The ability of listeners to detect level differences between two sinusoidal stimuli in a two-interval forced-choice procedure was measured as a function of duration and level in three conditions: (1) the pedestal was fixed in level and the stimuli in the two intervals had the same frequency of either 1 or 2 kHz (fixed-level condition); (2) the pedestal was roved in level over a 20-dB range from trial to trial, but the stimuli still had the same frequency of either 1 or 2 kHz (roving-level condition); and (3) the pedestal was roved in level over a 20-dB range and the two stimuli differed in frequency, such that one was around 1 kHz while the other was around 2 kHz (across-frequency condition). In the fixed-level conditions, difference limens decreased (improved) with both increasing duration and level, as found in previous studies. In the roving-level conditions, difference limens increased and the dependence on duration and level decreased. Difference limens in the across-frequency conditions were generally highest and showed very little dependence on either stimulus duration or level. The results may be understood in terms of different internal noise components with additive variances: In the fixed-level conditions, sensation noise, which is dependent on stimulus attributes such as duration and level, is dominant. In more difficult conditions, where trace-memory and/or across-channel comparisons are required, a more central, stimulus-independent noise dominates. (C) 2000 Acoustical Society of America.