. The Bell System technical journal . to Mr. W. of these Laboratories for the calibrations. They are pre-liminary in character and may need modification in the light ofsubsequent studies. With the aid of these calibrations, ear canal pressure levels may be * F. A. Coles, Hearing-Test Machines at the Worlds Fairs, Bell LaboratoriesRecord, 18: 290, June 1940. 556 BELL SYSTEM TECHNICAL JOURNAL calculated from the receiver voltage levels measured in the calculations for the reference level or condition of zero hearingloss as used in this paper are shown in Table 15. The resulti
. The Bell System technical journal . to Mr. W. of these Laboratories for the calibrations. They are pre-liminary in character and may need modification in the light ofsubsequent studies. With the aid of these calibrations, ear canal pressure levels may be * F. A. Coles, Hearing-Test Machines at the Worlds Fairs, Bell LaboratoriesRecord, 18: 290, June 1940. 556 BELL SYSTEM TECHNICAL JOURNAL calculated from the receiver voltage levels measured in the calculations for the reference level or condition of zero hearingloss 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 15Calibration of Hearing Test Equipment at the Reference Level Frequency 440 880 1760 3520 7040 Reference voltage level across receivers—dbabove one volt -104 -112 -115 -114 -76 705A receiver calibration—db above per sq. cm. per volt 133 134 133 134 98 Re ference ear canal pressure level—db dyne per sq. cm. 29 22 18 20 22. 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 othercurves. See text. the ear canal pressure levels corresponding to zero hearing loss onthe 2A and 6A audiometers and the minimum audible pressurecurve 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 lowestdial setting at which the tone is heard. Threshold would, on the average, be half adial step lower than the recorded setting. Hence the curves given here for zerohearing loss are db lower than those given for zero dial setting in the also footnote 2. L. J. Sivian and S. D. White, Minimum Audible S
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