[jacobolus]

When we have discordant sound (e.g. a collection of plucked strings where the fundamental frequency ratios are not in nice simple ratios, so that none of the overtones align), there’s a great deal of complexity to the sound, and you get interference patterns between them, similar to moiré patterns in images.

This causes “musical tension”.

Some types of discord are mild, and cause a bit of mild annoyance or “sadness” in the sound. Other types are aggressive and cause serious anxiety.

When you return most of the sounds to be in harmony, that tension is relieved. This causes a more positive emotional response. The greater the former tension, the more satisfying the release.

Imagine you’re in a crowd of applauding people, each clapping at a different rate, so that the sound is like a cacophony. Your brain can’t make out any pattern except a wave of sound. Now imagine the people start clapping in rhythmic unison, with some kind of structure. Suddenly your brain can make sense of the pattern.