International projectionist (Jan 1963-June 1965)

Volume 38 INTERNATIONAL PROJECTIONIST May, 1963 Number 5 Good Sound for Drive-ins The Paramount Consideration in motion-picture pi ejection is the quality of the picture itself — its brightness, uniformity and color of illumination, clarity of focus, reproduction of photographic contrasts, and rocksteadiness. The movies are primarily a visual art; and the picture on the screen is what the public pays for at the boxoffice. Nevertheless, the quality of the sound reproduction must in no wise be relegated to second place, for sound and picture go together to create a lifelike illusion and to tell a story in audiovisual terms. Bad sound reminds the audience that it is only looking at a picture. There are important technical differences between the reproduction of sound in a drive-in, with its hundreds or thousands of separate speakers, and the production of high-fidelity sound in a conventional theatre. The management of an indoor theatre need consider only one "focal point" of sound reproduction, namely, the stage-speaker installation which provides the sound for every patron in the house. The system of amplifiers is built up to match a speaker assembly having a power capacity and sound-radiating characteristic best ^Matching transformer Speaker voice coil i — wmrm Rheostat volume control Ivlatching transformer Speaker voice coil Potentiometer volume control for constant impedi ance / FIG. 3 — A potentiometer is preferred to a simple rheostat for an in-car speaker volume control because of its constant impedance and load on the sound amplifier regardless of volume setting. 4 By Robert A. Mitchell suited to the size and acoustic properties of that particular auditorium. Adequate Sound Power Essential A drive-in theatre must not only have amplifiers of sufficient power output to provide an adequately strong sound signal to all of its many hundreds of individual in-car speakers, but should also have a reserve power which will minimize the "power robbing" effects of accidental variations in electrical load. Such variations are certain to occur when the speaker volume controls are of the simple rheostat, rather than the constantimpedance potentiometer type. The in-car speakers purchased for drive-in use should be large enough to give an acceptably strong reproduction of the low-frequency bass tones in the sound. In no case should the cone of the speaker be under 4 inches in diameter: 5-inch cones are recommended, though the quality of the sound reproduction by a speaker does not depend on the size of the cone alone. It should always be remembered that the smallest, cheapest speakers require more watts of power from the amplifiers to give the same volume of sound, that they usually give "tinny," distorted sound, and that they are constructed in such a way that it is impossible to re-cone or otherwise repair them. Speakers of good quality are more economical in the long run, and certainly satisfy the patrons in a way that cheap speakers can never do. Assuming an average consumption of 0.25 watt by each speaker and associated matching transformer (measured at 1000 cycles, 90 per cent soundtrack modulation), the following amplifier power outputs should be provided for drive-ins of various sizes: 400-car theatres, 100 watts. 500-car theatres, 125 watts. 600-car theatres, 150 watts. 800-car theatres, 200 watts. 1000-car theatres, 250 watts. 1200-car theatres, 300 watts. 1500-car theatres, 375 watts. 2000-car theatres, 500 watts. In addition to the amplifiers needed to supply the minimum required audio power, there should be at least one stand-by amplifier of 25 watts output for emergency use. An amplifier of this size is large enough to serve 100 cars ( 1 to 4 ramps) in the event of amplifier breakdown. Each drive-in speaker is powered through a matching transformer usually located in the junction box on the speaker post. Accidental short circuits in the speakers or their connecting cords cannot, therefore, render other speakers on the same ramp inoperative. Leeway in Impedance Matching The way the speaker-matching transformers are interconnected depends upon the impedances of their primary (input) windings and upon the output impedance of the International Projectionist May 1963