Acoustical correlates of perceptual blend in timbre dyads and triads

Achieving a blended timbre for particular combinations of instruments, pitches, and articulations is a common aim of orchestration. This involves a set of factors that this study jointly assesses by correlating the perceptual degree of blend with the underlying acoustical characteristics. Perceptual blend ratings from two experiments were considered, with the stimuli consisting of: 1)~dyads of wind instruments at unison and minor-third intervals and at two pitch levels, and 2)~triads of wind and string instruments, including bowed and plucked string excitation. The correlational analysis relied on partial least-squares regression, as this technique is not restricted by the number and collinearity of regressors. The regressors encompassed acoustical descriptors of timbre (spectral, temporal, and spectrotemporal) as well as ones accounting for pitch and articulation. From regressor loadings in principal-components space, the major regressors leading to substantial and orthogonal contributions were identified. The regression models explained around 90% of the variance in the datasets, which was achievable with less than a third of all regressors considered initially. Blend seemed to be influenced by differences across intervals, pitch, and articulation. Unison intervals yielded more blend than did non-unison intervals, and the presence of plucked strings resulted in clearly lower blend ratings than for sustained instrument combinations. Furthermore, prominent spectral features of instrument combinations influenced perceived blend.