Controlling Perception Thresholds for Changing Timbres in Continuous Sounds

Perceptual dimensions underlying timbre and sound-source identification have received considerable scientific attention. While these scholarly insights help us in understanding the nature of sound within a multidimensional timbral space, they carry little meaning for the majority of musicians. To help address this, we conducted two experiments to establish listeners’ perceptual thresholds (PT) for changes in sound using a staircase-procedure. Unlike most timbre perception research, these changes were sonic manipulations that are common in synthesisers, audio processors and instruments familiar to musicians and producers, and occurred within continuous sounds (rather than between discrete pairs of sounds). In experiment 1, two sounds (variants of a sawtooth oscillation) both with the same fundamental frequency (F1: 80 Hz, 240 Hz or 600 Hz) were played with no intervening gap. In each trial, the two sounds’ partials differed in amplitudes or frequencies to produce a timbre change. The sonic manipulations were varied in size to detect thresholds for the perceived timbre change – listeners were instructed to indicate whether or not they perceived a change within the sound. In experiment 2, we modified stimulus presentation to introduce the factor of transition time (TT). Rather than occurring instantaneously (as in experiment 1), the timbre manipulations were introduced gradually over the course of a 100 ms or a 1000 ms TT. Results revealed that PTs were significantly affected by the manipulations in experiment 1, and additionally by TT in experiment 2. Importantly, the data revealed an interaction between the F1 and the timbre manipulations, such that there were differential effects of timbre changes on the perceptual system depending on pitch height. Musicians (n=11) showed significantly smaller PTs compared to non-musicians (n=10). However, PTs for musicians and non-musicians were highly correlated (r=.83) across different sonic manipulations, indicating similar perceptual patterns in both. We hope that by establishing PTs for commonly used timbre manipulations, we can provide musicians with a general perceptual unit, for each manipulation, that can guide music composition and assessment.