Adding two post-maskers together can reduce rather than increase masking

Evidence

The left-most data points in the two figures belong to a sinusoidal masker Ml. Proceeding from this masker to a narrow-band masker with 30- Hz bandwidth results in an increase of masked threshold of about 12 dB for masker Ml alone. Above a bandwidth of 100 Hz, masked threshold decreases to an almost constant value for bandwidths larger than 660 Hz. If the two maskers are presented simultaneously, post-masked thresholds remain almost independent of the bandwidth of masker Ml. This leads to a large decrement of post-masked threshold when the second masker M2 is added. This is in complete contrast to data for simultaneous masking. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 632

Categorization

• QUESTION: Why does post-masking behave opposite to simultaneous masking when two maskers are combined? The mechanism described in the next paragraph (temporal fluctuation reduction) deserves further study.