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1954-55 Theatre Catalog, 12th Edition, Page 283 (247)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 283
Page 283

1954-55 Theatre Catalog, 12th Edition, Page 283

THIS RARE photo shows the electrical equipment used in 1939 Academy of Music demonstration.

give some illustrative examples of how they are used. These examples are primarily of situations with which the author has personal experience.

Number of Channels

The number of channels will depend upon the size of the stage and listening rooms, and the precision in localization desired. Two channels give a large measure of the spacious effect desired for stereophonic reproduction, and will give fairly accurate localization for a small stage. Such a system on an ordinary-sized stage will give quite different localization impressions to observers in different parts of the auditorium, and is apt to suffer from the ffhole-in-thecenter" effect where all sounds at center stage seem to recede toward the back. Nevertheless, for a use such as rendition of music in the home, where economy is required and accurate placement of sources is not of great importance if the feeling of separation of sources is preserved, two-channel reproduction is of real importance.

That this is true is borne out by the current sponsored programs being broadcast by radio stations in various parts of the country using the FM transmitter for one channel and the AM transmitter for the other. Experience with this service in the writers home has demonstrated the great increase in enjoyment it provides. Various methods for utilizing a single carrier for this type of broadcasting have been proposed, using upper and lower sidebands separately, simultaneous AM and FM modulation, and modulating one channel on a sub-carrier which is then modulated with the other channel on a regular FM transmitter. For such service the idea of supplying only one low-frequency loudspeaker appears important. It is well to recognize that a poor crosstalk ratio between channels in such a stereophonic system is not serious, because the relative intensity levels in the two channels never become greatly different. Thus systems which could not be considered for separate programs may be usable for stereophonic reproduction.

Three channels appear to be a good economic choice for ordinary stages and auditoriums. Good accuracy of localization can be achieved for favorable observing position, with reasonable results at other seating locations. The center channel is a great aid for solo and closeup work, as well as removing the "holein-the-center" effect mentioned before. For unusually wide stages, additional channels have been found necessary. At present it may be taken as a rule of thumb that additional channels should be considered when stage dimensions require channels spaced more than 25 feet on centers.


PlacementeLoudspeaker placement is straightforward if considered for sound alone. The outside loudspeakers are placed at the outside edges of the Space considered the reproducing stage, since sound cannot be made to travel past the outside speakers. The center, or other


loudspeakers are placed at uniform spacing across the stage. It was stated that the close microphone position ordinarily used makes it possible to enhance depth effects. The source can therefore be made to appear in front of the loudspeakers, and they may be placed a few feet back of the front of the stage, In the Bell System demonstration at Carnegie Hall in 1940, the outside loudspeakers were spaced 40 feet on centers, and the front of each loudspeaker was 11 feet back of the decorative sound transparent front curtain. This curtain was illuminated in various simple color patterns during the performance an artifice which adds enjoyment when no picture accompanies the sound.

For sound-picture reproduction, the effect of the picture is great, and the precision of localization required is smaller. If the sound tends to be in the region of the visible source, it will be localized there. Consequently, here it is possible to create the illusion of sound outside the farthest loudspeaker.

When the stereophonic system is used for sound reinforcement. serious difficulty may be experienced in placing the loudspeakers where they will not obstruct the view. Fortunately here, also, the source is visible. In addition, it was shown that localization in the vertical plane is poor. The loudSpeakers can therefore be placed above or below the stage level without loss of illusion, provided high fidelity of repdoduction is maintained. It is also sometimes possible

to use a smaller loudspeaker in the center positions, without full low-frequency response, to give proper localization. One of the most successful stereophonic reinforcement systems was tested in the Hollywood Bowl in 1936, where the loudspeakers were mounted on a platform 45 feet abOVe the stage level. The system supplied almost uniform sound throughout the seating area, and considerable amplification even for the closest seats. Nevertheless, the illusion that the sound came directly from the orchestra in the shell was excellent. To preserve a good illusion the loudspeakers should have approximately the same spacing as the channel microphones. Characteristics-Since the illusion is caused by the receipt of multiple sound pulses, and in view of the observerposition effects discussed before, it is important that the loudspeakers give uniform angular coverage of the whole seating area. Actually greater energy should be supplied to seats at the side than to those in front of a loudspeaker, the inverse of the ordinary loudspeaker directional characteristic. Some toeing-in of the outside loudspeakers will help the average situation, In addition to these factors, de Boer also recommends minimizing sound projection to areas outside the audience to reduce wall reflections, and maintaining the quality of the several channels above 300 cycles as alike as possible. Quality differences will be interpreted in the stereophonic illusion as differences in direction.
1954-55 Theatre Catalog, 12th Edition, Page 283