The dependence of the distortion product 2f₁ - f₂ on primary levels in non-impaired human ears

The optimal intensity relation between the two primaries used to generate Distortion Product Otoacoustic Emissions (DPOAEs) in unimpaired human ears, over a clinically relevant intensity range, was evaluated using a commercially available clinical device. The ILO92 was used to determine the level of the DPOAE at 2f₁ - f₂ for 16 combinations of primary levels in the range of 40 to 80 dB SPL from 40 unimpaired, young adult ears. Data were collected between 1 and 6 kHz at 1, 1.5, 2, 3, 4, 5, and 6 kHz. The commonly used procedure of dropping data points less than 3 dB above the noise floor was compared to a power subtraction procedure. A multivariate ANOVA was performed to determine main effects of gender, ear, stimulus levels, frequencies, and interactions between stimulus levels and frequencies. An overall increase of DPOAE amplitude with increase in primary level was observed, along with a decrease of the optimal difference in primary levels as L₂ was increased. Although the power subtraction and 3-dB drop paradigms yielded similar results at high stimulus levels, the power subtraction paradigm provided a more realistic indicator of DPOAE level when low level primaries were used. Possible mechanisms responsible for the level dependence of the optimal relationship between primaries and implications for clinical choice of primary levels are discussed.