Audiometric ear canal probe with active ambient noise control

Abstract

An audiometric earphone system is introduced, which employs both passive and active acoustic noise attenuation. The audiometric earphone includes a miniature speaker, an error microphone, and a reference microphone placed in a foam plug. It is inserted into a subject's ear canal during a hearing test, and reduces the ambient noise to bone conduction levels. Passive attenuation is achieved by the foam plug, and active attenuation is achieved by a feedforward active noise control implementation. Initially, the earphone was tested in mechanical models, and its causality and optimal characteristics were investigated. A causal controller was obtained only in a mechanical model with enlarged spacing between the reference microphone and the speaker. Up to 30 dB attenuation of broadband noise in the enlarged mechanical model and 40 dB attenuation of tones in the regular-size mechanical ear model were achieved. The system was then tested for its ability to reduce noise audibility by placing the audiometric earphone in the ear canal of human subjects. An attenuation of more than 30 dB of 500 Hz tone was achieved using a hearing threshold test. Although the system can currently attenuate only periodic noise when placed in the ear canal, with advanced technology it will be possible to design practical audiometric earphones for hearing tests.