SEVENTY-FIVE YEARS OF THE TELEPHONE 235 



rived from this work in other fields. The condenser transmitter developed 

 for the ideal telephone transmission system was the pickup microphone used 

 in the introductory period of public address systems, radio broadcasting, 

 electrical recording for phonograph reproduction and both disc and film 

 recording for sound pictures. Subsequent other high quaUty micro- 

 phones^^~^^ in these fields and the succession of loudspeakers^^"^ of increasing 

 quality of reproduction owe their development to the same techniques which 

 were evolved to improve Bell's instruments. The same techniques of design 

 were applied also to the light valve^- ®^ for film records, the electrical re- 

 corders^- ^^' " for disc records and to the many types of reproducers.^^ Thus 

 this technology of telephone instruments has had widespread application in 

 the mass use of sound reproduction in the phonograph, sound pictures and 

 broadcasting. 



Another application which would have been especially pleasing to Dr. 

 Bell was that to the microphones and receivers of hearing aids^^"'^^ and to 

 the measurement of hearing impairments^ ^'^^. Also of interest to him would 

 have been the contributions which these measuring techniques have made 

 to the work on the nature of speech and hearing. 



In addition, these measurement tools and devices derived from them 

 have provided solutions to many problems of architectural acoustics, and 

 of noise and vibration reduction. 



In World War II, this technology made it possible to determine quickly 

 the desirable performance characteristics of microphones and receivers suit- 

 able for the high noise conditions of military applications such as in planes 

 and tanks, and to develop the structures to provide this performance and 

 meet the other miUtary requirements. In the submarine field, this tech- 

 nology was applied to develop rapidly the instruments and methods for the 

 measurement of underwater sound and to design and improve the various 

 acoustic devices employed in that field.^^ 



The analytical quantitative equivalent network method of design for per- 

 formance, which has been applied in such a refined manner and so success- 

 fully to electro-acoustic devices, has been extended to mechanisms outside 

 the acoustic field. Many of those who participated in pioneering this kind 

 of design in acoustic devices are now engaged in the Laboratories on the 

 development of improved mechanical devices. This method is a powerful 

 tool but requires a type of training which is beyond that generally offered 

 to mechanical designers; their studies might well be directed along the lines 

 of the material in some of the articles cited here. 



All this constitutes a wonderful illustration of the manner in which the 

 results of research and fundamental development in a particular field and 

 for a particular purpose can ramify into other fields, and have as by-prod- 



