Masked thresholds were measured for signal frequencies below, within, and above two 190‐Hz noise bands centered arithmetically at 1125 Hz. One noise band was computer synthesized and had amplitude fluctuations characteristic of analog noise bands typically used in masking experiments. The other noise band, matched in spectrum and level to the synthesized noise band, was generated by modulating the frequency of a sinusoid with a low‐pass filtered, clipped, Gaussian noise. The resulting waveform was free of envelope fluctuations. Masked thresholds in the FM noise were 5–10 dB lower than in the synthetic noise in the flat portions of the spectra. Synthesized noise resulted in trapezoidal appearing audiograms. The FM noise audiogram tended to be unimodal. These results are interpreted as evidence in support of envelope detection hypotheses which predict substantially improved performance in the detection of signals in noise when the noise has a constant envelope.