Psychoacoustic studies for development of the “binaural” Bone-Conducted Ultrasonic Hearing Aids

Ultrasound with a frequency greater than 20 kHz, which is generally recognized as a sound beyond the upper limit of human hearing, can be heard via bone-conduction. Such “audible” ultrasound through bone-conduction is referred to as bone-conducted ultrasound (BCU). It have been reported that profoundly hearing-impaired people can also hear a BCU and recognize part of the information on the modulating signal from the amplitude-modulated BCU. These perceptual characteristics were utilized in the development of a new hearing-aid system, Bone-Conducted Ultrasonic Hearing Aids (BCUHA), for the profoundly hearing-impaired. In this study, to verify the feasibility of “binaural” BCUHAs, we investigated whether listeners can use the interaural time differences in the amplitude envelopes (envelope-ITDs) and the intensity differences (IIDs) as cues for lateralization of BCUs. Results showed that listeners can detect changes in envelope-ITDs or IIDs of BCUs. Also, the discrimination thresholds of the polarities of IIDs were compensated by envelope-ITDs, i.e., the time-intensity trading was observed in the BCU perception. These findings indicate that the auditory system has the ability for lateralization using envelope-ITDs and IIDs of bilaterally presented BCUs. Further, it suggest that bilaterally presented BCUs are processed in the auditory pathway associated with lateralization in a similar manner to high-frequency amplitude-modulated sounds.