Fine structure of the 2 f1-f2 acoustic distortion products: effects of primary level and frequency ratios.

The fine structure of the 2 f1-f2 acoustic distortion product (ADP) was measured in humans with different primary level (L1/L2) and frequency (f2/f1, f2 > f1) ratios. The (L1/L2) ratio was varied under two conditions. In the first condition L1 was fixed at 50 dB SPL while L2 was varied from 30 to 75dB SPL in 5-dB steps. An upward frequency shift was observed in the ADP fine structure as L2 was increased. In the second condition, L2 was fixed at 50 dB SPL and L1 varied, and a downward frequency shift was observed. These opposing frequency shifts are predicted by a vector-sum model [Sun et al., J. Acoust., Soc. Am. 96, 2166-2174, 2175-2183 (1994)] and support the hypothesis that the ADP fine structure largely reflects place features of the area of overlap of the primary traveling waves. The mechanisms underlying the shifts in fine structure were further investigated by using three primary f2/f1 ratios: 1.11, 1.2, and 1.33. An orderly difference in the rate of fine-structure shift with level was observed as a function of f2/f1 ratio, with the largest rate of shift associated with the smallest frequency ratio. This observation, along with the fact that downward frequency shift (with L1 varied) is always at a larger rate than the upward shift (with L2 varied), suggests that ADP levels and fine structure are strongly influenced by the nonlinear compression present in the mechanics of the basilar membrane in the region of overlap between the primary traveling waves.