Effects of frequency separation of primary tones on the amplitude of acoustic distortion products

Abstract

The effects of frequency separation of equilevel primary tones, f1 and f2, on the amplitude of distortion‐product emissions (DPEs) at 2f1 − f2 was investigated in 10 ears of five humans with normal hearing. The DPEs at 1, 2.5, and 4 kHz in response to primaries at 65, 75, and 85 dB SPL were generated with f2/f1 ratios varying in 0.02 steps from 1.01 to either 1.41 (4 kHz), 1.59 (2.5 kHz), or 1.79 (1 kHz). Results indicated that, in generating the maximum DPE amplitude, f2 − f1 frequency separation was systematically related to stimulus frequency and level. Thus, for all levels of stimulation, f2/f1 ratio was inversely related to DPE frequency so that higher ratios were most effective in generating DPEs at 1 kHz, while lower ratios elicited large emissions at 2.5 and 4 kHz. In addition, for all three DPEs, in obtaining maximum responses, primary‐tone level was directly related to f2/f1 ratio in that lower ratios were most efficacious in generating DPEs to 65‐dB stimuli, while higher ratios elicited larger DPEs in response to 75‐ and 85‐dB primaries. On the average, however, across all frequency‐amplitude stimulus conditions, maximal DPE amplitude was best obtained using an f2/f1 ratio of 1.23. Overall, DPE findings describing the outcome of varying level, frequency separation, and frequency region of primary tones were consistent with those reported in the psychophysical literature for normal‐hearing humans. [Work supported by NINCDS.]