Detecting temporal onset and offset asynchrony in multicomponent complexes.

Abstract

The ability of listeners to detect asynchrony in either the temporal onset or offset of components in multicomponent complexes was measured. The listener discriminated a standard complex, one in which all components were synchronous, from an asynchronous complex. In the initial experiments, asynchrony was created by starting (onset experiments) or ending (offset experiments) the harmonics at times drawn from a Gaussian distribution. In later experiments, asynchrony was created by starting or terminating only certain components before or after the other components in the complex. One complex consisted of 20 harmonic components with a fundamental of 200 Hz. Another multicomponent complex used components spaced at equal intervals in logarithmic frequency (200 to 4000 Hz). The parameters investigated were rise or decay time of components, duration of a complex, and frequency position of displaced components. The obtained thresholds were different for onset and offset asynchrony. For onset asynchrony in harmonic complexes, the thresholds were a nonmonotonic function of rise time with a minimum of 0.2 ms obtained for a rise time of 1 ms. For offset asynchrony, thresholds were generally monotonic with increases in decay time and ranged from 0.45-1.3 ms. Experiments with a much shorter signal duration demonstrated that forward and backward masking played little role in the observed differences in thresholds. Onset thresholds for harmonic stimuli measured as a function of the frequency region of the asynchronous component(s) showed a minimum of 0.2 ms at about 2000 Hz. The thresholds for offset were about ten times larger. For logarithmic complexes, in some conditions, thresholds were larger by nearly two orders of magnitude. Experiments in which the listeners discriminated changes either in the onset or offset envelope of the wideband stimulus suggest that detection of temporal asynchrony depends on comparison between frequency channels rather than on differences in the temporal onset or offset of the overall energy of the sounds.