De institutione musica I.8: Quid sit sonus, quid intervallum, quid consonantia.
A sound, then, is a falling of the voice that is emmeles, that is, fit for melody, on a single pitch. I do not now wish to define sound in general, but that which in Greek is called phthongos, so named from its likeness to speaking. An interval is the distance between a high sound and a low. Consonance is the mixture of a high sound and a low, falling upon the ears sweetly and uniformly. Dissonance is the harsh and unpleasant blow upon the ear of two sounds mixed together. For when they are unwilling to blend with each other, and each strives in a way to arrive whole, and one hinders the other, both are conveyed to the sense unpleasantly.