Multicomponent acoustic distortion product otoacoustic emission phase in humans. I. General characteristics.

Phase characteristics and latency of 2f1-f2, 3f1-2f2, and 2f2-f1 acoustic distortion product otoacoustic emissions (DPOAEs) have been analyzed in 12 adults with normal hearing. Phase-versus-frequency functions (PFF) have been recorded at each of ten pairs of primary frequencies, by two methods, depending on which one of the primary frequencies f1 or f2 was swept in frequency. Whichever sweep method was used, local phase irregularities were found in the PFF and were shown to be independent of the phase variability due to the measured noise, to be reproducible and frequency specific. Phase irregularities were significantly greater in subjects exhibiting spontaneous otoacoustic emissions. The latency calculation was unaffected by phase irregularities provided that a suitably large range of f2/f1 ratio was used. The DPOAE latencies that were obtained at low ratios of primaries were significantly longer than those obtained at higher ratios. For all DPOAE components and sweep methods used, the latency decreased as frequency increased from 750 Hz to 6 kHz. Substantial intersubject differences were found. They might be due to two main factors: differences in middle ear acoustic properties and in cochlea mechanical characteristics. The influence of pressure in the outer ear canal increased the DPOAE latencies recorded with both the f1 and f2 sweep methods at low frequencies. Men showed significantly longer 2f2-f1 DPOAE latencies than women, especially at low frequencies, which can be attributed to the difference in length of the cochlea according to gender. Lastly, the latencies measured using the f2 sweep were consistently longer than the latencies obtained using an f1 sweep for lower sideband DPOAEs, but no difference was observed for upper sideband DPOAEs.