Behavioral hearing thresholds between 0.125 and 20 kHz using depth-compensated ear simulator calibration

The purpose of this study was to obtain behavioral hearing thresholds for frequencies between 0.125 and 20 kHz from a large population between 10 and 65 yr old using a clinically feasible calibration method expected to compensate well for variations in the distance between the eardrum and an insert-type sound source. Previous reports of hearing thresholds in the extended high frequencies (>8 kHz) have either used calibration techniques known to be inaccurate or specialized equipment not suitable for clinical use.

Hearing thresholds were measured from 352 human subjects between 10 and 65 yr old having clinically normal-hearing thresholds (<20 dB HL) up to 4 kHz. An otoacoustic emission probe fitted with custom sound sources was used, and the stimulus levels individually tailored on the basis of an estimate of the insertion depth of the measurement probe. The calibrated stimulus levels were determined on the basis of measurements made at various depths of insertion in a standard ear simulator. Threshold values were obtained for 21 frequencies between 0.125 and 20 kHz using a modified Békésy technique. Forty-six of the subjects returned for a second measurement months later from the initial evaluation.

In agreement with previous reports, hearing thresholds at extended high frequencies were found to be sensitive to age-related changes in auditory function. In contrast with previous reports, no gender differences were found in average hearing thresholds at most evaluated frequencies. Two aging processes, one faster than the other in time scale, seem to influence hearing thresholds in different frequency ranges. The standard deviation (SD) of test-retest threshold difference for all evaluated frequencies was 5 to 10 dB, comparable to that reported in the literature for similar measurement techniques but smaller than that observed for data obtained using the standard clinical procedure.

The depth-compensated ear simulator-based calibration method and the modified Békésy technique allow reliable measurement of hearing thresholds over the entire frequency range of human hearing. Hearing thresholds at the extended high frequencies are sensitive to aging and reveal subtle differences, which are not evident in the frequency range evaluated regularly (≤8 kHz). Previously reported gender-related differences in hearing thresholds may be related to ear-canal acoustics and the calibration procedure and not because of differences in hearing sensitivity.