Unanesthetized cats with chronically implanted epidural electrodes in the region of primary auditory cortex were stimulated with frequency‐modulated (FM) noises of varying bandwidths. The average evoked responses to these stimuli were compared with responses to tone bursts of the same center frequency and intensity. Two different noise stimuli were used: (a) bursts with rise/fall times the same as the tone burst; (b) transitions from tone to noise and back with transition times equal to tone‐burst rise time. It was found that: (1) the magnitude of the early response components increases with the bandwidth of the modulating noise, the relationship being fitted well by a power function; (2) there is a smaller power‐law type of increase in response amplitude with rms rate, noise bandwidth being held constant; (3) responses to transitions from tone to noise were quite marked, often exceeding burst responses, while responses to transitions from noise to tone were only rarely observed. These results are discussed in terms of the activity of single units in auditory cortex. A simple neuronal model is proposed to explain and unify the findings. The results are also compared with psychological loudness summation studies.