Purpose: Distortion product otoacoustic emissions (DPOAEs) are a by-product of active cochlear processes that lead to the compressive nonlinearity of healthy ears. The most commonly studied emission is at the frequency 2f1-f2, but there has been recent interest in using the quadratic distortion product at the frequency f2-f1 to detect cochleopathies including endolymphatic hydrops. Before the DPOAE at f2-f1 can be applied clinically in any capacity, optimal stimulus parameters for its elicitation must be established.
Method: We investigated stimulus parameters for the DPOAEs at f2-f1 and 2f1-f2 in 23 adults with normal hearing. Logarithmically swept tones between approximately 0.6 and 20 kHz (L1 = L2 = 70 dB SPL) served as the higher frequency stimulus (f2). DPOAEs were measured for 6 f2/f1 ratios: 1.14, 1.18, 1.22, 1.30, 1.32, and 1.36.
Results: Both DPOAEs were consistently measurable. In line with previous investigations, the highest levels of the DPOAE at 2f1-f2 were generated between f2/f1 ratios of 1.14-1.22, with a peak in the level ratio function at 1.22. In contrast, f2-f1 was less influenced by ratio, although the narrowest ratio (1.14) produced slightly higher levels across frequency.
Conclusion: The DPOAE at f2-f1 is measurable in individuals with normal hearing up to f2 of 20 kHz at narrow f2/f1 ratios. Measurements at additional stimulus levels and in subjects with hearing impairment will be needed before clinical implementation.