FM-tone fluctuation strength declines with center frequency above 1 kHz while AM does not

Evidence

The data displayed in Fig. lO.4a suggest that the fluctuation strength of amplitude-modulated pure tones depends very little on their centre frequency; despite the large interquartile ranges, the medians indicate the tendency for amplitude-modulated tones at very low (125 Hz) and very high (8 kHz) frequencies to produce less fluctuation strength than AM tones at medium frequencies. However, the results shown in Fig. lO.4b for FM tones indicate a clear dependence of fluctuation strength on centre frequency so that, although the fluctuation strength of FM tones is almost constant up to about 1 kHz, it decreases approximately linearly with the logarithm of the centre frequency towards higher frequencies. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1398