Time-domain observation of otoacoustic emissions during constant tone stimulation.

Observation of the otoacoustic emissions (OAEs) evoked during a continuous single stimulus tone have been made on humans using a nonlinear residual time domain technique. The technique, described in this paper, involved the digital summation of responses to contiguous stimulation intervals, some of which included short bursts of a suppressor, or probe, tone. Stimulus intervals are constructed so that both the stimulus and probe tones summed to zero cyclically, leaving a residual response. This residual is attributable to the nonlinearity of the whole acoustic response, as measured in the ear canal, to the stimulus and probe tone complex. A theoretical treatment of this paradigm is presented examining the relation of this residual to the OAE evoked by the stimulus tone. It is shown experimentally that the residual, found at the stimulus tone frequency, has a latency and saturating input-output growth functions indicative of an OAE. The detailed OAE amplitude-versus-frequency variations, and the general latencies of the OAEs in two human ears were measured using both the constant tone evoked residual method described and the click evoked delayed emission method. The results from both methods are in agreement. The frequency-dependent properties of the suppression of the OAE were investigated using various stimuli to probe frequency ratios. The continuous tone time domain residual method has advantages for the observation of stimulus frequency OAEs and for relating these to any distortion product simultaneously generated.