The masked threshold’s high-frequency slope steepness depends on masker level

Evidence

All masked thresholds show a very steep rise from low to higher frequencies before the maximum masking is reached. The slope of this rise seems to be independent of the level of the noise masker, and the maximum always is reached 3 dB below the level of the masking noise. Beyond the maximum, the masked thresholds decay towards lower levels quite quickly for low and medium masker levels. At higher masker levels, however, the slope towards high frequencies becomes increasingly shallow. Therefore, the frequency dependence of the masked threshold is level-dependent or nonlinear. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 421