158 BELL SYSTEM TECHNICAL JOURNAL 



made to determine when any complex sound is equally loud to a 

 700-cycle tone. Such measurements can be made although the ob- 

 servational error is rather large and the judgment of various indi- 

 viduals is sometimes quite different, which means only that loudness 

 as measured by various individuals is different. For use in engineering 

 work, however, the average of a large number of individuals can be 

 taken and this loudness will have a definite determinable value. For 

 example in Fig. 6, the loudness of the A sound when it is 60 units 

 above the threshold is 72, since it sounds as loud as a 700-cycle tone 

 which is 72 units above its threshold. The loudness of complex 

 sounds usually increases faster with increases in intensity than that 

 of pure tones. This would be expected since the threshold is de- 

 termined principally by the loudest frequency in the complex sound 

 and as the intensity is increased the other frequencies begin to add 

 to the total loudness. 



Since pure tones of different pitches which are the same number 

 of units above the threshold sound equally loud their loudness L 

 can be represented by the formula 



L=L + 20 1og/> 



where p is the root mean square value of the pressure amplitude 

 produced in the ear by the tone and Lq is the number of units from 

 the 1-dyne line to the minimum audibility curve. The values of Lq 

 can be read directly from the chart in Fig. 1. 



Measurement of Degree of Deafness 



The choice of the loudness and pitch units used above leads to a 

 rational definition of the degree of deafness. 



The number of possible pure tones that can be sensed by a deaf 

 person is considerably smaller than that mentioned above obtained 

 from the normal auditory-sensation area. A logical way of defining 

 the amount of hearing is : To give the per cent, of the total number of 

 distinguishable pare tones audible to a person with normal hearing, 

 that can be sensed by the deaf person. 



Some definition of this sort will be very helpful in clearing up 

 the confusion that now exists in court cases involving the degree of 

 deafness. It is well known that there are a number of laws which 

 prevent people who have more than a defined amount of deafness 

 from doing certain classes of work. For example, one cannot oper- 

 ate an automobile if he has a certain per cent, of deafness. At the 

 present time, there is a large variation between the standards set up 

 by the various doctors in different parts of the country. 



