TELEPHONE TRANSMITTERS AND RECEIVERS 623 



dependent upon the efficiency with which these instruments made 

 these conversions. Experimental activities were, therefore, soon 

 directed to increasing the efficiency of these instruments and especially 

 to getting a more efficient transmitter than the forerunner of the pres- 

 ent telephone receiver which initially was used both as a transmitter 

 and a receiver. The outcome was the carbon contact transmitter 

 which provided a means for drawing upon an outside source of energy 

 in the process of converting sound waves into electrical waves and thus 

 combined in the transmitter the function of a converter of energy 

 with that of an amplifier. 



Since that time numerous important improvements have been 

 effected in both the carbon transmitter and the magnetic receiver but 

 the general principles of both are still employed today in the best 

 practical instruments for commercial telephony. Both of these 

 instruments employ vibrating diaphragms which, like other mechani- 

 cal vibrating systems, have regions of maximum response due to reson- 

 ance between the mass and elasticity of the diaphragm. With these 

 resonant effects inherent in the structure, it was natural to place them 

 in the frequency range so as to obtain the maximum benefit. In 

 accordance with this, the reproductions of speech sounds obtained 

 with these resonances located at different points were listened to and 

 the judgment reached, taking into account both the intelligibility and 

 naturalness of the reproduced sounds, that they should be placed 

 around 1,000 cycles. It was found that a material shift in the point 

 of maximum response of the circuit to a lower value made the output 

 sounds "boomy" and to a higher value rendered them "thin." While 

 precise means for measuring the effects were not available at that time, 

 subsequent work has substantiated this choice as a wise one. Inves- 

 tigations of the frequency components of speech sounds have shown 

 that the principal components of about half the vowel sounds lie below 

 1,000 cycles and of the other half are about equally divided above and 

 below this point. Articulation tests have demonstrated that the 

 frequency range which covers about an octave each side of 1,000 cycles, 

 namely from about 500 to 2,000 cycles, includes the more important 

 frequency components in speech from the standpoint of intelligibility. 

 The frequencies below this range are important primarily for natural- 

 ness and those above for intelligibility and also for naturalness. The 

 location of the region of maximum response of the telephone circuit 

 in the neighborhood of 1,000 cycles emphasizes then this 500 to 2,000- 

 cycle range and meets well the requirements of both intelligibility and 

 naturalness. 



In addition to the diaphragm resonances there are also inherent 



