RESULTS OF THE WORLD'S FAIR HEARING TESTS 



557 



curves and the Fair curve are based on ear canal pressures measured 

 at the ear opening in the manner just described. The minimum 

 audible pressure curve is based on pressures measured about 1 cm. 

 from the ear drum, which correspond more nearly to ear drum pressure 

 levels. The two types of measurements are undoubtedly quite com- 

 parable below 1000 cycles. For frequencies above 5000 cycles and 

 possibly around 2000 cycles it is believed that the pressure at the ear 

 opening is somewhat smaller than the corresponding ear drum pressure. 

 A comparison of the Fair data with data from two other surveys 

 of hearing is shown in Fig. 7. One curve shows the mean threshold 



58 

 mo 



0-6 



^< 



UlU 



100 



200 500 1000 2000 



FREQUENCY IN CYCLES PER SECOND 



5000 



10.000 



Fig. 7 — Comparison of several surveys of hearing, giving mean ear canal pressure level 



for men aged 20-29. 



pressure for men in the 20-29 age group from the Fair data. Another 

 gives values for the same age and sex group in a survey conducted in 

 1936 by the United States Public Health Service using the 2A 

 Audiometer. 12 fhis curve is for a somewhat selected group, including 

 only individuals who stated when the test was made that they believed 

 their hearing was normal. The third curve is for members of Bell 

 Telephone Laboratories in the same age and sex group, who were 

 tested in 1931 with a 2A Audiometer.^* In comparing these results, 

 it should be remembered that differences may be due to three general 

 causes. The groups of people tested may have differed in hearing 

 acuity. The calibrations by which the ear canal pressures were 

 established are subject to error, especially at high frequencies. The 

 conditions of the test, including technique, concentration of subjects, 

 receiver fit, and background noise, were not alike in all cases. Con- 



^^ W. C. Beasley, National Health Survey, Hearing Study Series, Bulletin 5, Table 

 3, The United States Public Health Service, Wash. D. C. (1938). 



'^ H. C. Montgomery, " Do Our Ears Grow Old," Bell Laboratories Record, 10: 

 311 (1932). Note that median values were given in this reference, differing slightly 

 from the mean values used here, 



