The effect of sound intensity on f1-sweep and f2-sweep distortion product otoacoustic emissions phase delay estimates in human adults.

Phase measurements of distortion product otoacoustic emissions (DPOAE) provide an estimate of round-trip travel times in the cochlea. This study examined differences in f1- and f2-sweep round-trip delays estimated from DPOAE phase responses in 20 normal-hearing adult human subjects as a function of f2 frequency and sound intensity. Eight different f2 frequencies ranging from 1.1-13 kHz were presented. For both the f1- and the f2-sweep stimulation conditions the f2/f1 ratios were between 1.1 and 1.3. Primary intensity levels for f2 were varied in 5-dB steps from 30-50 dB SPL (where f1 was 15 dB > f2). Delays in the f2-sweep condition were equal to or longer than travel times in the f1-sweep condition. Round-trip delays showed a significant intensity dependence in both the f1- and f2-sweep conditions (p < or = 0.01). In both conditions, the delay increased as stimulus intensity decreased. Delay estimates in the f2-sweep condition were more strongly intensity dependent than estimates in the f1-sweep condition at f2 frequencies above 1.6 kHz. The mean difference in f2- and f1-sweep delays at low intensities ranged from 15.9 periods at the 9.2 kHz f2 place, to 2.5 periods at the 1.6 kHz f2 place. The intensity dependence of round-trip delay estimates in both conditions may be attributed to intensity-dependent changes in the cochlear filter response time related to the sharpness of tuning of DPOAE responses. The steeper intensity dependence and longer delays observed in the f2-sweep condition may similarly be attributed to a greater proportion of the f2-sweep response being composed of the filter response time.