The limitations of monaural sound are what bothered Alan Blumlein, the inventor of stereophonic sound. Enjoying a movie at the theater he was unhappy because the voices of actors from the screen's left and right were misplaced, always coming from the center where a single mono speaker sat. When the same actors spoke in the center of the screen it all made sense, but moving from side to side it seemed wrong to Blumlein. And so he placed the one to the left side, added a second loudspeaker on the right and stereo was born.
If the new stereo speakers shared reproduction of a voice or instrument with equal content and loudness, placement would be in the middle between the two speakers, forming a phantom center channel that mimicked the original single speaker. The reason this works is simple. A true mono source, one voice or instrument directly in front of you, produces equal sound in both ears. We interpret this as being centered. If the sound is slightly louder in the left ear than the right, we know placement is in the left direction. If the sound from the left reaches that ear first, the right ear a smidge later, this is further verification as to placement. Thus, if one speaker or two speakers provides equal sound arriving at the same time in both ears, we do not know the difference; it will always be perceived in the middle.
But reproducing with two what originally took only one is not a productive goal unless there are other advantages: like three dimensional sound. Blumlein wanted the audio placement of on-screen actors to follow their movements and it was obvious to him a left or right speaker served that purpose. The genius of the idea came not from a left or right mono speaker producing directional sound but everything in between. A single voice can seamlessly move from left to right and everywhere in between by varying the levels in each speaker; the middle position a result of equal sound pressure in both.
Now that we understand how stereo sound gives us left, right and everything in between, floating in space appearing to be unattached to a physical box, it's time to look at depth and how it is achieved.
