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BELL SYSTEM TECHNICAL JOURNAL 



the mind can perceive, and how it reacts to somewhat larger distortions. 

 This information is utilized in determining a reasonable basis of design 

 both for separate instruments and for transmission systems as a 

 whole, to give a proper balance between cost and performance. 



I can only indicate a few of the important results of our investiga- 

 tions. One of the first steps was to determine in a quantitative way 

 the performance of our ears as machines. It was obviously important 

 to know how faint a sound the ear can hear, and also how loud a 





1000 

 5 00 



100 

 50 



0.1 

 0.05 



0.01 

 0.005 



0.001 

 0.0005 



0.0001 

 0.00005 



100 500 1000 5000 10,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 5 — Auditory sensation area for the typical ear of a young adult. 



sound the ear can tolerate. With the advent of the vacuum tube, 

 it was possible to develop methods of accurately measuring the 

 intensity of faint sounds and of readily producing such sounds. 

 Figure 5 gives the results of a large number of measurements made to 

 determine the limits of hearing. This graph is called the auditory 

 sensation area. The lower solid curve represents the minimum sound 

 that an average young person can hear. The abscissa gives the 

 frequency of the pure tone, and the ordinate the sound pressure in 

 dynes per square centimeter. The top solid curve represents the 



