Detection of inharmonicity in dichotic pure-tone dyads

Thresholds for the detection of quasi-sinusoidal frequency ratio fluctuations were measured with stimuli consisting of dichotic dyads of simultaneous pure tones. The two component tones of each dyad were slowly modulated in frequency, in such a way that the ratio of their instantaneous frequencies oscillated (or not) around some standard frequency ratio (SFR). As in a previous study [Demany and Semal (1988) J. Acoust. Soc. Am. 83, 687-695], it was found that smaller oscillations could be detected when the SFR was precisely an octave (2/1) than when it was slightly smaller or larger (2/1 +/- 50 or 100 cents). Similar 'harmonicity effects' were obtained here for SFRs in the vicinity of a fifth (3/2), a twelfth (3/1), or a double octave (4/1). However, these harmonicity effects were generally less pronounced than those observed in the vicinity of an octave. Each of our four subjects provided evidence for a central sensitivity to the octave harmonicity, but the same consistency could not be found with respect to other kinds of harmonicity.