Ultrasonic Acoustic Heterodyne Transmission Into the Human Auditory and Vestibular Systems

It is well-known that airborne sound induces vibration of the eardrum, the coupled middle ear bones, and the inner ear. Sound transmission to the inner ear is attenuated by damage or dysfunction in the eardrum or ossicular chain. Corrective devices often use contact shakers to directly vibrate the temporal bone of the skull, delivering sound. We investigate an alternative, noncontact method of sound transmission that uses ultrasonic signals to transmit sound into the auditory and vestibular systems. Minimal literature exists describing ultrasonic hearing, largely due to attenuation of air-conducted frequencies above 20 kHz. High-amplitude airborne sound incident upon the skull can induce temporal bone system vibrations along an unconventional structural path. Finite-element-based acoustic modeling of the auditory and vestibular anatomy reveals resonant behavior in structural components of the middle and inner ear at ultrasonic frequencies. These “built-in sound amplifiers” can be leveraged to compensate for impedance mismatches experienced in airborne ultrasound transmission. By heterodyning (amplitude modulating) a targeted ultrasonic carrier signal with an audio signal, the nonlinearities of acoustic propagation and the auditory and vestibular sense organs allow interpretation of heterodyne signals. These techniques provide a foundation to improve a wide variety of communication equipment, including hearing aids, without interfering with balance sensations.