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 · Les Espaces Acoustiques (1974-1985) · 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.