Ultrasonic masker clarifies ultrasonic perception in man

Bone conduction enables ultrasound to be heard. Although several hypotheses about ultrasonic perception have been presented, the perception mechanism of bone-conducted ultrasound has not yet been established. In this study, to investigate ultrasonic perception, the amount of masking produced by 27-, 30- and 33-kHz bone-conducted ultrasonic maskers for air-conducted high-frequency sounds was measured in the frequency range of 8-18 kHz at 1-kHz intervals. The results showed that the air-conducted signals in the frequency range of 10-14 kHz were strongly masked by the ultrasonic maskers. When the masker intensity increased from 5 to 10 dB SL, the growth of masking was more than 10 dB in the frequency range of 9-15 kHz, and the masking spread strongly to lower frequencies. Furthermore, the dynamic range for bone-conducted ultrasound was clearly narrower than that for air-conducted high-frequency sounds. These results suggest that perception of bone-conducted ultrasound depends on inner hair cell activity induced by ultrasound, even without modulation being present, and does not depend on enhancement by the outer hair cells in the basal turn of the cochlea.