Postmasking threshold decay is not exponential despite fitting one approximately.

Evidence

The broken lines in Fig. 4.22 illustrate an exponential decay with a time constant of 10 ms. Comparison of the broken lines and the solid lines indicates that the effect of postmasking cannot be described aoS an exponential decay. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 515