Thresholds for the detection of inharmonicity in complex tones

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Abstract
Thresholds were measured for the detection of inharmonicity in complex tones. Subjects were required to distinguish a complex tone whose partials were all at exact harmonic frequencies from a similar complex tone with one of the partials slightly mistuned. The mistuning which allowed 71% correct identification in a 2-alternative forced-choice task was estimated for each partial in turn. In experiment I the fundamental frequency was either 100, 200, or 400 Hz, and the complex tones contained the first 12 harmonics at equal levels of 60 dB SPL [sand pressure level] per component. The stimulus duration was 410 ms. For each fundamental the thresholds were roughly constant when expressed in Hz, having a mean value of .apprx. 4 Hz (range 2.4-7.3 Hz). In experiment II the fundamental frequency was fixed at 200 Hz, and thresholds for inharmonicity were measured for stimulus durations of 50, 110, 410 and 1610 ms. For harmonics above the 5th the thresholds increased from < 1 Hz to .apprx. 40 Hz as duration was decreased from 1610-50 ms. For the lower harmonics (up to the 4th) threshold changed much less with duration, and for the 3 shorter durations thresholds for each duration were roughly a constant proportion of the harmonic frequency. Inharmonicity is detected in different ways for high and low harmonics. For low harmonics the inharmonic partial appears to stand out from the complex tone as a whole. For high harmonics the mistuning is detected as a kind of beat or roughness, presumably reflecting a sensitivity to the changing relative phase of the mistuned harmonic relative to the other harmonics. The results are discussed in relation to theories of pitch perception.

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