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1954-55 Theatre Catalog, 12th Edition, Page 282 (246)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 282
Page 282

1954-55 Theatre Catalog, 12th Edition, Page 282

tre, this is a. truly serious problem. Here the very factors which produce the stereophonic effect prove a disadvantage in some aspects, and measures must be taken to compensate them, Source Position Shift as Observer Moves-The effects so far described characterize listening at the position of Figure 2, or other listening positions on the center line where the distances to the side loudspeakers are equal. They also apply to other observing positions qualitatively, but as the observer moves away from the center large shifts of virtual source position may occur. The stereophonic feeling of spaciousness is preserved, and virtual sources continue to move, but they are not localized at the same place on the stage by all listeners as they would be on a real stage. Figure 3 illustrates what is happening for a source at center of the pickup stage, and a typical setup. Observer 1 receives identical direct sound pulses from the two side channels. Ever here, hOWever, the center-channel sound arrives slightly ahead of that from the sides, and at greater amplitude. In practice, the center channel is operated

at lower gain than the side channel to correct for this.

Observer 2 at the right, receives pulses from the three loudspeakers with the relative times and intensity level shown. It is seen that the righthand loudspeaker now contributes both more intense signal and an earlier signal than before; and both of these effects are known to make localization tend in this direction. This is indeed the case, and as the observer moves to the right the virtual source position moves in the same direction. Note that the differences in time are several milliseconds. Qualitatively (again based upon personal experience) it is found that a considerable shift takes place for small observer deviations from the center, where relative intensity changes are small. These must be ascribed to changes in arrival time. For any given observer position these shifts can be compensated by changes in channel gains, and appear to become relatively constant at anything over a few milliseconds. Obviously the effects of intensity increase can be overcome by unbalancing the channel gains,

Methods of Reducing Shifts e The United States Patent 2,137,032 contains

THE HOLLYWOOD Bowl shell with decorative cloth columns hiding loudspeaker platform in 1536.

a suggested method of alleviating these troubles. The loudspeakers would be so designed as to project a delayed signal and one of reduced intensity in the forward direction compared to the side directions. This would tend to equalize conditions for the various observing positions.

Suppose that observer 2 remains at the right while the source moves to the left. The intensity increases in the left channel, but more important, the arrival times become more nearly equalized, and the virtual source moves toward the left. Only the intensity change is duplicated in the bridged channel, so that there is definite advantage in the real system considering all observing posi> tions. If the source moves to the right, the arrival time disparity is aggravated; but since there appears to be a limit to the effect of arrival time, this negative effect is smaller than the positive ad vantage for movement to the left, and an overall gain results.

ONE OF THE Iowelroquency loudspeakers on tho elevated platform. The loudspeaker was In loot high and 12 toot across the mouth of the ham.

If the observer turns his head to follow movement of the virtual source. the effect is to oppose the movement, since the ear on the side of the head in the direction of movement in effect turns away from the loudspeaker of increasing intensity, and the opposite ear turns toward the loudspeaker of decreasing intensity. Since the sound tends to move "too fast" toward the microphone being approached because of the combined effects of intensity and arrival time, this is an advantageous compensating factor, considering all seats in the auditorium.


The practical art of applying stereophonic reproduction for public use is now building up rapidly. This paper is concerned primarily with the underlying principles, but it seems useful to

VIEW OF the temporary platform erected to carry the loudspeakers. It was IIO loot long and [5 tea! high. and was used in ISJS demonstration.

1954-55 Theatre Catalog, 12th Edition, Page 282