The effect of conductive hearing loss on the masking-level difference: insert versus standard earphones

Previous studies have indicated that the masking-level difference is reduced in listeners having conductive hearing loss, even when sensation level is controlled. According to the crosstalk hypothesis, this deficit can be explained in terms of reduced binaural cues at the level of the cochleas, due to acoustic contributions from the bone conduction route. It is possible that such a deficit can be overcome through the use of insert earphones, because such transducers provide a high degree of interaural attenuation. In this study, NoSo and NoS tau thresholds were measured in normal-hearing subjects and in subjects having hearing loss due to otosclerosis, using both standard TDH earphones encased in NAF-48490-1 circumaural cushions and deeply seated Etymotic 3A insert earphones. Masking noise was presented both at a fixed sound-pressure level and at a fixed sensation level. The results did not indicate consistently larger MLDs with insert earphones for either group of subjects, for either type of presentation level. The MLDs of the hearing-impaired subjects often remained abnormally small even when the insert transducers were used and the noise was presented at a fixed SL. Whereas acoustic crosstalk appeared to have a role in the results obtained, other factors (perhaps related to long-term hearing threshold asymmetry and/or interaural phase mismatch) also were apparently involved.