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1954-55 Theatre Catalog, 12th Edition, Page 280 (244)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 280
Page 280

1954-55 Theatre Catalog, 12th Edition, Page 280

the observer. He therefore receives three faithful copies of the sound at each ear in rapid succession. The time diderences between these pulses are too short to allow the ear to distinguish them as separate; consequently the hearing mechanism fuses them into an illusion of a single sound pulse coming from a virtual sound source located somewhere in the space between the outer loudspeakers. The time differences are short, but still long compared to the maximum of 700 uses to which the ears are accustomed in normal listening. Thus this type of listening falls outside of normal experience, but fortunately the brain is able to form a single concise impression from what might be expected to be a confusing set of signals sent by the ears. The closest parallel is reverberation. But while there are distinct similarities, the three direct-sound pulses arrive ahead of any reflections other than the floor reflections which do not have individual directivity. In addition, they are separate and distinct, of high fidelity, and in compact directive pattern The reverberation follows as a usmear" of echoes of random directivity, and does not create a virtual source illusion. The problem then in stereophonic reproduction is to produce multiple. sound images at the cars of the observer which


will fuse in such a way as to give the desired illusion of sound origin.

Angular Perception

v-Intensity DifferenceseWhat are the characteristics of the direct-sound pulses which cauSe them to give the observer the sensation of angular localization of the virtual source? The most obvious difference is intensity of sound projected by the several loudspeakers. These differences are caused by the varying distances of the source from the various microphones. When the source moves close to a microphone the output of the loudspeaker is greater than that or" the other loudspeakers, and localization tends in its direction. The virtual source therefore. moves in the same. direction as the, real source, and with proper system design can be made to have essentially proportional movement. In the original paper Dr. Steinbergr and this writer discussed this in detail and proposed a theory for thc effect of these intensity differences based upon the total loudness that would be produced in each car by the total direct; sound from all loudspeakers, taking into account the directivity of hearing caused by the. shape. of the head. While the agreement between the theory and experimental results was by no means

ELECTRICAL EQUIPMENT used in the 1832 systom. installed in the basunent of Philadelphia Academy of Music. The author can be seen at righl.

perfect, and the differences were pointed out, the theory did appear to account for the main effect. This theory has been questioned by other experimenters, principally, it is believed, because of the common confusion between the mechanisms of ordinary binaural hearing and stereophonic hearing which the disscussion above should have now dispelled.

While a true understanding of the process is highly desirable, for the purposes of this paper it is not necessary to be certain of the precise physiological and psychological mechanism involved. It is well established that intensity differences in the channels are an extremely important contributor to angular perception. With positions of source and observer fixed so that all other factors are constant, variation of the gain'controls in the channels can shift the virtual source to any angular position in the reproducing area. This is true for any combination of source and observer positions. In practice this is important because gain is easily controlled, to correct faults in pickup, or to enhance angular movement. The bridge-microphone system of Figure 1 operates on this basis, since the only differences that can be given the loudspeaker outputs must be obtained from electrical controls in the channels. As this is written many pictures are being made itstereophonici, by the use of volume controls in bridged channels from sound tracks originally recorded for single channel or tfmonophonicli reproduction. The pseudostereophonic system has its place; but it is not a satisfactory substitute for a real stereophonic pickup. It does not have the benefit of the other aids to angular or depth perception described; and in particular it can be. used on only a single source at one time so that an individual source and ffpan-out" must be supplied for each sound.

Quality Differences

If the microphones have varying directivity, with frequency, there are quality differences as well as intensity differences in the channels as the source moves. Angular localization is definitely affected by this. It has been found that the higher frequencies where the head has relatively high directivity, contribute most to stereophonic localization. Localization tends toward the loudspeaker givingggreatest high-frequency output, if the overall loudness is the same.

The veryvlow frequencies contribute essentially nothing to stereophonic locah ization. For example, poor localization results if MOO-cycle low-pass filters are inserted, and no diffcrencc in localization is produced by eliminating frequencies below 301) cycles. It has, been found that much of the stereophonic effect is prescrvcd if low frequencies are reproduced from only one low-frequency unit and side channels reproducing only frequencies above 300 cycles are employed, This is of great practical value for economical stereophonic reproduction such as home music'systems. For the flexibility and high- fidelity

1954-55 Theatre Catalog, 12th Edition, Page 280