by Danlee Mitchell and Jack Logan, Ph.D.

The harmonic profile or sound quality of a sound source is timbre or tone color in music. Certain descriptive words may be used to express the effect of musical timbre or tone color such as: dark - brilliant; opaque - transparent; rich - mellow; fuzzy - clear; dull - sharp; complex - simple, et al. As a metaphor, musicians consider sound in the same manner that painters consider "color" (hence the term "tone color") and respond to the mixing of sound sources much the same as a painter responds to the mixing of oils. Music timbre contributes greatly to the effect of mood in music.
Timbre is determined by the harmonic profile of the sound source. Every sound source has an individual quality that is determined by its harmonic profile.
Timbre influences human mood. Sound sources which have a complexity of harmonic profile enjoy a psychological "richness" of sound. Timbre stimulates human energy levels without regard to rhythmic or harmonic saturation. Sound sources that have simple harmonic profiles have "darker" timbres and tend to soothe human emotions.
Bright or rich timbres coupled with loud dynamics affect moods of vigor, turmoil, conflict, and valor. The same timbres coupled with soft dynamics affect moods of sensuality, passion and compassion.
Dark or simple timbres coupled with loud dynamics are encountered only occasionally in music and affect moods of starkness and loneliness. The same timbres coupled with soft dynamics affect moods of mystery and terror. These combinations are subjective to all listeners but are well understood by theater and movie composers.
Timbral combinations provide unique possibilities for music. Often composers assign successive segments of melody or other material to different instruments. Certain orchestral combinations of instruments sound very much like other non-orchestral instruments. Instruments of music have long been associated with the natural world. Flutes, for example, very often represent the songs of birds.