Very brief test tones stay within a critical band only at high centre frequencies.

Evidence

Additional measurements were produced at an 8-kHz centre frequency of the masker with a deviation of ±2 kHz. In this condition, test-tone bursts of only I-ms duration still have a spectral width within the critical bandwidth. Therefore, measurements up to 512 Hz modulation frequency can be made. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 557