“Musical systems around the world and across historical eras have been diverse, but octaves are commonly a feature of them. The acoustic structure of octaves is always the same: The frequency of a note in one octave is half the frequency of the same note in the octave above,” and that fact of physics has led to the general assumption that “octave equivalence” is universal. But research with an indigenous Bolivian ethnic group that has limited outside contact indicates otherwise. – Quanta Magazine