression...An observer in the room is listening with two ears, so that echoes reach him with the directional significance which he associates with the music performed in such a room...When the music is reproduced through a single channel the echoes arrive from the same direction as the direct sound so that confusion results. It is a subsidiary object of this invention so to give directional significance to the sounds that when reproduced the echoes are perceived as such."
In other words, if you can record not only a sound, but also the direction in space that that sound comes from, and can do so for every sound wave making up the soundstage, including all the reflected sound waves (the reverberation or "echoes"), then you can reproduce a facsimile of the original soundstage, accurate in every detail. In addition, because the spatial relationship between the direct and reflected sounds will be preserved, that reproduced stereo soundstage will give a realistic illusion of depth.
The patent goes on to explain that "It will be clear that obliquity of the direction of sound wave propagation relative to the microphones...will produce differences of intensity at the loud speakers [sic] so as to give an impression to an observer of oblique sound incidence."
If you have two independent information channels, each feeding its own loudspeaker, then the ratio of the signal amplitudes between those two loudspeakers will define the position of a virtual, or phantom, sound source for a centrally placed listener equidistant from them. For any ratio of the two speakers' sound levels, this virtual source occupies a dimensionless point located somewhere on the line that joins their acoustic centers. The continuum of these points, from that represented by maximum-left/zero-right to that represented by zero-left/maximum-right, makes up the conventional stereo image.
If there is no recorded reverberant information, then the brain will place the virtual image of the sound source in the plane of the speakers; if there is reverberation recorded with the correct spatial relationship to the corresponding direct sound---if it is "coherent"---then the brain places the virtual image behind the speakers, the exact distance depending on the recorded direct-sound/reverberant-sound ratio.
Note that this illusion requires processing by the brain. Some people cannot perceive stereo reproduction at all---they always hear two loudspeakers and no phantom images between them. And as Gordon correctly points out, the tradeoff with stereo reproduction is that the reverberation enveloping the real-life listener of necessity folds into the frontal image. But for studio-based, nonclassical music recordings, this has not been a major factor in limiting stereo's acceptance. Only now, driven by the film industry's need for more than two information channels to compensate for the lack of three-dimensional visuals, has true surround music-only playback become practicable.---John Atkinson
Footnote 1: Available for $27 (AES members) or $30 (Nonmembers) from the Audio Engineering Society, 60 East 42nd Street, New York, NY 10165-0075. Tel: (212) 661-2355.---JA