Any sound narrower than a critical band produces identical loudness regardless of its width.

Evidence

For the same overall intensity and centre frequency, any stimulus narrower than a critical band, even a pure tone, produces a pattern of specific loudness corresponding to that shown for the noise one critical band in width. The SPL falling into one critical band is responsible for the specific loudness produced. Therefore, bands narrower than a critical band are represented by a pattern of specific loudness that is independent of bandwidth. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1288

Categorization

• DEFINE: Clarifies that the critical band is the width within which loudness equals that of a pure tone.