AM noise masking depth is frequency-independent while FM tones are strongly frequency-dependent

Evidence

For an amplitude-modulated broad-band noise, the magnitude of the masking depth of the temporal masking pattern, LJ.L, is largely independent of frequency. For amplitude-modulated pure tones, some frequency dependence occurs because of the nonlinearity of the upper slope in the masking pattern. In addition to those factors, the magnitude of the masking depth shows a strong frequency dependence with frequency-modulated tones. This means that, for describing the fluctuation strength of AM tones and FM tones, the model can be modified as follows: instead of one masking depth of the temporal masking pattern LJ.L, all of the magnitudes of LJ.L occuring are integrated along the critical-band rate scale. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1426