Distortion products and the perceived pitch of harmonic complex tones

Combination tones (CTs) produced by acoustic stimuli have been studied by many authors as an efficient way to learn about the numerous physiological nonlinearities involved in auditory processing. As far as perception is concerned, CTs have been proposed as a basis for the perception of the pitch of the missing fundamental (Helmholtz, 1877). It has since been shown that the missing fundamental can still be perceived when CTs are cancelled (Schouten, 1938). However, this does not preclude a possible contribution of CTs to the pitch of the missing fundamental in normal listening conditions. To our knowledge, relatively few CT data are available which could shed direct light on the contribution of CTs to pitch perception. Most psychophysical distortion studies employed acoustic stimuli made of 2 tones and measured the different kinds of CTs produced (e.g. Goldstein, 1967). When a harmonic tone complex is used instead, one might predict that a complete distortion spectrum (DS) that reconstitutes the lower part of the harmonic series would be generated. Fletcher (1924) proposed the existence of such a DS but it is unclear how his data were collected and at which level (possibly more than 120 dB SPL). Greenwood (1972) showed that sounds with a continuous spectrum could produce noise distortion bands but generalisation of his results to harmonic sounds remains theoretical. In the current study, combination tones produced by harmonics 15 to 25 of a missing 100Hz fundamental were measured psychophysically at a moderate sound level.