Why do we prefer some musical chords over others?

Why are some musical chords so inherently pleasing while others sound so obviously dissonant?

A study of a group of people with the genetic condition amusia, which causes sufferers to incorrectly perceive pitch, may have the answer: Nice-sounding, or consonant, chords have a property called “harmonicity” while dissonant chords lack it. The study was published Monday in the journal Proceedings of the National Academy of Sciences.

The finding contradicts the theory to which most acoustics researchers subscribe, that a property called “beating” is responsible for dissonance. Beating occurs when different frequencies present in the same musical chord interfere with each other’s perception in the cochlea because the frequencies are very similar. The effect causes listeners to report a sound as unpleasant because the sound appears to increase and decrease in volume over time. The theory arose in large part because dissonant chords almost always cause some amount of beating.

But does beating cause the dissonance, or do beating and dissonance simply coexist? To find out, researchers from New York University and the University of Montreal created synthetic sounds that separated out beating from dissonance and played them for a group of people with amusia and a collection of control subjects. Aside from having problems perceiving pitch, studies have shown that amusics also fail to distinguish between consonant and dissonant sounds.

Both the control subjects and the people with amusia disliked sounds that caused beating. But only the control subjects rated consonant and dissonant chords differently. This suggests that, while beating may play a role in sounds being more or less dissonant, it is not the main cause of the difference between consonant and dissonant sounds.

So what is? The authors of the study point out that beating is not the only aspect that tends to distinguish consonant and dissonant chords. Every note that makes up a chord is itself made up of a number of different frequencies. In consonant chords, the frequencies all add up to make something that sounds like a single tone even though it is made up of many—creating what’s called a “harmonic spectrum.” This does not occur in dissonant chords, which have an inharmonic spectrum. The researchers also tested whether amusics could differentiate between harmonic and inharmonic chords. They could not.

As a result, they argue, this difference, and not the beating, is likely to be the cause of consonance and dissonance.

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