Question: How adn why does the human ear detect dischord in music?

Carolyn McGettigan answered on 16 Mar 2010:

I was actually delighted to see this **excellent** question come up. I did a course in the Psychology of Hearing at Cambridge, which was given by Prof Brian Moore. He was probably the most inspirational teacher I had at uni. This question gave me an excuse to revisit his textbook.

Your question is a good illustration of the fact that things like ‘loudness’ and ‘pitch’ aren’t actually basic acoustic properties of sound. They are *perceptual* phenomena that have to do with how our ears and brains process sounds. Some of the very most basic properties of sound like the amplitude and frequency of a waveform are related to these properties, so a simple sine wave with a high frequency (lots of repetitions of the wave per second) will sound like it has a higher pitch than one with lower frequency. When a sound reaches the basilar membrane in your inner ear (in the cochlea), the membrane vibrates at a place along its length that is selectively sensitive to the frequency of the wave, and this vibration causes nervous impulses to be fired up to the brain. But, like all interesting things, hearing musical pitch is much more complicated than that! Let’s keep that in mind while we tackle the main bit of the question.

I’d like to re-word your question slightly to read “Why do discords sound dissonant?” Hope you don’t mind…

Let’s imagine the discord has been played on a piano. The sounds made by individual keys on a piano are complex tones, that is, they are made up of individual waves at several frequencies. The lowest of these waves is called the ‘fundamental’ frequency, which gives the perceived ‘pitch’, and the higher ones (harmonics) are whole multiples of this value. So, for example the A above middle C on your piano has fundamental frequency 440Hz and harmonics of 880, 1320, 1760… and so on. When several complex tones are played together, like a 3-note chord on the piano, we tend to prefer combinations of notes whose fundamental frequencies are in simple ratios to each other. For example, a fundamental frequency ratio of 2:1 is an octave, 3:2 is a fifth and 5:4 is a major third. Notes played together with simple ratios in their fundamental frequencies, like 3:2, tend to sound very pleasant to our ears. When the ratios get more complex than this, we tend to hear dissonance.

WHY?
It could be because some of those harmonics (the higher frequency components of the notes) are ‘clashing’ with each other on the basilar membrane and causing interference because they are very slightly different.

BUT dischords are produced even when two notes that make an ‘unpleasant’ combination are presented simultaneously to separate ears, so they’re not even competing on the same basilar membrane. Argghh!

There are lots of ideas why this might happen, including that it might have something to do with neural firing rates being ‘synchronised’ for consonant combinations of notes and not for dissonant ones. However, I like the suggestions that take account of our perceptual experience. A lot of the sounds that we hear all the time, like speech (which we even hear from within the womb late in our mother’s pregnancy) are harmonic, that is they have these simple ratios of frequency components in their waveform. So, maybe we just get used to listening to these and we use these learned patterns when we process things like music. This ‘experience’ account also fits with the fact that music from some non-Western cultures, such as Indian music, contain smaller divisions in their scales, like ‘quarter tones’. Westerners tend to find them quite weird simply because we aren’t used to them. Some 20th Century classical composers also make great use of dissonances and dischords that people do find pleasant, because they have grown used to unconventional chordal style. All of these learning processes likely take place further along the auditory system, in the cerebral cortex (which is where I do my work!).

I hope that makes some sense to you. I thoroughly recommend Brian Moore’s ‘Introduction to the Psychology of Hearing’ if you want to find out more.