Sweet Sounds and Sour Notes

Kim Krieger

A mathematical model may explain how the nerves in your ear sense harmony, a team of biophysicists reports. The model suggests that pleasant harmonies cause neurons to fire in regular patterns whereas discordant notes stimulate messier neuron activity.

Strike the middle C on a piano and hold it. Count two white keys to the right and hit the A. The bright and pleasing sound of a major third fills the air. That unmistakable sensation of musical harmony depends on the frequencies of the sound waves that make the two notes. Consonant chords consist of musical notes whose frequencies form simple ratios such as 2/1 for an octave, 3/2 for a major fifth, or 5/4 for a major third. Dissonant chords have frequency ratios of big numbers such as 16/15 or 45/32. But scientists don’t know precisely how the ear and brain sense this mathematical difference.

Now, Bernardo Spagnolo, a biophysicist at the University of Palermo in Italy and collaborators at Lobachevsky State University of Nizhni Novgorod in Russia have come up with a simple neurological model that does the trick. A sound wave sets your eardrum vibrating, which ultimately causes a spiraling membrane within the inner ear called the basilar membrane to vibrate, too. Exactly where along its length the membrane jiggles depends on the frequency of the sound, with higher frequencies causing jiggling farther along the tapering membrane. Those vibrations stimulate neurons that convey the frequency information to the brain.

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