Development of a Bone-conducted Ultrasonic Hearing Aid for the Profoundly Deaf: Assessments of the Modulation Type with Regard to Monosyllable Articulation and Confusion Analyses

Bone-conducted ultrasound (BCU) can be experienced as sound even by the severely hearingimpaired. We have developed a novel hearing aid using BCU perception (BCU hearing aid: BCUHA) for the profoundly hearing-impaired. Results of evaluation tests suggested its practicability, however, there is room for improvement in terms of articulation. In the BCUHA, ultrasonic sinusoids with a frequency of about 30 kHz are amplitudemodulated by speech and presented to the mastoid by a vibrator. Generally, two sounds are perceived by the BCUHA: one is a high-pitched tone due to the ultrasonic carrier, with a pitch corresponding to a 8–16 kHz air-conducted (AC) sinusoid, and the other is the envelope of the modulated signal. As a method of amplitude modulation (AM), double-sideband with transmitted carrier (DSB-TC) modulation has been used. 1-3) However, by the DSB-TC, the high-pitched tone due to the ultrasonic carrier is strongly perceived especially when the modulation depth is low. In previous studies, we proposed a new AM method, transposed modulation (TM), to reduce the high-pitched tone. 4) Since the peak of power at the carrier frequency is suppressed by the TM, the high-pitched tone due to the carrier is reduced. Furthermore, the intervals between peaks of the envelope of the modulated signal are the same as the intervals between the peaks of the modulator signal, thus; the pitch due to the envelope is expected to be similar to that of the modulator signal. The evaluation tests showed that the TM has some advantages in terms of the sound quality over the other AM methods. 4, 5) On the other hand, it is thought that the TM speech essentially contains some distortion because of its rectification process. Such distortion seems to produce some deterioration in articulation, however, no significant differences were observed in word intelligibility scores between the TM and the DSB-TC. In this study, to observe distortion of speech associated with the TM precisely, mono-syllable articulation tests were conducted for DSB-TC and the TM and their confusion matrices were compared.