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BELL SYSTEM TECHNICAL JOURNAL 



octaves; that is, each unit corresponds to 1/100 of an octave. The 

 results of this line of investigation have an important bearing on the 

 physiological theory of hearing which I cannot enter into, and another 

 important result has been the development of methods of determining 

 the degree of impairment of hearing. 



In telephony we are, of course, not directly concerned with simple 

 sounds, but with the highly complex sounds of speech, and these are 



16.35 33 



FREQUENCY IN CYCLES PER SECOND 



261 522 1044 2089 ^177 8356 16,712 33,424 



100 200 300 400 500 600 700 800 900 1000 1100 



PITCH IN CENTI-OCTAVES 



Fig. 6 — Minimum changes in intensity level in db that the ear Is able to appreciate 

 at various positions in the auditory area. 



on actual telephone circuits generally associated with extraneous 

 sounds which we may group under the broad term of noise. Further, 

 telephone instruments are not perfect, and could be made to approach 

 perfection only at a great expense. In order to arrive at a quantitative 

 understanding of the importance of departures from perfection in 

 telephone transmission elements and conditions of use, we have in 

 very general terms proceeded as follows: We set up transmission 

 systems so nearly perfect that even the keenest ear could not find a 



