Harmonic spacing on a log scale determines masking-pattern dip size

Evidence

Whereas a flute produces primarily one single component, the fundamental, a trumpet produces many harmonic partials and therefore elicits a much broader masking effect than a flute. Figure 4.10 shows thresholds of pure tones, masked by a complex tone composed of a 200-Hz fundamental frequency and nine higher harmonics, all with the same amplitude but random in phase. The masked thresholds are given for sound pressure levels of 40 and 60 dB of each partial. On the logarithmic frequency scale, the distance between the partials is relatively large at low frequencies, but becomes very small between the ninth and tenth harmonic. Accordingly, the dips between the harmonics become smaller and smaller with increasing frequency of the test tone. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 449