
If one thing oscillates twice as fast as another, together they make a very sweet chord, called an octave. We say the frequency ratio is 2:1.
Simple frequency ratios like 3:2 or 4:3 or 5:3 or 5:4 make harmonious chords.
Complicated ratios like 23:17 sound dissonant!
Music is the pleasure the human mind experiences from counting without being aware that it is counting. — Gottfried Leibniz, around 1700.