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A simple mathematical model of auditory nerve activity was used to evaluate the increase in critical bandwidth with masker intensity suggested by psychophysical detection experiments. Given the masker spectrum, the model allows calculation of the excitation areas [i.e., the ranges of characteristic (best) frequencies of excited auditory nerve fibers] as a function of intensity. The results of a recent study of detection thresholds for a tone centered in the gap of a notched noise masker [D. L. Weber, J. Acoust. Soc. Am. 62, 124–129 (1977)] were analyzed. For narrow notch widths, the excitation areas completely overlap in the notch region at all intensities. The masking effect is then essentially the same as for a single wide‐band masker, and is characterized by a constant signal‐to‐noise ratio as a function of masker level. At wider notch widths, the excitation areas are separated by a gap that is inversely proportional to masker intensity. This leads to a nonlinear increase in detection threshold with masker intensity. The model thus predicts Weber's findings and supports the contention that critical bandwidth increases with masker intensity. [Work supported by NIH.]