Aural Combination Tones and Auditory Masking

In experiments mapping pure-tone masked audiograms at low to moderate masker intensities, in which the signal is either a pure tone or a narrow band of noise, a notch appears on the high-frequency side of the masker with a low point at a frequency one critical bandwidth from the masker frequency. The evidence indicates that this notch is caused by the detection, on the lower-frequency side of the masker, of the cubic combination tones that are produced by the addition of masker and signal. At higher masker intensities, the difference tone may also be involved in accordance with the conclusions of Wegel and Lane [Phys. Rev. 23, 266–285 (1924)]. Similar results are obtained when a very narrow band of noise masks a pure-tone signal. The implication that the same process that creates combination tones creates combination bands further supports other evidence as to the mechanical nature of combination tones. The immediate further implications that combination bands or tones arising from complex maskers should produce low-frequency masking effects in the same way as external stimuli also has been tested and confirmed over a wide range of masker intensities and spectra. The maskers consisted of pairs of tones, tones plus narrow bands of noise, pairs of narrow bands of noise, and single bands of noise. Some of the implications of these results for cochlear physics and critical bandwidth are outlined.