Combination Tones and Their Origin

Abstract

Combination tones corresponding to f1 − k (f2 − f1), with k a small positive integer, are often audible during stimulation by the two frequency components f1 and f2 (f1 < f2). In this paper primarily the cubic difference tone (CDT) 2f1 − f2 (k = 1) is studied. The research reported is directed at the type of nonlinearity generating the CDT, and at the site of CDT generation. The first experiment shows that the generation of the CDT is affected by a dip (a threshold elevation in a narrow frequency region). The CDT was perceived only when the level of f2 exceeded the elevated threshold. In the second experiment the cancellation method is used. The results suggest that the high-frequency slope of the pattern of stimulation upon which the nonlinearity operates is comparable with the slope revealed in masking experiments. In the last experiments the cancellation method is reconsidered. Estimates of the CDT level found by various other measuring methods, in which the probe tone was presented nonsimultaneously with the stimulus f1, f2, were all lower than the cancellation level. The results are discussed in terms of a nonlinearity intimately coupled with a frequency selectivity. A simple compressing type of nonlinearity is proposed which accounts rather well for the CDT data. The lower estimates of the CDT level found for nonsimultaneous probe tones are attributed to suppression effects. These suppression effects might be introduced by the same nonlinearity.