Two experiments are reported in which difference limens (DLs) were measured for onset times of a 1000-Hz tone pulse. An adaptive two-alternative forced-choice procedure and (mostly) welltrained subjects were used. In the first experiment, DLs were measured for the rise time of linear onset ramps at rise-time values between 10 and 60 msec. The DLs follow Weber's law up to a rise time of about 50 msec, and do not support the notion that rise times are perceived in a categorical manner. In the second experiment, DLs were obtained for linear, exponential, and raisedcosine onset envelopes at rise-time values between 10 and 40 msec. When energy differences in the critical band around 1000 Hz are computed for just-discriminable onsets, values between 0.7 dB (10-msec rise time) and 0.3 dB (40-sec rise time) are found. These equivalent intensity DLs show the same "near miss to Weber's law" behavior as do intensity DLs for pure tones.Recognition of a musical instrument by its sound is based on quasi-stationary attributes such as tone spectrum, as well as on transient attributes such as tone attack and decay. It has been shown (Berger, 1964;Clark, Luce, Abrams, Schlossberg, & Rome, 1964; Saldanha & Corso, 1964) that subjects' abilities to identify musical instruments correctly by their sound decreases significantly when sounds are presented without attack.Further insight into the perceptual role of a tone attack was obtained through studies of differential sensitivity for a tone's onset time. A central question in these studies was whether a tone's rise (onset, attack) time forms a perceptual continuum in which, for instance, Weber's law would hold, or whether the physical onset-time continuum maps perceptually into a small number of subjective categories, resulting in so-called categorical perception. Especially during the last few years, there has been a renewed interest in categorical perception for nonspeech signals. Rosen and Howell (1981) determined that categorical discrimination functions could not be obtained for music-like stimuli spaced with equal intervals of the rise time, as had been reported by Cutting and Rosner (1974). Their subject's responses appeared rather to follow Weber's law; that is, difference limens (DLs) for rise time increased proportionally with increasing rise-time durations. These findings were confirmed by Kewley-Port and Pisoni (1984).Smurzyriski (1985) that subjects trained in auditory perception, when presented with 300-Hz sawtooth-wave bursts with six different rise times between 10 and 60 msec, generated typical confusion matrices with responses clustered around the main diagonal, rather than two sharply defined response categories ("pluck" and "bow"), as Cutting and Rosner (1974) had found. Further analysis of these identification data revealed that average received (or mutual) information (Garner & Hake, 1951) in that experiment amounted to 1.3 bits. This is significantly more than the prediction of a categorical perception model that divides the 10-to 6O-msec rise-time range in...