Much of the world’s music uses more than one note at a time: it is polyphonic, meaning that it has many voices. Western music has one of the most highly developed polyphonic traditions, although it’s also strong in, for example, the vocal harmonies of sub-Saharan Africa. How do we fit notes together? Why do some seem to sound good and pleasing when played at the same time, and others jarring? In music we speak of these two things as consonance and dissonance, but they are among the most misunderstood concepts in music. Many people who dislike modernist classical music, such as the atonal music of Arnold Schoenberg and his followers, will swear that this is because it is too full of dissonance. They may not realize, first, that they probably rejoice in the abundant dissonance one can find in the music of Beethoven and Chopin, and second, that what we perceive as dissonance is mostly determined by culture rather than by acoustic physics.
There is a genuine sort of physiological dissonance, which comes from the “rough” sound of two notes with slightly different pitches. This in itself seems to suggest that any two notes will sound OK together if they’re not too close in pitch. But in fact, because every note from a musical instrument is a complex tone of many harmonics, working out how much of this sensory dissonance there is in any two-note chord is complicated. If we do that, however, we find that apart from intervals of octaves and fifths, there is not much difference between the amount of dissonance in intervals we conventionally call consonant and those we call dissonant. It’s much more a question of what we’ve become accustomed to.