Two-Channel Stereo Recording

Two-channel stereo is the longest established and simplest of sound field imaging techniques. The use of two microphones and two loudspeakers allows spatial information in the horizontal plane to be approximately recorded.

The effectiveness of stereo depends both on the relative position of the two microphones and on the position of the loudspeakers in relation to the listener. At best it can be impressive, but many "stereo" reproduction setups effectively destroy the effect - notably portable "stereo" players with loudspeakers a foot (60 cm) or less apart played across the room from the listener. Headphones reproduce recordings from many stereo configurations exceptionally well, but with some recording techniques it's possible to experience a disconcerting "hole in the head" effect via headphones, whereby sounds seem to be positioned within the head rather than in front of the listener.

Stereo works - when it works - by providing information about the relative loudness - and possibly timing - of sounds arriving from different positions in the sound field. At its simplest, two directional microphones are placed in exactly the same position - within half an inch (1 cm) or so - angled so that their pickup patterns overlap by a specified amount. Sounds coming from directions other than straight ahead are picked up by one microphone more strongly than by the other due to the shape of the pickup pattern, so the resulting two tracks differ in volume at that point in time, the difference reflecting the off-axis angle the sound came from. This technique has the benefit that recordings made with it reduce to mono perfectly, but it's limited in other ways, notably the maximum width of sound field it can handle and the kinds of microphones that can be used.

A more sophisticated arrangement uses two microphones angled as before, but also spaced a short distance apart. In addition to the differential strength of pickup from different directions, there's also a differential delay resulting from the extra distance the sound waves have to travel between the nearer and further microphone. This is a much more flexible arrangement as both the angle and the distance between the microphones can be adjusted. It can accommodate a wide range of sound field widths and almost any directional microphones can be used. However, recordings made with some of its variants cannot be reduced to mono without introducing unpleasant artefacts due to the time delays.