Mitigation of excessive acoustic compliance and trapped volume insertion gain in ear-sealing listening devices: Toward a safe, full-frequency-response hearing aid

Abstract

When a sound producing device is sealed in the ear canal, acoustical compliances resulting from pressurization of the trapped volume lead to dramatic boosts in SPL, up to 60 dB, especially at low frequencies. This has been found to result in listener fatigue, and to trigger the acoustic (stapedius) reflex, as well as producing temporary threshold shift. Repeated exposure can cause temporary threshold shift to become permanent. Hearing aids avoid this problem by suppressing frequencies below about 300 Hz, where the effect is most pronounced. Other devices, such as ear buds and professional in-ear monitors, offer wider frequency response and thus expose listeners to potentially dangerous sound pressures. The acoustical compliance and trapped volume insertion gain is measured for ear buds and hearing aids by comparing SPL, measured in the ear canal, for sealed and unsealed conditions. New ear sealing technology is demonstrated that allows release of the excess acoustical compliance and thus mitigates the trapped volume insertion gain: (1) a vent covered with a flexible membrane, and (2) an inflatable bubble seal. This novel technology has allowed the creation of a hybrid device with hearing aid functionality that also has the broad frequency response of professional in-ear monitors.