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1954-55 Theatre Catalog, 12th Edition, Page 279 (243)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 279
Page 279

1954-55 Theatre Catalog, 12th Edition, Page 279

FIGURE 1 is an actual 3-chnnnel eteroophonic system. A practical system gives a multiple reproduction of the original sound which the observer interprets as coming from single source.

The arrival-time effect is aided by the quality differences at the ears caused by sound diffraction. Quality differences also change most rapidly near the median direction; consequently, angular localization is much less precise at the. side than in front or back.

Changes in both arrival time and quality are relatively small as a source is elevated in front of an observer. Therefore, the ability to distinguish angle in the vertical direction is relatively poor.

The statement is made in Figure 1 that the observer cannot turn to face the source. While systems have been constructed with servo connections between observer and dummy, thereby localization, this is not practicable for a system used with multiple observers, or with a recording link,

Depth Localization

Perceiving the position of a sound source- in space involves the determination of distance as well as angle. The car has no mechanism corresponding to that of the eye for converging- on the source, and must depend on less definite clues. In the absence of reverberation, the Only information given is intensity and quality. From past experience the car can form an approximate idea of distance from its interpretation of the absolute loudneSS of a sound, and from its judgment of quality differences produced by atmospheric absorption. These comparisons are made with a mental image of what the sound should be. In the presence of reverberation, the ear can judge distance based on the ratio of direct to reverberant sound. Since neither of these methods is precise, judgment of distance is much less accurate than perception of angle. Probably everyone has had the experience of badly misjudging the distance of a sound heard for the first time, whereas no difficulty was experienced in determining its direction.

Fundamental Ditterence from

Stereophonic Sound

This discussion of the determining physical factors underlying ordinary binaura] hearing has been given at some length to lay a foundation for the discussion of those underlying stereophonic reproduction. There are basic differences which have been almost universally overlooked. When this confusion is cleared up, stereophonic reproduction can be used with much greater ease and satisfaction.


It has become customary to describe stereophonic reproduction as follows: A Screen consisting of an extremely large number of extremely small microphones is hung in front of the sound source. Each microphone is connected to a cor 'HGURE 3 lhows the effects of changing listening location. A: the observer moves aw from the center-line of the auditorium the noun increaseIn in intensity from the "near" loud-pecker.





responding extremely small loudspeaker in a screen of loudspeakers hung before the audience. Then the sound projected at the audience will be a faithful copy of the original sound, and an observerwill hear the sound in true auditory perspective. It is then stated that such an impractically large number of channels is not needed and that good auditory perspective can be achieved with only two or three channels. These are true statements, and the natural interference from their juxtaposition is that far less than faithful itspace" reproduction of sound will give localization by ordinary binaural mechanisms.

When we proposed this theory early in our studies of stereophonic phenomena, we realized that there were fundamental differences which were not fully understood, and pointed out the multiple source effect in connection with our loudness calculations. Apparently this has not been sufficiently emphasized. The experience of the intervening 20 years has convinced this writer that this natural inference is mistaken, and has


caused the confusion postulated in the previous section.

The myriad loudspeakers of the screen, acting as point sources of sound identical with the sounds heard by the microphones, would project a true copy of the original sound into the listening area. The observer would then employ ordinary binaural listening, and his ears would be stimulated by sounds identical to those he would have heard coming from the original sound source. As shown in Figure 1, this means one direct-sound pulse to each ear for a single pulse from the source.

Operating Conditions

Figure 2 illustrates the conditions for a typical setup where only three channels are used. This arrangement does indeed give good auditory persepective, but what has not been generally appreciated is that conditions are now so different from the impractical "infinite screen" setup that a different hearing mechanism is used by the brain. Each individual loudspeaker sends a pulse to

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1954-55 Theatre Catalog, 12th Edition, Page 279