Self-demodulation characteristics of amplitude-modulated bone-conducted ultrasound in the human body presented to the neck, trunk and arm

Bone-conducted ultrasound (BCU) can be clearly perceived and can transmit speech information using amplitude-modulation (AM). BCU can also be perceived when presented to body parts like the neck, trunk and arms. When AM-BCUs propagate in the biological tissues with nonlinearity, like the cartilage, modulator signals may be self-demodulated. This demodulated-sound may affect BCU hearing. To examine the availability of the demodulated-components, difference limens for frequency (DLFs) were measured with/without the low-pass-masking noises that masked the demodulated-components. Practical frequency-discrimination ability was obtained even at distant body parts, whereas DLFs increased when the demodulation components were masked. Additionally, to elucidate the demodulation mechanisms in the human body, vibrations were measured around the cartilage. Significant demodulated-components were generated at especially the cartilage near the outer ear canal. These results indicated the availability of demodulated-components to improve hearing of the distantly-presented BCU, and provide useful information to optimize the novel BCU devices. (C) 2020 The Japan Society of Applied Physics