Comparing stimulus-frequency otoacoustic emissions measured by compression, suppression, and spectral smoothing.

Stimulus-frequency otoacoustic emissions (SFOAEs) have been measured in several different ways, including (1) nonlinear compression, (2) two-tone suppression, and (3) spectral smoothing. Each of the three methods exploits a different cochlear phenomenon or signal-processing technique to extract the emission. The compression method makes use of the compressive growth of emission amplitude relative to the linear growth of the stimulus. The emission is defined as the complex difference between ear-canal pressure measured at one intensity and the rescaled pressure measured at a higher intensity for which the emission is presumed negligible. The suppression method defines the SFOAE as the complex difference between the ear-canal pressure measured with and without a suppressor tone at a nearby frequency. The suppressor tone is presumed to substantially reduce or eliminate the emission. The spectral smoothing method involves convolving the complex ear-canal pressure spectrum with a smoothing function. The analysis exploits the differing latencies of stimulus and emission and is equivalent to windowing in the corresponding latency domain. Although the three methods are generally assumed to yield identical emissions, no equivalence has ever been established. This paper compares human SFOAEs measured with the three methods using procedures that control for temporal drifts, contamination of the calibration by evoked emissions, and other potential confounds. At low stimulus intensities, SFOAEs measured using all three methods are nearly identical. At higher intensities, limitations of the procedures contribute to small differences, although the general spectral shape and phase of the three SFOAEs remain similar. The near equivalence of SFOAEs measured by compression, suppression, and spectral smoothing indicates that SFOAE characteristics are not mere artifacts of measurement methodology.