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394 CINEMATOGRAPHIC ANNUAL
vibrates as nearly like a rigid plunger as possible. This is accomplished by so shaping the center portion of the diaphragm that it is relatively stiff compared with the edge. Furthermore the coil which drives it is fastened around the outside of the stiffened central portion. The coil which is rigid and very light is made self-supporting. This form of mounting enables the coil to radiate heat readily, and therefore permits a large power input to the receiver without overheating.
The outstanding feature of this type of receiver is the high efficiency with which it converts electric energy into acoustic energy. In experimental models efficiencies as high as 50% have been realized. When you consider that a receiver of 100% efficiency would result in an increased sound intensity of three decibels, which is only a comfortably perceptible difference, it is not likely that much economy would be gained from higher efficiencies.
This type of receiver, when used, is attached to a horn which partially isolates a column of air from the surrounding medium. This column of air affords an acoustic coupling between the receiver and the space in which the sound is to be reproduced.
The horn is designed in such a way as to avoid interference between air waves as they pass through the chamber and the throat of the horn. The'horn used is shown in Fig. 11; its design and construction is referred to technically as "exponential," which qualifies its shape.
MAINTAINING SYNCHRONISM
Synchronization between sound record and photographic record is an inherent requirement of sound pictures. In projection this is usually accomplished by mechanically coupling the picture projection machine with the sound recorder. Synchronization in itself is not sufficient, however, for there is still the problem of speed control.
Since musical pitch depends upon the frequency, it is necessary in reproducing music with fidelity of pitch, that the sound record be run at identically the same speed as that at which it was made. To accomplish this, some reproducing systems make use of a synchronous motor or certain types of induction motors whose speed characteristics are inherently constant under certain given conditions. However, variations of load, supply voltage and frequency may produce noticeable variations in pitch of