Adding multiple maskers raises masked thresholds most when each contributes equally

Evidence

The increment f1LT of masking when two maskers are presented, in relation to that produced by only one masker, is indicated in Fig. 4.39 as a function of the level difference of the masked thresholds produced by each of the two maskers separately. … The expected maximum at a level difference f1LT(1-2) = 0 shows up clearly in both cases. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 620