How does your ear pull apart the notes of a chord?

Your ear sorts a chord into its notes mechanically, before your brain has done anything clever. Inside the cochlea, named after the Latin word for a snail shell, runs the basilar membrane, a ribbon that is narrow and stiff at its base and wide and floppy at its far end, the apex. Sound sets a travelling wave rolling along it, and each spot responds best to one frequency: high notes make the stiff base vibrate most, low notes the floppy apex. A three-note chord therefore makes the ribbon vibrate most at three different places at once.

Along the membrane sit about 3,500 inner hair cells in a single row, and different frequencies end up exciting different nerve fibres on their way to the brain. Which fibres fire tells the brain which pitches are present. Scientists call this map of frequency onto place tonotopy.

The man who first watched it happen was Georg von Békésy, a Hungarian physicist who got interested in ears while studying telephone signal quality for the Post Office. He dissected cochleas from human cadavers, sprinkled silver flakes on the membrane and photographed it under strobe light. It won him the 1961 Nobel Prize.

The map has a limit. Two tones closer together than a so-called critical band can't be resolved by the ear: instead of two clean notes, you hear beating and roughness.