148 



BELL SYSTEM TECHNICAL JOURNAL 



Limits of the Field of Audition 



The new tools which have made possible more accurate measure- 

 ments in audition are the vacuum tube, the thermal receiver and the 

 condenser transmitter. When connected in a proper arrangement 

 of circuits, the vacuum tube is capable of generating an oscillating 

 electrical current of any desired frequency. This electrical vibration 

 is translated into a sound vibration by means of the telephone receiver. 

 Between the receiver and the oscillator, a wire network called an 

 attenuator 28 is interposed which makes it possible to regulate the 



.0001 



300 



16 



400 



32 64 



500 600 

 C 



128 



100 



156 512 1024 2048 4096 8192 16384 32768 Frequency 



800 900 1000 1100 1200 1300 1400 1500 Pitch 



C ' C ' c 3 c* c 5 c s f Musical Notation 



Fig. 1 



volume of sound. The theory 54 ~ 55 of the thermal receiver has been 

 worked out so that it is possible to calculate its acoustic output from 

 the electrical energy it is absorbing. In this way, it is possible to 

 calculate the pressure variation produced in the outer ear canal 

 when a tone is being perceived. A detailed description of the ap- 

 paratus and method used in such measurements was given in a paper 

 presented before the National Academy of Science, November 14, 

 1921. 55 Such a combination of apparatus which has been calibrated 

 is called an audiometer and is suitable for measuring abnormal as 

 well as normal hearing. A receiver more rugged than the thermal 

 may be substituted when its efficiency compared to the thermal 

 receiver is known for all frequencies. By using such an audiometer 

 the average absolute sensitivity for approximately 100 ears which 

 were considered to be normal was determined. The lower curve in 



