Tonal masker spread inverts from low to high frequencies as masker level rises

Evidence

The different behaviour of the high and low frequency slopes at low levels produces an effect that is somewhat unexpected. At low levels, a greater spread of masking towards the lower frequencies than towards the higher frequencies occurs. At high levels, this behaviour is reversed, so that a greater spread of masking is found towards higher frequencies than towards lower frequencies. While the effect at higher levels is well known from masking with narrow-band maskers, the effect at low levels is rather unexpected. Between these two level ranges, i.e. near a masker level of about 40 dB, the masking patterns are approximately symmetrical. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 443

Categorization

• DEEPTHINKING: An unexpected, non-monotonic inversion of masking asymmetry — the kind of counterintuitive auditory phenomenon that rewards careful reflection and connects to how excitation patterns change with level.
• XREF: Builds directly on the level-dependent cochlear excitation nonlinearities familiar from masking literature (e.g. Moore’s ‘An Introduction to the Psychology of Hearing’).