Bone-conducted ultrasound (BCU) is perceived even by the profoundly sensorineural hearing-impaired and has been applied to a novel hearing aid. The vibrator is usually pressed against a part of the cranial bone behind the ear (mastoid process); however, BCU presented to parts of the body distant from the head can be perceived. In this study, to assess the basic properties of distantly presented BCU hearing, hearing thresholds were measured when 30 kHz tone bursts were presented to the neck and the upper and lower arms in normal-hearing participants. Furthermore, temporal modulation transfer functions (TMTFs), which reflect the temporal resolution of hearing, were estimated for the distantly presented 30 kHz carrier. The results showed that BCU presented to parts of the body distant from the head, including the lower arm, can be perceived at least by normal-hearing persons, whereas the threshold increased depending on the distance from the head. Moreover, the temporal resolutions of the hearing of BCU distantly presented to the neck and the upper and lower arms were comparable to that distantly presented to the mastoid process of the temporal bone.

We investigated the linearity of bone-conducted ultrasound transmission through a living human head. The frequency response functions and the coherence functions of head vibration at different signal levels were examined. Vibrations in the heads of six subjects were measured using a small accelerometer in the ear canal under bone-conducted tone stimulation from audible high frequencies to ultrasonic frequencies. Stepped sine signals from 10 to 45 kHz in 0.5 kHz steps at two different signal levels were used. The signal levels were set to the levels corresponding to 5 and 10 dB sensation level (SL) at 30 kHz in each subject. The result suggests that the head vibration increases almost linearly across an audible to ultrasonic frequency range and there was no significant difference in nonlinear behavior between the audible and ultrasonic sound transmissions.

Ultrasonic vibration generates a sensation of sound via bone-conduction. This phenomenon is called bone-conducted ultrasonic (BCU) hearing. Complex sounds can also be perceived by amplitude-modulating a BCU stimulus (AM-BCU). The influence of the modulation frequency on the sensitivity to detecting amplitude modulation of sinusoidal carriers of 10, 20, and 30 kHz was examined to clarify the characteristics of the perception of amplitude modulation over the sonic or audio-frequency range and the ultrasonic range. In addition, the detection sensitivity for single-sideband modulation for a 20 kHz carrier was measured. Temporal modulation transfer functions (TMTFs) obtained at each carrier frequency suggest that the auditory system has the ability to process timing information in the envelopes of AM-BCUs at lower modulation frequencies, as is the case with audio-frequency sounds. The possible influence of peripheral filtering on the shape of the TMTF at higher frequencies was examined.

Bone-conducted ultrasound (BCU) is perceived even by the profoundly hearing impaired and a novel hearing aid has been developed. In the BCU hearing aid, the ultrasonic carrier is amplitude-modulated by speech signals and presented to the mastoid process of the temporal bone by a vibrator. However, BCU presented to parts of the body distant from the head can be perceived. In this study, vibration at the external auditory meatus was measured when amplitude-modulated BCU was presented to distal parts of the body. The results showed spectrum peaks corresponding to the carrier frequency (30 kHz) and the modulation frequency. The peak level of the carrier frequency tended to decrease depending on the distance between the external auditory meatus and the stimulation part basically. On the other hand, the peak level of the modulation frequency did not change with the distance between the external auditory meatus and the stimulation part, whereas the effect of modulation frequency was significant.

In bone conduction, vibrators are usually pressed onto the mastoid process, however, bone-conducted ultrasound (BCU) presented to body parts distant from the head can also be perceived. To elucidate mechanisms of the distantly presented BCU perception, transmissions of BCU were investigated on living human bodies. First, vibration at the external auditory meatus were measured when 30 kHz BCU was presented to distal parts. Second, vibrations of the upper limb were measured at nine surface points lined up straight from the forearm to shoulder when 30 kHz BCU was presented to nearby the wrist. The results showed that prominent spectrum peaks corresponding to the stimulus-frequency were observed at all parts. The vibration tended to decrease depending on the distance between the stimulus and measurement points, however, some exceptions existed. Efficiency of the BCU transmission seems to vary depending on factors like shape of the body, acoustical properties of the tissue, and coupling of the vibrator and the skin.