Why is dissonance noisy?

Nature explores the strange mathematics of yuck – the neurological reason why we find dissonant music hard to listen to:

Consonant chords are, roughly speaking, made up of notes that ‘sound good’ together, like middle C and the G above it (an interval called a fifth). Dissonant chords are combinations that sound jarring, like middle C and the C sharp above (a minor second). The reason why we should like one but not the other has long vexed both musicians and cognitive scientists.

Yet when [University of Montreal neuroscientist Marion] Cousineau and colleagues asked amusic [or “tone-deaf”] subjects to rate the pleasantness of a whole series of intervals, they showed no distinctions between any of the intervals. In contrast, normal-hearing people rated small intervals (minor seconds and major seconds, such as C–D) and large but sub-octave intervals (minor sevenths (C–B flat) and major sevenths (C–B)) as very unpleasant.

Those preferences seem to stem from the so-called harmonicity of consonant intervals. Notes contain many overtones — frequencies that are whole-number multiples of the basic frequency in the note. For consonant intervals the overtones of the two notes tend to coincide as whole-number multiples, whereas for dissonant intervals this is no longer the case: they look more like the irregular overtones for sounds that are ‘inharmonic’, such as metal being struck.

The control group preferred consonant intervals with these regular harmonic relationships over artificial ‘consonant’ ones in which the overtones were subtly shifted to be inharmonic while the basic tones remained the same. The amusics, meanwhile, registered no difference between the two cases: they seem insensitive to harmonicity.

Diana Deutsch, a music psychologist at the University of California at San Diego, says that the work is “of potential interest for the study of amusia”, but questions whether it adds much to our understanding of normal hearing. In particular she wonders if the findings will survive in the context of everyday music listening, where people seem to display contrary preferences. “Rock bands often deliberately introduce roughness and dissonance into their sounds, much to the delight of their audiences”, she says.