Bias due to noise in otoacoustic emission measurements

Measurements of otoacoustic emission (OAE) magnitude are often made at low signal/noise ratios (SNRs) where measurement noise generates bias and variability errors that have led to the misinterpretation of OAE data. To gain an understanding for these errors and their effects, a two part investigation was carried out. First, the nature of OAE measurement noise was investigated using human data from 50 stimulus-frequency OAE experiments involving medial olivocochlear reflex (MOCR) activation. The noise was found to be reasonably approximated by circular Gaussian noise. Furthermore, when bias errors were taken into account, measurement variability was not found to be affected by MOCR activation as had been previously reported. Second, to quantify the errors circular Gaussian noise produces for different methods of OAE magnitude estimation for distortion-product, stimulus-frequency, and spontaneous OAEs, simulated OAE measurements were analyzed via four different magnitude estimation methods and compared. At low SNRs (below -6 dB), estimators involving Rice probability density functions produced less biased estimates of OAE magnitudes than conventional estimation methods, and less total rms error-particularly for spontaneous OAEs. They also enabled the calculation of probability density functions for OAE magnitudes from experimental data.