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BELL SYSTEM TECHNICAL JOURNAL 



densers and inductance coils having a frequency selectivity which 

 was the complement of that of the rest of the system. 



The telephone receiver was a bipolar type having a special con- 

 struction which was designed to broaden the range of frequency 

 response. 



The reproducing efficiency of the system from the mouth of the 

 speaker to the ear of the listener for each frequency is shown in Fig. 2. 



800 1200 1600 2000 2400 2800 3200 3600 WO 



Fig. 2. 



The pitch or frequency of the tone is given on the X axis. The 

 ordinates represent amplitude ratios or the number of times the 

 amplitude of the tone reaching the ear was greater than that which 

 entered the transmitter. It will be seen that this high quality system 

 has practically a uniform response for all frequencies throughout the 

 speech range. 



In order that its uniformity may be appreciated, a comparison 

 curve is given. This curve shows the deviation in the sensitivity 

 of a typical individual ear from the average sensitivity of a large 

 number of ears. The ordinates represent the ratio of amplitudes 

 at the various pitches which was necessary to bring the tone to the 

 threshold of audibility. It is evident that this deviation is much 

 larger than the departure of the high quality circuit from uniformity. 



To show that this particular individual's curve is typical, the 

 curves for both ears of 20 women are given in Fig 3. For convenience 

 these curves, are plotted on logarithmic paper. If an arithmetic 



