Masking is graded, with partial masking reducing loudness rather than audibility.

Evidence

If the masker is increased steadily, there is a continuous transition between an audible (unmasked) test tone and one that is totally masked. This means that besides total masking, partial masking also occurs. Partial masking reduces the loudness of a test tone but does not mask the test tone completely. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 398

Categorization

• DEFINE: Introduces ‘partial masking’ as distinct from total masking — a loudness reduction without full inaudibility.