We reopen the question of an appropriate representation to describe the masking of a sine tone midway in frequency between two sine maskers. We suggest that when the maskers are closely spaced in frequency signal detection is mediated by differences in the the stimulus envelope caused by the target signal. We show that in previous two-masker experiments detection threshold was limited by stimulus duration. Our experiments for long stimulus durations find thresholds that are approximately independent of the phase angle between the target and the maskers. Because the different phase angles correspond to very different stimulus envelopes the observed invariance is not easy to understand. For a general phase angle the presence of the signal causes both periodic changes in the envelope and periodic frequency modulation. The experimental detection data do not allow one to distinguish between these two effects.