“Over the past century, researchers have tried to relate the perception of dissonance to the underlying acoustics of the signals,” says psychoacoustician Marion Cousineau of the University of Montreal in Canada. In a musical chord, for example, several notes combine to produce a sound wave containing all of the individual frequencies of each tone. Specifically, the wave contains the base, or “fundamental,” frequency for each note plus multiples of that frequency known as harmonics. Upon reaching the ear, these frequencies are carried by the auditory nerve to the brain. If the chord is harmonic, or “consonant,” the notes are spaced neatly enough so that the individual fibers of the auditory nerve carry specific frequencies to the brain. By perceiving both the parts and the harmonious whole, the brain responds to what scientists call harmonicity.
In a dissonant chord, however, some of the notes and their harmonics are so close together that two notes will stimulate the same set of auditory nerve fibers. This clash gives the sound a rough quality known as beating, in which the almost-equal frequencies interfere to create a warbling sound. Most researchers thought that phenomenon accounted for the unpleasantness of a dissonance.
But Cousineau and her colleagues suspected that beating might not be the whole story. In a previous paper, cognitive neuroscientist Josh McDermott of New York University in New York City isolated the acoustic factors of harmonicity and beating, and then tested subjects on their preferences. Subjects that were attracted to harmonicity, he found, were drawn to consonant sounds more consistently than they disliked beating.
To put the beating hypothesis to an even more rigorous test, Cousineau and McDermott teamed up to study a group of subjects with a condition called amusia, an inherited inability to distinguish pitch, recognize melody, or sing in tune. Amusics also can’t distinguish consonance from dissonance. The investigators reasoned that if beating really does explain why people don’t like dissonance, and the amusics are unmoved by dissonance, then they presumably wouldn’t respond to the beating either.