The Influence of Individual Ear Canal and Eardrum Characteristics on Speech Intelligibility and Sound Quality Judgments.

The present investigation examined whether it is preferable, based on intelligibility and quality judgments, for a hearing aid to mimic an individual's external ear. Continuous discourse and multispeaker babble were recorded at the eardrums of ten subjects with various ear canal and eardrum characteristics and various hearing sensitivity. These recordings were manipulated for each subject in order to produce four hearing aid configurations. The hearing aid responses included an average ear canal response, an average ear canal response and compensation for individual eardrum impedance, individual ear canal data and no compensation for individual eardrum impedance, individual ear canal data and compensation for individual eardrum impedance. Finally, a condition was included for each subject that mimicked a poor hearing aid with increased gain in the low and mid -frequencies and a sharp reduction in gain at high frequencies. This final condition produced a 50% accuracy score when compared with the hearing aid condition that included individual ear canal data and compensation for eardrum impedance. Subjects rated sixteen presentations of each condition in terms of absolute sound quality (0 to 100%) and intelligibility (0 to 100%). Results indicated that it was preferable based on intelligibility judgments for a given hearing aid to mimic the external ears of three subjects. For four of the subjects mimicking the external ear was not preferable, but it also was not detrimental. And for two of the subjects, poorer intelligibility ratings resulted from mimicking the individuals' external ears. One subject was chosen because her external ear response was identical to that of KEMAR (Knowles Electronic Manikin for Auditory Research). No differences in ratings were found between this subject's listening conditions. A lack of difference between the individual external ear response and the average (KEMAR) response resulted in a lack of difference in ratings across listening conditions, although the presence of a difference between the individual external ear response and the average (KEMAR) response did not guarantee a difference in ratings across listening conditions. Results indicated that quality and intelligibility preferences differed for each subject and appeared to be related to certain subject variables including external ear characteristics and amount of dB difference in particular frequency regions between conditions.