A Sound Projector System for Use in 

 Motion Picture Theaters ^ 



By E. O. SCRIVEN 



Synopsis: The general problem involved in the design of a system 

 suitable to be used to record and reproduce sounds such as are required for 

 "talking" motion pictures is outlined. The general method of attack is 

 indicated. There follows a description of the several pieces of apparatus 

 which comprise the theatre equipment, including a discussion of some of 

 their salient features and of the part each plays in the sound projector 

 system. 



IN order to reproduce in a theater the pictorial record of events 

 accompanied by the sound associated with those events, it is, of 

 course, necessary to add equipment to that installed to produce only 

 the silent motion picture. It is the purpose of this paper to outline 

 and discuss briefly the major items of such equipment as developed 

 by Bell System engineers. 



In the design of sound equipment a primary requisite is that there 

 shall be freedom from distortion. Distortion may be of the sort 

 which is independent of load and is evident in that the intensity of 

 some portion or portions of the sound spectrum is increased or de- 

 creased in comparison with the rest; or there may be the distortion 

 which is a function of the level at which the device is operated and is 

 characterized by the reduction of a pure tone into fundamental and 

 one or more harmonics. This latter condition is most often the conse- 

 quence of operating a vacuum tube amplifier above its proper energy 

 handling capacity. 



It is the resonances of vibrating strings or reeds or air columns or 



vocal cords that give us the music we record but we are careful that a 



minimum of the resonances of the recording system itself shall go 



into the record, and that any resonances of the reproducing system 



shall not appear in the output of the sound projectors. Aside from 



the effects of overloading, the prevention of distortion is largely a 



matter of getting away from resonance phenomena since it is the 



characteristic of the resonant system to respond with disproportionate 



amplitude to stimuli in the region of its own natural period. The 



whole story of the passage from sound energy through the various 



recording and reproducing devices back to sound energy again is 



one of contest with this fundamental physical phenomenon. 



1 Presented before Society of Motion Picture Engineers at Lake Placid, New 

 York, September, 1928. 



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