Masking-period patterns require masker masking and test bursts under an eighth of the period

Evidence

From the curves it becomes clear that a masking-period pattern can be measured only in that frequency range in which the masker produces a masking effect. … The test signal has to be short in relation to the masker period, i.e. shorter than one eighth of the masker period. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 569

Categorization

• QUESTION: Why exactly one-eighth of the period as the cutoff? Worth chasing the underlying reasoning for that specific bound.