558 



BELL SYSTEM TECHNICAL JOURNAL 



sidering all the possible causes of variation the curves seem to be in 

 fairly good agreement. 



In order to give a preliminary picture of the prevalence of deafness 

 in terms of free field intensity and frequency, the distribution curves 

 of Fig. 5 were converted into the contour lines shown in Fig. 8. For 



140 



20 



100 



2 80 



60 



uj 20 

 > 



t 



?-20 



20 



50 



100 200 500 1000 2000 5000 10,000 20,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 8 — Contour lines above which lie a given percentage of thresholds for 

 the age group 10-59. 



the low frequencies, this conversion was made by applying the differ- 

 ences between the minimum audible pressure and minimum audible 

 field curves of Sivian and White " to the ear canal pressure levels of 

 Table 15. For the high frequencies, the conversion was based on a 

 free field calibration of the receivers. The free field intensity levels 

 apply for the condition in which the observer faces the source and 

 listens with both ears. The resulting contours purport to show the 

 percentage of people in the population within the age range 10 to 59 

 years who cannot hear tones below the given level. The boundary 

 lines forming the auditory sensation area represent the picture of the 

 limits of useful hearing, based upon earlier studies.^* 



The solid portions of the contour lines represent the distributions 

 " H. Fletcher, "Auditory Patterns," Reviews of Modern Physics, 12: 47-65 (1940). 



