Two simultaneous maskers partition the spectrum where each dominates

Evidence

At lower frequencies, masking will be defined by the low-frequency masker. For frequencies around 1 kHz, masking will be defined by the narrow-band noise. There is a region, however, for which the masking effect produced by the low-frequency tone and the masking effect produced by the narrow-band noise, each of them presented separately, produce an equal shift of threshold. This may be, in the case mentioned, in the neighbourhood of 800 Hz. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 617