160 BELL SYSTEM TECHNICAL JOURNAL 



shows a common type of deafness in which the sensitivity to the high 

 frequencies suddenly decreases, as is indicated by the rise in the 

 minimum audible intensity curve when the frequency exceeds 3,000 

 cycles per second. The sensation area for this person is 94 per cent, 

 of that for the average. Consequently, his per cent, of hearing is 

 94 per cent. It is also convenient to speak of the per cent, of hearing 

 for each pitch. It is evident that the logical definition for this is the 

 ratio of the widths of the sensation area for the person tested and 

 normal person, measured along the ordinate drawn at the frequency 

 in question. 56 For example, in this audiogram the person had more 

 than 100 per cent, hearing for most of the pitch range. At 4,000 

 cycles, however, the per cent, hearing was only 60 per cent. This 

 means that for this pitch, the person when compared with one having 

 normal hearing could sense only 60 per cent, as many gradations in 

 tonal volume before reaching the threshold of feeling. 



The second chart corresponds to a type of deafness that is not so 

 common. It shows relatively large losses at the lower frequencies. 

 The per cent, hearing in this case is seen to be 64 per cent. 



The third type is very common and corresponds to a general lowering 

 of the frequencies throughout the entire pitch range. In these first 

 three cases, the deaf persons could carry on a conversation without 

 any difficulty whatever. In the last two of these, difficulty was 

 experienced in understanding a speaker at any considerable distance. 

 In the first case, the person could not hear the steam issuing from a 

 jet or any other high hissing sound. However, he could hear and 

 understand speech practically as well as anyone with normal hearing. 



The fourth case shows a falling off at the high frequencies, but 

 this loss in hearing proceeds gradually as the pitch increases rather 

 than abruptly as in the first case. As indicated in the figure the per 

 cent, of hearing is 58 per cent. 



The fifth case is one of extreme deafness and is typical of such 

 cases. The per cent, of hearing is only 12 per cent. The last case 

 shows not only the minimum audibility curve, but the quality of the 

 sensation perceived. As indicated on the chart, in certain regions 

 noises are heard when the stimulus is a pure tone. When computing 

 the per cent, of hearing in such cases, it seems reasonable to take 

 only the area where sensation of good quality is perceived. In some 

 cases, this poor quality extends through practically the whole area 

 and although the person hears sounds, he is unable to properly in- 

 terpret them. Consequently, from a practical point of view, his per 

 cent, of hearing is very low. For such cases, deaf sets or other aids to 

 the hearing do not give any satisfactory help. 



