Consonance and Dissonance

In the previous lesson, you heard that the octave and the fifth blend, while the minor second and the tritone rub. Musicians call this consonance and dissonance. These aren't value judgments: a dissonance isn't a mistake, it's tension. All the music you love plays with both, and learning how much of each to use completely changes the way a loop breathes.

Stable or tense: listen first

Listen to this series of intervals, all built on C4 and played two per bar. They go, roughly, from the most blended to the most tense. Try to feel the moment when your ear starts wanting something to move.

You heard it: the first ones sound like a single whole, the last ones like two notes arguing. Traditionally, musicians sort intervals into three families, and that grouping matches what you just heard fairly well.

• The perfect consonances are the unison, the octave and the perfect fifth. They're so stable that they sound almost empty, with no major or minor color. The perfect fourth is a special case: very open when you hear it on its own, it sounds less stable when its lower note is also the bass of the chord.
• The imperfect consonances are the thirds and sixths, major or minor. They're stable, but they have a color: they're what makes a chord bright or dark.
• The dissonances are the seconds, the sevenths and the tritone. The minor second, the major seventh and the tritone bite the hardest; the major second and the minor seventh are gentler.

Why some notes blend

The explanation lies in the vibrations. When you play an octave, the top note vibrates exactly twice as fast as the bottom one: their waves line up again on every cycle. For a fifth, the ratio is close to three to two; for a fourth, four to three; for a major third, five to four. The simpler the ratio, the more the two sounds have in common, and the more they blend.

For a minor second, the ratio is complicated, and the two frequencies are so close that they get in each other's way: your ear hears a kind of roughness, a slight buzz. In the part of the course on sound, we'll see why these ratios are only approximations inside your software, and what happens when two nearly equal frequencies mix. For now, remember that a simple ratio gives you blending and a complicated ratio gives you rubbing.

The two intervals that sting

The minor second is the harshest dissonance. Two neighboring rows played together, E4 and F4 for example, produce an immediate clash. As a melody, though, the same distance is the gentlest there is: a note that slides a half step to its neighbor feels like it has arrived. So the same interval can be very harsh when its notes sound together and very tender when they follow each other.

Also listen to the major seventh, which uses the same two letter names, F and E, but an octave further apart. The rubbing is still there, but softened, almost dreamy. Remember this principle, because you'll use it often: spreading two dissonant notes apart softens their clash. We'll come back to it with seventh chords.

The tritone rubs less than the minor second, but it's unstable: it sounds neither open like the fifth nor colored like the third. It feels like a question waiting for its answer, and that's exactly what makes it useful.

Useful tension

A dissonance asks for something to follow. When your ear hears a clash, it expects one of the notes to move toward a consonance. That movement is called a resolution. Listen to the tritone formed by B3 and F4: the B3 moves up a half step to C4, the F4 moves down a half step to E4, and the tension turns into a major third that sounds like relief.

This small movement is one of the engines of tonal music. We'll meet it again in the lesson on tension and resolution, where it explains why some chords make you want to go to others.

Dissonance has other uses too. A melody note that rubs against a held pad grabs attention, and that's often what you want for the high point of a phrase. A major second added to a chord, as we'll see with suspended chords, gives it air without making it harsh. And in dark music, a dissonance is sometimes left hanging for a long time without resolving: that's precisely what creates the unease.

In the low end, everything gets muddy

Here's the most common trap in the studio. A chord that sounds great in the midrange becomes blurry, even dirty, when you move it down two octaves. Listen to the same C major chord, first in close position around middle C, then in close position two octaves lower, then spread out differently.

In the low end, two notes a third apart are only seventeen hertz from each other, between C2 and E2, and your ear struggles to tell them apart: they smear together. Around C4, the same third is nearly seventy hertz wide, and the two notes stay clearly separate. It gets even worse with a sub or an 808, which already fill that whole zone on their own.

The practical rule is simple. In the low end, keep intervals wide, an octave or a fifth, rarely less. Save thirds, seconds and sevenths for the midrange and the highs. That's also the order in which nature stacks the harmonics of a note: widely spaced at the bottom, closer and closer together toward the top.

You now know what intervals are worth when you hear them two at a time. In the next lesson, we'll stack them to build our first chords.