Just-noticeable level variations always exceed just-noticeable level differences

Evidence

Although the data for just-noticeable level variations or just-noticeable level differences depend somewhat on the measurement technique used, the values determined for variations are always larger than those produced for justnoticeable differences. … The data produced for justnoticeable frequency modulation (open circles in the right part in Fig. 7.4) show no dependence on level. The same is true for the just-noticeable frequency difference (dots) although the data are about a factor of 3 lower than for frequency modulation. The interquartile ranges indicate very clearly that the two sets of data do not overlap, i.e. just-noticeable frequency modulation and just-noticeable frequency difference not only lead to different results but may also be produced by different signal processing in our hearing system. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1034

Categorization

• XREF: Observation tasks (detecting modulated sounds) vs. discrimination tasks (comparing two static sounds) call on different processing — connects to signal detection theory separation of detection from discrimination.
• QUESTION: The claim that frequency modulation and frequency difference may involve different signal processing is a fascinating open question worth chasing.