According to history, Pythagoras divided his followers into two groups: an inner circle called the mathematikoi (‘mathematicians’) and an outer circle called the akousmatikoi (‘listeners’). The akousmatikoi were never allowed to see their teacher, and had to listen to his lectures from behind a veil. Some two and a half millennia later their name was purloined by the French composer Pierre Schaeffer and used to describe a form of music that would have seemed every bit as unearthly to them as the ‘music of the spheres’ that Pythagoras claimed to hear: acousmatic music, defined today as that kind of electronic music that exists only in recorded form in a fixed medium, and is intended to be heard over loudspeakers. In...