The goal of a series of listening tests was to better isolate the principal dimensions of timbre, using a wide range of timbres and converging psychophysical techniques. Expert musicians and nonmusicians rated the timbral similarity of three sets of pitched and percussive instruments. Multidimensional scaling analyses indicated that both centroid and rise time comprise the principal acoustic factors across all stimulus sets and that musicians and nonmusicians did not differ significantly in their weighting of these factors. Clustering analyses revealed that participants also categorized percussive and, to a much lesser extent, pitched timbres according to underlying physical-acoustic commonalties. The findings demonstrate that spectral centroid and rise time represent principal perceptual dimensions of timbre, independent of musical training, but that the tendency to group timbres according to source properties increases with acoustic complexity.