Ok, time to open what will hopefully be a much smaller can of worms than my last discussion post.
I wonder if many of you composers have considered how much the role timbre plays in harmony. Obviously, those interested in orchestration have timbre very much in mind when writing. But I have lately been giving much thought to how individual timbres affect the quality of a given harmony in ways that aren't often discussed.
Most of you are probably very familiar with the natural harmonic series. When dissonance occurs between two or more notes, the intensity of the dissonance is determined not just by the relationships between the fundamental notes, but also the relationships of all the harmonics in each note.
When we learn about traditional theory, this fact plays only a very small role. It usually only manifests itself as a rule of thumb regarding spacing in a chord: "user larger harmonic intervals for lower pitches." Why is this a rule of thumb? The reason lies in the spacing of the harmonic series. Since our hearing tops out somewhere around 20khz and the harmonic series is more closely spaced the higher you go, lower pitches have many more overtones that fall within the natural range of hearing. More overtones means more potential for clashes between pitches in the lower registers.
If this isn't clear, try the following experiment: Go to your piano, and play a harmonic major third at middle C and the E above it. listen to the harmony, and then play it an octave lower to compare. Repeat at each successively lower octave, and note how much denser and less "pleasant" it becomes in the lowest piano registers. The same experiment can be done in reverse, by picking a dissonant interval, and moving it progressively higher to hear the dissonance become progressively less harsh.
Now let's examine the next facet of this idea. We know that different instruments have different harmonic makeups (timbre). This is how we can distinguish an oboe from a flute by ear. The timbre of an instrument is determined by the distribution of the harmonic series. If we know a little bit about the harmonic distribution of different instruments' sounds, we can use that as an in-road to some interesting orchestration experiments.
To be continued in part II