Stereophonic Reproduction

Author: N.H. Crowhurst

Stereophonic reproduction

The stereophonic technique of reproduction takes the approach of endeavoring to recreate an original sound field, avoiding the necessity for wearing headphones. To do this, the recording microphones are spaced a given distance apart and the loudspeakers are spaced a similar distance apart. This, to some extent, recreates the original time and phase differences that set up the directional effect in the sound waves. It cannot completely achieve the desired result because the acoustic characteristics of the listening room as well as those of the studio are present. (The original sound field will never be fully recreated because of the additional reverberation added in the listening room.) If the reverberation in this listening room is reduced by putting very absorbent material on all walls, the floor, and the ceiling, the sound is still unnatural because then the reverberation would not appear to come from all directions in the same way as it did in the original performance.

In practice, the program is recorded in a studio that has less reverberation than the final reproduction should have, which permits the listening room characteristics to complete the sound pattern. It is better to allow the sound pattern to be slightly different from the original, but still a credible reproduction of it, than to try to reproduce the original sound exactly and fail in some more important aspect.

Stereo has its problems

For this last reason, it is particularly important in stereophonic reproduction to pay particular attention to how transients reach the listener and the method by which the reproducer systems throws them out into the room. If the transient pattern is a realistic recreation of the original, the reverberation effects will be relatively unimportant.

The ability of our hearing faculty to locate sound sources depends on three main differences in the sound at our two ears: intensity, time, and quality. If one sound is stronger at one ear than the other, the apparent direction is toward the side where it is received the strongest. If the sound is heard by one ear a fractional interval before the other, it will seem to come from the direction where it was heard earliest. Finally the hearing faculty notices the frequency content reported by both ears. It will also interpret a clearer sound, because more of the higher frequencies are present, as meaning that the sound originates from that side,

The hearing faculty uses all of these differences at once, but the relative importance it attaches to each will depend on room acoustics. In a big, reverberant room with a lot of echos, whether these are noticeable separately or just add confusion, time difference becomes almost meaningless. So our hearing concentrates on the other differences. In a smaller room, where reverberation is almost absent (and by the very size of the room, intensity must equalize somewhat), the hearing faculty gets extra critical about time differences.

This means that, in a large room, wide-spaced speakers fed with audio in which the intensity corresponding to different locations is emphasized will give the best stereo. In a small room, wide spacing can result in unnaturally large time differences for the room acoustics. Thus a system with loudspeakers close together, which relys on the way they radiate rather than on relative intensity, will do the best job.

Best arrangement for stereo depends on listening room