242 



BELL SYSTEM TECHNICAL JOURNAL 



would accomplish this involves many considerations it will be assumed in 

 this discussion that an ideal transmittion system is one in which the sound 

 waves impressed on the listeners' ears in the home are an exact replica of the 

 sound waves striking the microphone in the distant studio. Limitations 

 inherent in the human ear, in the program material to be transmitted, and 

 in the usual listening conditions, however, make such ideal transmission 

 unnecessary. In expressing the requirements for satisfactory transmission, 

 frequency range, attenuation distortion, delay distortion, nonlinearity and 

 noise are used as indices of quality. 



Fig. 5 — Monitoring position at Washington, D. C. 



Before taking up the transmission requirements of a program circuit, it is 

 important to consider further the fundamental factors that are involved in 

 fixing the characteristics considered desirable for the entire system. Ac- 

 cording to Harvey Fletcher,^ the zone of audibility of the average normal 

 human ear for pure or single frequency sounds is the area within the curve 

 of Fig. 6. The abscissas represent frequency and the ordinates show the 

 range of intensity recognizable as sound, between the lower limit or threshold 

 of audibility and the upper limit where the sensation of pain is felt. It is 

 seen that the extreme frequency range shown on the chart is from about 20 



