Use of passive hearing protectors and adaptive noise reduction for field recording of otoacoustic emissions in industrial noise

Distortion Product Otoacoustic Emissions (DPOAEs) can detect noise-induced hearing loss in-field, but their data extraction is very sensitive to background noise. This paper investigates how passive and active noise reduction enhance DPOAE recording based on data collected in white noise from 54dB(A) to 90dB(A). Despite considerable high-frequency attenuation from a proper placed DPOAE probe, 54dB(A) background noise deteriorates the test outcome substantially. More low-frequency attenuation by an extra passive earmuff enables measurements in white noise levels of 70dB(A). The relationship between external sound level and noise recorded by the DPOAE system has been statistically modeled. Additionally, the upper limits of attenuation improvement are analyzed by quantifying residual physiological noise. Furthermore, for an earplug integrating microphone and speakers of the DPOAE measurement probe, adaptive noise reduction processing on the DPOAE signal is used to improve the Signal-to-Noise ratio. The adaptive noise reduction (ANR) is implemented using the NLMS algorithm to filter out the ambiant noise, measured by the first microphone measuring the DPOAE signal, with a second miniature microphone mounted flush with the external faceplate of the isolating DPOAE probe. Simulated data shows that DPOAE response extraction is possible in an environment with noise levels exceeding 70dB(A).