De institutione musica I.31: Quid contra Platonem Nicomachus sentiat.
But Nicomachus does not think this was truly said, for consonance is not of like sounds but rather of unlike sounds coming into one and the same concord. If a low sound is mixed with a low, it makes no consonance, since likeness does not produce this concord of singing, but unlikeness, which, though it differs in the single voices, is joined in the mixed. But Nicomachus rather thinks that consonance arises from this: There is not, he says, only one stroke that sends forth a simple measure of the voice, but a string once struck, driving the air many times, makes many voices. But because the swiftness of the stroke is such that one sound in a way overtakes another, the difference is not perceived and they come to the ears almost as one voice. If, therefore, the strokes of the low sounds are commensurable with the strokes of the high sounds, as in the proportions we mentioned above, there is no doubt that the commensuration itself is mingled together and makes one consonance of the voices.