Orchestration often involves selecting instruments for concurrent presentation, as in melodic doubling or chords. One evaluation of the aural outcome of such choices is along the continuum of "blend": whether the instruments fuse into a single composite timbre, segregate into distinct timbral entities, or fall somewhere in between the two extremes. This study investigates, through perceptual experimentation, the acoustical correlates of blend for 15 natural-sounding orchestral instruments presented in concurrently-sounding pairs (e.g. flute-cello, trumpet-oboe, etc.).
Ratings of blend showed primary effects for centroid (the location of the midpoint of the spectral energy distribution) and duration of the onset for the tones. Lower average values of both centroid and onset duration for a pair of tones led to increased blends, as did closeness in value for the two factors. Blend decreased (instruments segregated) with higher average values or increased difference in value for the two factors. The musical interval of presentation slightly affected the relative importance of these two mechanisms, with unison intervals determined more by lower average centroid, and minor thirds determined more by closeness in centroid. The contribution of onset in general was slightly more pronounced in the unison conditions than in the minor third condition. Additional factors contributing to blend were correlation of amplitude and centroid envelopes (blend increased as temporal patterns rose and fell in synchrony) and similarity in the overall amount of fundamental frequency perturbation (decreased blend with increasing jitter from both tones).
To confirm the importance of centroid as an independent factor determining blend, pairs of tones including instruments with artificially changed centroids were rated for blend. Judgments for several versions of the same instrument pair showed that blend decreased as the altered instrument increased in centroid, corroborating the earlier experiments. Other factors manipulated were amplitude level and the degree of inharmonicity.
A survey of orchestration manuals showed many illustrations of "blending" combinations of instruments that were consistent with the results of these experiments. This study's acoustically-based guidelines for blend augment instance-based methods of traditional orchestration teaching, providing underlying abstractions helpful for evaluating the blend of arbitrary combinations of instruments.