Stereophonic

(Greek: stereos = solid; phone = sound) Generally, a term used to refer to the spatial distribution of sound, normally using audio technology. More specifically, a form of reproduction which records, transmits and reproduces the original sound with two channels, regardless of the number of loudspeakers used. Also abbreviated to stereo.

In electroacoustic reproduction, there are various ways a stereophonic image may be produced, and the degree to which each method achieves a true two-dimensional stereo perspective of laterality and depth will vary. A simple monophonic source may be mixed from one channel to another (see mixing, pan), with amplitude differences of the signal in each channel producing the effect of lateral movement. Multiple monaural sources may be mixed so that they occupy different positions laterally; this is the basis of multi-track recording. These artificial stereo images reflect mixing processes rather than recording processes.

Two stereophonic recording processes are that of binaural recording and the Kunstkopf method using only two microphones which are placed in the ears of the recordist or those of an artificial head. The signals are usually reproduced with no mixing. Because these methods involve time and phase differences, as well as pinna reflections and Sound Shadowing of high frequencies - all of the methods of human sound localization - these methods are better able to convey the depth and perspective of normal hearing.

See: Binaural Hearing, Projicience, Stereoacusis.

Stereophonic reproduction was developed in the early 1930s by Bell Telephone Labs in North America and by the British sound engineer, A.D. Blumlein in Europe. The Blumlein microphone technique uses two figure-of-eight microphones (see directivity) placed at 90° to each other, nearly touching. Alternatively, two cardioid mikes are placed optimally at 120°. This method provides less control over individual components of the original sound (e.g. instruments or sections of an orchestra) than the multi-track method which is associated with North America. However, its image is often heard as being more natural and faithful to the original acoustic space. In addition, it spreads the stereo image over a wider area, gives a sense of depth, and is less dependent on the listener being about equal distances from each loudspeaker, as in most multi-track recordings or those which were created with pan-pot mixing.

Quadraphonic and octophonic reproduction is also a form of stereophonic reproduction, the main difference being the increase in the lateral dimension from about 180° to 360°. Also, if the speakers are at different heights, the vertical dimension may also be reproduced (this technique being called periphony). Compare: Diffusion.