Detection of a temporal fluctuation in the frequency ratio of two simultaneous tones

Abstract

Listeners were presented with two simultaneous frequency‐modulated pure tones, each at 45 dB SPL. The two tones were heard dichotically and their carrier frequencies were separated by either 1 oct (1200 cents) or a slightly different interval (1200 ± 25, 50, or 100 cents). Each carrier frequency was sinusoidally modulated by ± 5% at a rate of 2 Hz. Using a 2IFC procedure, we measured just‐noticeable phase differences (jnpd's) between the two modulations. When the modulations were in phase, the two tones maintained a steady frequency ratio (the ratio of their carrier frequencies). Dephasing the modulations produced a quasisinusoidal frequency ratio fluctuation, which increased in amplitude with the phase difference. The results show that, for two tones with carrier frequencies below 2 kHz, jnpd's tend to be smaller when the interval formed by the carrier frequencies is equal or close to 1 oct than when it is ± 100 cents from 1 oct. This indicates that deviations from an octave interval may be easier to detect than deviations from an inharmonic interval even when monaural periodicity cues of the octave relationship are absent. However, the observed octave effects only occurred within a limited and listener‐dependent frequency domain.