Two instruments play the same note at the same volume. You can still tell them apart. That is timbre: the spectral shape that makes a violin a violin and an oboe an oboe.
Harmonic partial series above a single fundamental
In the 1970s, Gerard Grisey and Tristan Murail built entire compositions from the physics of the overtone series. Harmony became the analysis of sound itself, not a system of functional chords.
Random Forest models trained on biosignal data from 80 real listening sessions found that spectral features, not harmonic ones, best predicted both valence and arousal. Every top-5 feature was timbral.
Gerard Grisey ยท Les Espaces Acoustiques (1974-1985) ยท 17:45
Top predictive features from the Random Forest models trained on ~2,773 audio windows from 80 contemporary music clips. All five leading features are spectral.
A vibrating string naturally divides into exact fractions of itself โ a half, a third, a quarter, and so on. Each division vibrates at an integer multiple of the fundamental frequency and produces a distinct pitch called a partial (or harmonic). The diagram below shows what this looks like physically: the gray dots are nodes, points on the string that stay perfectly still while everything else vibrates.
Some of these partials land between the keys of a standard piano. A piano divides the octave into 12 equal steps (equal temperament), but the harmonic series follows the physics of vibrating strings โ not a 12-step grid. Partials 7, 11, 13, and 14 (highlighted in orange below) produce pitches that sit in the cracks between piano keys. To play them on a piano you would have to use the nearest key, which is noticeably out of tune with the pure harmonic. Spectral composers like Grisey treated these "off-key" partials as deliberate timbral colors rather than approximating them into familiar chords.