MECHANICAL PARAMETERS OF HEARING BY BONE CONDUCTION

A new powerful, magnetostrictive bone conduction (BC) vibrator was described. At low output levels, it had a wide frequency range (5 Hz-above 20 kHz), and for midfrequencies undistorted outputs reach up to 50 g in terms of acceleration. BC thresholds and SL [sensation level] (the latter determined by AC [air conduction]/BC cancellations) were measured [in man] with this vibrator. Both varied with contact force, but much more markedly with contract area, in unoccluded and occluded ears. Sound attenuation boxes, after von Bekesy, were used in the unoccluded state. Results of the AC/BC cancellations permitted determination of the external-ear and the inner-ear BC components. With the ear unoccluded, variations with contact area (and/or force) were mainly related to changes of the inner-ear responses, but with the ear occluded, they were related to those of the BC-generated sound pressures in the ear canal for f [frequency] < 2 kHz. At AC and BC threshold SPL [sound pressure level] in the ear canal had identical values and did not vary with contact area or force for f < 2 kHz. For a comparable small contact area, BC acceleration thresholds were similar to those specificed in ANSI [American National Standards Institute] S3.13-1972. For larger contact areas they were lower, by as much as 25 dB, at some frequencies. Acceleration may not be a valid indicator of the BC input to the skull.