Distortion-product otoacoustic emission input/output functions as a function of frequency in human adults

Two issues concerning distortion-product otoacoustic emission (DPE) input/output functions were addressed in the present study. The first was to characterize those functions from threshold to 65 dB SPL. The second concerned the extent to which DPE input/output functions vary as a function of frequency. DPE input/output functions were generated at 20 frequencies from 537 to 10,009 Hz from eight normal-hearing subjects. Replicate functions were generated within an experimental session and with more than 4 months separating sessions. Given the impressive reproducibility of the DPE measurements, the input/output functions recorded can be interpreted as the consequences of stable physiologic processes and not unimportant variability (measurement noise). These input in dB/output in dB functions were generally linear functions with slopes less than unity indicating compression of the DPE output as a function intensity of the input. The slopes of the DPE input/output functions increased as a function of frequency to approaching one at the highest frequencies tested. DPE magnitudes tended to be more similar across frequencies at high intensities. At the lower intensities, DPE magnitudes varied as a function of input frequency, explaining the difference in slope of the input/output functions. There were a small number of markedly nonmonotonic, nonlinear functions. The functions presented included a greater number of intensities and frequencies and were less susceptible to explanations implicating the variability of the measurements than previous studies. The results, however, were quite consistent with those studies.