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BELL SYSTEM TECHNICAL JOURNAL 



calculated from the receiver voltage levels measured in the tests. 

 Such calculations for the reference level or condition of zero hearing 

 loss as used in this paper are shown in Table 15. The resulting refer- 

 ence ear canal pressure levels are plotted in Fig. 6. For comparison, 



TABLE 15 

 Calibration of Hearing Test Equipment at the Reference Level 



CO 

 < UJ 



UJ o 



100 



200 



500 1000 2000 5000 



FREQUENCY IN CYCLES PER SECOND 



10,000 



Fig. 6 — Ear canal pressure level for certain reference conditions. The measure- 

 ments for the M. A. P. curve were made nearer the ear drum than those for the other 

 curves. See text. 



the ear canal pressure levels corresponding to zero hearing loss on 

 the 2A and 6A audiometers" and the minimum audible pressure 

 curve derived by Sivian and White ^^ are shown. The audiometer 



1" J. C. Steinberg and M. B. Gardner, "Auditory Significance of Hearing Loss," 

 Jour. Acous. Soc. Amer., 11: 270 (1940). 



In using the Audiometer it is customary to record as the hearing loss the lowest 

 dial setting at which the tone is heard. Threshold would, on the average, be half a 

 dial step lower than the recorded setting. Hence the curves given here for zero 

 hearing loss are 2.5 db lower than those given for zero dial setting in the reference. 

 See also footnote 2. 



" L. J. Sivian and S. D. White, "Minimum Audible Sound Fields," Jour. Acous. 

 Soc. Amer., 4: 288-321 (1933). 



