Baiduc, R.R., & Dhar, S. (2018). "Exploring Optimal Stimulus Frequency Ratio for Measurement of the Quadratic f2-f1 Distortion Product Otoacoustic Emission in Humans." JSLHR, 61 (7): 1794-1806 .

A publication in the Journal of Speech, Language, and Hearing Research by Drs. Baiduc and Dhar, which explores optimal stimulus frequency ratios for the f2-f1 distortion product otoacoustic emission. 

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.