Level variations within a sound and differences between sounds activate distinct hearing processes.

Evidence

Two different kinds of sound changes are discussed in this chapter. One is the variation that may be compared to variation in water level: there is always some water but the level varies as a function of time. In acoustics, modulations are typical changes of the sort we call variations. The other kind of change is that of differences. One apple may be different from another apple. In this case, we compare one piece with another piece. In acoustics, this means that we compare one sound with another sound presented after a pause. Because these two kinds of changes may activate different processing features in our hearing system, the first by direct and quick comparison and the second by activating and introducing memory, it is necessary to differentiate strictly between the two kinds of changes. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1000

Categorization

• DEFINE: Establishes the crucial distinction between just-noticeable variations (within-sound modulation) and just-noticeable differences (between-sound comparison), foundational for loudness and pitch measurement methodology.