Distortion product emissions in humans. I. Basic properties in normally hearing subjects.

Distortion product otoacoustic emissions (DPEs) at the 2f1-f2 frequency were recorded from 44 normal ears in response to equilevel primary tones. Detailed testing included the recording of DPE "audiograms" in 100-Hz steps from 1 to 8 kHz at three primary-tone levels (65, 75, and 85 dB sound pressure level [SPL]). In addition, response-growth or input-output (I/O) functions depicting the relationship of the amplitudes of DPEs to primary-tone levels, ranging from 25 to 85 dB SPL in 5-dB steps, were also tested for 11 frequencies distributed at quarter-octave intervals over the identical frequency range. The average DPE "audiogram" illustrating the frequency response of these emissions demonstrated a bilobed contour having a low-frequency maximum at approximately 1.5 kHz and a high-frequency peak that plateaued at about 5.5 kHz. The two maximum regions were separated by a minimum around 2.5 kHz. Depending on the frequency region, the average I/O functions exhibited detection "thresholds" at 3 dB above the noise floor at primary levels between 35 and 45 dB sound pressure level. The dynamic range of the emitted response between detection "threshold" and maximum amplitude varied over a 40-dB extent of the stimulus-level dimension. Approximately one third of the ears exhibited irregular DPE "audiograms" in which emitted responses were significantly reduced in restricted regions tested by low, medium, or high frequencies. When the 44 ears were separated into two groups representing more-normal and less-normal responses, the irregular "normal" ears demonstrated increased variability, especially in high-frequency regions. Mean age did not explain the differences noted between the two types of normally hearing subjects. However, across ears, DPE amplitudes and "thresholds" for the highest frequencies tested were correlated significantly with age in that the oldest individuals showed higher "thresholds" and lower amplitudes.