The Sound of Music
The world of music was built by people who wanted to create sounds that made people feel very comfortable. They have found that in the human brain, we like the combinations of sound waves with very simple frequency ratios. For example, A4 (440 waves per second) and A5 (880 waves per second) for a ratio of 1:2, to create the feeling of formality, or A4 with E5 (660 waves per second) to show ethereal. A comfortable piece of music should stay within the range of 1:1 unison, 2:1 octave, 3:2 perfect fifth, 4:3 perfect fourth, 5:3 major sixth, 5:4 major third, 8:5 minor sixth, 6:5 minor third. All the other frequency ratios are used to create occasional instability that contrasts the stability.