Evaluation of masking effects induced by sub-threshold bone-conducted ultrasound

Recent studies have shown that the human auditory-system has a certain sensitivity to ultrasound presented via air- or bone-conduction. However, such ultrasonic perception is generally weak, and there’re significant variabilities in detection thresholds among reports and subjects. Some studies indicated that even ultrasound that cannot be clearly detected can be perceived as an “unpleasant pressure sensation in the ear,” suggesting that ultrasounds affect the inner ear even at levels below the so-called threshold. This study aims to clarify the effects of bone-conducted ultrasounds (BCUs) below the threshold on human hearing by investigating their masking effect on air-conduction (AC) hearing. We observed masking of 30-kHz BCU on AC maskee from 12 to 19 kHz. The masker intensities were set at ™3, ™2, ™1, 0, +1, and +5 dB relative to the threshold for a 30-kHz BCU tone-burst with 1600-ms duration. The results showed that BCU at ™2 dB or below didn’t affect AC perception, while BCU at ™1 and 0 dB lowered the AC threshold. Further, at higher masking intensities than these, the AC threshold increased. These results suggest that even BCU at levels below the threshold can affect the inner ear, and BCU induce stochastic resonance in the auditory periphery.