The influence of noise on the measured amplitudes of distortion-product otoacoustic emissions

This paper describes the influence of noise on the measured amplitudes of tonal signals, as determined using a narrowband spectral analysis, that is, the technique typically used to measure distortion-product otoacoustic emissions (DPOAEs). At small signal-to-noise ratios, background noise results in overestimation of DPOAE amplitude and, thus, substantially influences the measured properties of the low-level portions of DPOAE-growth functions, in particular the apparent slope of the functions in this region. It is shown that, because of the influence of noise, the algorithm for the objective estimation of detection thresholds of DPOAEs, and of the slopes of DPOAE- growth functions, described by Nelson and Kimberley (1992), will tend to underestimate these values. This systematic underestimation is presumably the reason why many of the DPOAE-detection thresholds and growth slopes presented in that study were considerably lower than those reported in previous studies using similar measuring equipment and paradigms but different detection- threshold and growth-slope estimation techniques. In the present paper, a simple equation allowing an estimated correction for the effects of noise on measured DPOAE amplitudes is presented. Finally, an alternative strategy for the estimation of DPOAE thresholds, one that is less prone to the influence of noise, is suggested.