Design considerations for robust noise rejection in otoacoustic emissions measured in-field using adaptive filtering

Otoacoustic emission (OAE) measurement is a sensitive and effective technique to monitor changes in the inner ear potentially induced by noise exposure. However, outside a controlled testing environment, measurements are challenging since the level of ambient noise might be higher than the low-level OAE response. Therefore, an OAE system was designed, suitable to measure OAEs repeatedly on an individual worker in noisy test conditions. This system features a left and right earpiece, each equipped with a pair of miniature loudspeakers, an external and an internal microphone. In addition to the passive attenuation of the earpiece, adaptive filtering on the distortion product OAE (DPOAE) signals is used to further enhance the ratio between the measured OAE signal and interfering noise. The adaptive filtering technique uses the sound captured by the ipsilateral external and internal microphones as well as from the contralateral internal and external microphones. In this paper, the accuracy of DPOAE signals are studied using different combinations of the four microphones in single and dual adaptive filter topologies, as well as the benefits of adding a fixed transfer function in the adaptive filtering algorithm topology to estimate the acoustic path. Side-by-side comparison shows that a dual-stage adaptive filtering algorithm, using a combination of the contralateral internal microphone with the ipsilateral external microphone, is the most promising approach to denoise the DPOAE signal.

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