Sound from Ultrasound: Characterization of a MEMS-based personal-audio device in human temporal bones

Emerging MEMS-based audio devices generate ultrasonic acoustic output, but little is known about long-term biological effects of such exposures, which vary in frequency, intensity, duration, and coupling pathway. In this study, a novel transducer was characterized for its ultrasonic acoustic output, generating amplitude-modulated (AM) pressure pulses at a carrier frequency of approximately 200 kHz and operating frequency of approximately 100 kHz. To understand ultrasonic pressure wave transmission through the human middle and inner ear, sound pressure levels in the ear canal (PEC) and mechanical vibration velocities at the stapes (VST) and promontory (VPRM) were measured in human cadaveric temporal bones across frequencies up to 240 kHz. This represents the first such measurements at ultrasonic frequencies in human temporal bone specimens. Results demonstrated relatively consistent PEC measurements at ultrasonic frequencies across specimens, with minimal inter-specimen variation. At the operating frequency (~100 kHz), overall PEC was 82.2 dB SPL (SNR = 40.6 dB), while at the carrier frequency (~200 kHz), PEC increased to 90.6 dB SPL (SNR = 34.8 dB). VST and VPRM measurements at the operating frequency remained relatively independent of audio stimulus-drive frequency and voltage, with comparable magnitudes, suggesting that bone conduction pathways contribute significantly to inner ear ultrasonic exposure. When compared to existing safety guidelines, this device appears to meet specified criteria; however, given the limited scientific basis for these guidelines and unknown long-term effects, cautious application is recommended.