310 



ARCHITECTURAL ACOUSTICS 



MICROPHONES 



The action of the carbon microphone has been described in Sec. 9.2A. 

 The action of the telephone receiver has been described in Sec. 10.2. A 

 simple telephone circuit employing a carbon microphone, a telephone 

 receiver and a battery is shown in Fig. lO.lly^. Employing these three 

 elements and a transformer there are large numbers of possible circuits. 

 In local transmission the electrical output of the microphone is sufficient 

 for the telephone receiver to generate sound of ample loudness for intelligent 

 transmission of speech. In long distance telephony, vacuum tube repeaters 

 are used at regular intervals to restore the level of transmission to normal. 

 The electroacoustic transducers of the telephone, namely: the micro- 

 phone and telephone receiver, have been described in this book. The 

 consideration of circuits, repeaters, automatic exchanges, etc., are outside 

 the scope of acoustics and the reader is referred to books ^^"* on these 

 subjects. 



B. Binaural Reproduction. — An ideal binaural sound reproducing sys- 

 tem ^^ is shown schematically in Fig. 12.18 

 which indicates that the desired objective is 

 obtained by effectively transferring the audi- 

 tor to the point of scenic action through the 

 intermediary of a double recording and repro- 

 ducing channel. Two microphones Mr and 

 Ml simulate the ears of a dummy, each 

 receiving the component of the original 

 sound that would normally be received were 

 the dummy a human being. Each compo- 

 nent is reproduced through a separate audio 

 channel, each channel terminating in a high 

 quality telephone receiver. Each of the re- 

 ceivers is placed on the proper ear by the 

 auditor and the sound produced in each of 

 his ears will be identical to what would have 

 been produced at the original set had he 

 been there at the time. 

 The advantages of this system are quite obvious; the binaural effect 

 is practically perfect, and the reverberation characteristic of the set (which 

 should be designed to conform to the scene) is transferred unadulterated 

 to the listener. 



DUMMY 



AMPLIFIER 



AMPLIFIER 



TELEPHONE 

 RECEIVERS 



Fig. 12.18. Schematic arrange- 

 ment of the apparatus for a 

 binaural reproducing system. 



^^^ Johnson, " Transmission Circuits for Telephonic Communication," D. Van 

 Nostrand Co., New York. 



32 Olson and Massa, Jour, Soc. Mot. Pic. Eng., Vol. 23, No. 2, p. 63, 1934. 



