CHAPTER X 

 MISCELLANEOUS TRANSDUCERS 



10.1. Introduction. — Interest in the science of sound reproduction has 

 been stimulated during the past two decades by the almost universal use 

 of the phonograph, radio and the sound motion picture. The two most 

 important acoustical elements in electrical reproduction of sound are loud 

 speakers and microphones. For this reason, considerable space has been 

 given in this book to complete discussions of the most common instruments. 

 There are innumerable electroacoustic, mechanoacoustic and electrome- 

 chanoacoustic transducers in use to-day for all types of applications. In 

 general, most of these can be reduced fundamentally to one of the systems 

 described in Chapter VI. Therefore, the major portion of the applications 

 discussed in this text will be confined to sound reproduction. In addition 

 to loud speakers and microphones, the following transducers are in common 

 use in certain types of sound reproduction : telephone receivers, mechanical 

 phonographs, phonograph pickups, electrical musical instruments and 

 hearing aids. It is the purpose of this chapter to consider typical examples 

 of these transducers. 



10.2. Telephone Receivers. — A telephone receiver is an electroacoustic 

 transducer actuated by energy in the electrical system and supplying energy 

 to an acoustic system. 



A. Bipolar Telephone Receiver. — The bipolar telephone receiver is a 

 telephone receiver in which the alternating force, due to the alternating cur- 

 rent in the electromagnet, operates directly upon a diaphragm armature of 

 steel. A cross-sectional view of a bipolar telephone receiver is shown in 

 Fig. 10.1. The steel diaphragm is spaced a small distance from the pole 

 pieces which are wound with insulated wire. A permanent magnet sup- 

 plies the steady flux. 



The force upon the diaphragm, in dynes, has been derived m Sec. 6.3A. 

 The force from equation 6.9 is 



^"^ ^ttRM R,R,J "^ R^'J 



lirN^i^rnai, COS 2(j}t 



R^A 

 lit 



