In the conventional air-conduction hearing path, airborne sound induces vibration of the eardrum, the coupled middle ear bones (ossicles), and the fluids of the inner ear. The vibrations are transduced by the cochlea for transmission to the brain. The ossicular chain acts as an impedance matching network which corrects for the approximately 1:3450 characteristic acoustic impedance mismatch between air and the endolymph and perilymph in the inner ear spaces. Damage or dysfunction of the eardrum or ossicular chain results in a drastic reduction of sound transmission to the inner ear. Corrective devices typically involve the use of direct-contact shakers which vibrate the temporal bone of the skull (which contains the eardrum, middle ear, and inner ear). Direct vibration of the bone induces vibrations in the middle ear fluid at a much more favorable 4.6:1 bone-to-fluid impedance mismatch.

We investigate an alternative, noncontact method of sound transmission to the inner ear which uses ultrasonic signals to transmit sound into the auditory and vestibular systems. Very little literature exists describing the performance of the auditory system at ultrasonic frequencies, in large part due to the very high attenuation of frequencies above 20kHz. Further discouraging research into high-frequency auditory phenomena is the pervasive belief that the basilar membrane of the cochlea and its coupled auditory nerve is a physical Fourier analyzer, and that no sensing capacity exists above 20kHz.

While the air-conduction hearing path is not sensitive to ultrasonic vibrations, high-amplitude airborne sound signals (such as those generated in the near-field of ultrasonic transducers, which can easily reach sound pressure levels of 100-120dB) incident upon the head can induce temporal bone vibrations. Acoustic modeling of the vestibular anatomy with finite-element techniques reveals that acoustic modes exist in the middle and inner ear spaces at frequencies in the range of off-the-shelf ultrasonic transducers. These “built-in sound amplifiers” can be leveraged to compensate for losses in airborne sound transmission and impedance mismatches, delivering the ultrasonic sound signal to the inner ear. Transmission of audio-frequency sound with this method can be accomplished with heterodyning (amplitude modulating) an ultrasonic carrier signal with an audio signal. The resulting heterodyne signal contains so-called “heterodyne frequencies” corresponding to the sum and difference of the inputs. The nonlinearities of acoustic propagation and the structure of the anatomy result in demodulation of the heterodyne signal within the temporal bone. These techniques can be used to improve a wide variety of communication equipment, including hearing aids.


Ultrasonic Acoustic Heterodyne Transmission Into the Human Auditory and Vestibular Systems

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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