Four subjects were run in a 2IFC psychophysical study in auditory masking. The signal consisted of a 100‐msec tone burst delivered in phase to the two ears. Signal frequencies between 250 and 4000 Hz were used. The masking noise had spectrum levels of 45, 25, and 15 dB SPL. For each signal frequency, the signal level was found that producedP(c) of 0.54 in wide‐band noise at each spectrum level. The high‐ and low‐frequency cutoffs of the noise were then varied independently (band limited), and changes in detectability were observed. Detectability remained constant until the cutoff of the noise was raised or lowered to a critical value; from that point on, detectability improved. The function traced by improvement in detectability represents the “critical band” for the stimulus conditions used. The width of the critical band was directly related to stimulus intensity for all signal frequencies.