Optimal L-1-L-2 primary tone level separation remains independent of test frequency in humans

Previous studies described a systematic asymmetry of the level of the 2f(1) -f(2) distortion product otoacoustic emission (DP) in the space of the prim ary tones levels L-1 and L-2 in normal-hearing humans. Optimal primary tone level separations L-1-L-2, which result in maximum DP levels, were close t o L-1 = L-2 at high levels, but continuously increased with decreasing stim ulus level towards L-1 > L-2 (Gaskill and Brown, 1990, J. Acoust. Sec. Am. 88, 821-839). At these optimal L-1-L-2, however, not only DP levels in norm al hearing were maximal, but also trauma-induced DP reductions. A linear eq uation that approximates optimal L-1-L-2 level separations thus was suggest ed to be optimum for use in clinical applications (Whitehead et al., 1995, J. Acoust. Sec. Am. 97, 2359-2377). It was the aim of this study to extend the generality of optimal L-1-L-2 separations to the typical human test fre quency range for f(2) frequencies between 1 and 8 kHz. DPs were measured in 22 normal-hearing human ears at 61 primary tone level combinations, with L -2 between 5 and 65 dB SPL and L-1 between 30 and 70 dB SPL (f(2)/f(1) = 1. 2). It was found that the systematic dependence of the maximum DP level on the L-1-L-2 separation is independent on frequency. Optimal L-1-L-2 level s eparations may well be approximated by a linear equation L-1 = a L-2+(1-a) b (after Whitehead et al., 1995) with parameters a = 0.4 and b = 70 dB SPL at f(2) frequencies between 1 and 8 kHz and L-2 levels between 20 and 65 dB SPL. Below L-2 = 20 dB SPL, the optimal L-1 was found to be almost constan t. Following previous notions (Gaskill and Brown, 1990), an analysis of bas ilar membrane response data in experimental animals (after Ruggero and Rich , 1991, Hear. Res. 51, 215-230) is further presented that relates optimal L -1-L-2 separations to frequency-selective compression of the basilar membra ne. Based on the assumption that optimal conditions for the DP generation a re equal primary tone responses at the f(2) place, a linear increase of the optimal L-1-L-2 level separation is graphically demonstrated, similar to o ur results in human ears. (C) 2000 Published by Elsevier Science B.V.