Limitations in the use of distortion product otoacoustic emissions in objective audiometry as the result of fine structure

There is strong evidence for a link between intact cochlear function and otoacoustic emissions (OAE). However, all attempts to find a close correlation between auditory thresholds and amplitudes of distortion product otoacoustic emissions (DPOAE) have failed. As an explanation for these findings, we have studied DPOAE fine structure and its dependence on increasing primary sound levels. Errors due to different calibrations of equipment for measuring DPOAE and auditory thresholds were also investigated. DPOAE were measured in 16 subjects using a frequency range of 500-1000 Hz. Frequencies changed in 12.5 Hz steps at primary levels of 55, 60, 65 and 70 dB SPL. DPOAE amplitudes were found to vary by up to 20 dB for a frequency step of 50 Hz. Some fine structures showed narrow dips that shifted in frequency and diminished in amplitude with increasing primary levels. These findings demonstrated that sampling DPOAE amplitudes at widely spaced frequencies gave incomplete information about true course. DPOAE growth functions measured close to a dip in the DPOAE fine structure were rendered useless by interference with either the frequency shift or amplitude variations of the dip at different primary levels.