The effect of modulation rate on the detection of frequency modulation and mistuning of complex tones

Experiment 1 measured frequency modulation detection thresholds (FMTs) for harmonic complex tones as a function of modulation rate. Six complexes were used, with fundamental frequencies $\mathrm{(F}\mathrm{0s})$ of either 88 or 250 Hz, bandpass filtered into a LOW (125–625 Hz), MID (1375–1875 Hz) or HIGH (3900–5400 Hz) frequency region. The FMTs were about an order of magnitude greater for the three complexes whose harmonics were unresolved by the peripheral auditory system $\text{(F0=88\hspace{0.05em}}\mathrm{Hz}$ in the MID region and both $F\mathrm{0s}$ in the HIGH region) than for the other three complexes, which contained some resolved harmonics. Thresholds increased with increases in FM rate above 2 Hz for all conditions. The increase was larger when the $\text{F0}$ was 88 Hz than when it was 250 Hz, and was also larger in the LOW than in the MID and HIGH regions. Experiment 2 measured thresholds for detecting mistuning produced by modulating the $F\mathrm{0s}$ of two simultaneously presented complexes out of phase by 180 degrees. The size of the resulting mistuning oscillates at a rate equal to the rate of FM applied to the two carriers. At low FM rates, thresholds were lowest when the harmonics were either resolved for both complexes or unresolved for both complexes, and highest when resolvability differed across complexes. For pairs of complexes with resolved harmonics, mistuning thresholds increased dramatically as the FM rate was increased above 2–5 Hz, in a way which could not be accounted for by the effect of modulation rate on the FMTs for the individual complexes. A third experiment, in which listeners detected constant (“static”) mistuning between pairs of frequency-modulated complexes, provided evidence that this deterioration was due the harmonics in one of the two “resolved” complexes becoming unresolved at high FM rates, when analyzed over some finite time window. It is concluded that the detection of time-varying mistuning between groups of harmonics is limited by factors that are not apparent in FM detection data.