Air-bone gap distributions in normal-hearing subjects.

Unmasked (UM) and masked (M) AC and BC thresholds were obtained on 48 normal-hearing young adults. The UMAC and MBC thresholds were in agreement with standardized reference levels. The mean air-bone gaps (A-B G's) were less than 1 db for the three comparisons (UMAC minus UMBC; UMAC minus MBC; MAC minus MBC). The distribution of the individual Ss' A-B G's for each AC minus BC threshold comparison resembled a slightly flattened normal curve showing that some Ss had a positive while some had a negative A-B G. Over the 3 comparisons per S, 34% of the Ss exhibited negligible A-B G's of +/- 2.5 db, while 17.4% exhibited A-B G's greater than or equal to 7.5 db. The common rule that Ss with a normal conductive hearing mechanism have an A-B G of 0 db can only be used for mean data and not be applied to individual Ss tested in clinical practice or used for subjective BC calibration.