Time-windowing of click-evoked otoacoustic emissions to increase signal-to-noise ratio.

Abstract

To investigate the effects of decreasing the response-window duration on the signal-to-noise ratio (S/N) of click-evoked otoacoustic emissions (CEOAEs). The ILO88 (Otodynamics, Ltd.) was used to measure CEOAEs from 149 normal adult ears, and 75 adult ears with high-frequency sensorineural hearing loss. Data were collected using the default response window of 2.5 to 20.5 msec post-click. Each response was rewindowed, post-hoc, from 2.5 to 7.5 msec, 2.5 to 9 msec, 7.75 to 14.25 msec, and 13 to 19.5 msec post-click. For each window, spectra of the CEOAE and of the background noise were determined. The S/N was estimated by subtracting the noise level from the CEOAE amplitude. The 13- to 19.5-msec window contained little CEOAE energy relative to earlier windows. Relative to the 2.5- to 20.5-msec window, the 2.5- to 7.5- and 2.5- to 9-msec windows reduced noise levels more than CEOAE amplitudes, yielding increased S/N, and greater "reproducibility" values. The increased S/N of the 2.5- to 7.5- and 2.5- to 9-msec windows allowed measurement of greater CEOAE-amplitude reductions in the impaired ears relative to the normal ears. With short-duration windows, click-presentation rate could be increased, allowing more responses to be averaged in a given time, thus further decreasing noise levels. Although click rate was not varied in the present study, the decrease of noise levels is predictable. Accounting for this factor, it is expected that a specified S/N would be obtained about five times faster using the 2.5- to 7.5-msec window with a 7.5-msec interstimulus interval, than when using the default window. Decreasing the response-window duration substantially increases the measurement efficiency of CEOAEs in adults, and thus may enhance clinical-test performance.