The critical modulation frequency and its relationship to auditory filtering at low frequencies

If the thresholds for detecting sinusoidal amplitude or frequency modulation of a sinusoidal carrier with frequency fc are expressed in terms of the respective modulation indices, m and beta, the ratio beta/m decreases as the modulation frequency increases, and approaches an asymptotic value of unity. The modulation frequency at which the ratio first becomes unity is called the critical modulation frequency (CMF). It has been suggested that the CMF is reached when the spectral sidebands in the stimulus first become detectable and that the CMF corresponds to half the value of the critical bandwidth (CB) at fc. In this paper it is demonstrated that the CMF is confounded as a measure of frequency selectivity at low frequencies, since, for modulation frequencies around the CMF, the sideband that is most detectable changes with fc. For values of fc above 250 Hz, the lower sideband is most detectable. For values of fc below 200 Hz, the upper sideband is most detectable. These findings can account for the fact that the CMF flattens off at low carrier frequencies, reaching an asymptotic value of about 40 Hz, whereas the auditory filter bandwidth continues to decrease down to very low center frequencies.