Abstracts of Technical Articles from Bell System Sources 



Modern Theater Loud Speakers and their Development} C. Flanna- 

 GAN, R. Wolf, and W. C. Jones. Although many of the basic ideas 

 involved in the operation of present-day loud speakers were conceived 

 during the early stages of the development of the telephone, it was not 

 until the advent of the vacuum tube amplifier that these principles 

 were applied to the design of structures capable of delivering sufficient 

 acoustical power to be audible throughout a room or auditorium. 

 Having reached this stage, however, the developments that culminated 

 in the sound reproducing systems employed with present-day sound 

 pictures came in rapid succession. These developments have em- 

 braced all phases of loud speaker design, with the result that systems 

 are now available that convert from 25 to 50 per cent of the electrical 

 input into acoustical output, and maintain conversion efficiencies of 

 this order of magnitude over a frequency range of 50 to 10,000 cps. 

 These systems are so designed as to be capable of reproducing the 

 recorded sound at intensities that not only greatly enhance the 

 dramatic effect of the presentation in the theater, but also open 

 entirely new fields in recording. All these improvements have been 

 attained with a reduction in distortion and improved fidelity of the 

 reproduced sound. The directional properties of the loud speakers 

 also have been markedly improved, with the result that the better 

 quality of reproduction achieved is available throughout the entire 

 seating area and the undesirable beam effects previously experienced 

 have been eliminated. 



Power System Faults to Ground — Part I: Characteristics} C. L. 

 GiLKEsoN, P. A. Jeanne and J. C. Davenport, Jr. The results of 

 an extensive oscillographic study of power-system faults to ground 

 are presented herewith. While this study was made primarily to 

 obtain data useful in inductive coordination problems, the results are 

 believed to be of general interest as well. They provide data on such 

 items as frequency of occurrence of ground-current disturbances, 

 their monthly distribution, duration, cause, method of clearance, and 

 wave-trace characteristics. Data on fault resistance are given in 

 part II, a companion paper. 



' Jour. S. M. P. E., March 1937. 

 ^Elec. Engg., April 1937. 



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