It’s a combination of biology, maths and physics.
If you play a tone with a frequency of 440Hz then you get an A. Doubling the frequency to 880Hz changes the note to an A one octave above.
Our ears hear this as essentially the same note, just pitched up. Within this range, certain ratios sound more pleasing than others; the most important being the ‘perfect fifth’ (3:2) and the ‘major third’ (5:4).
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Dividing the octave into 12 isn’t the only possible system (a lot of folk music uses a five-note octave), but 12 divides neatly by 2, 3, 4 and 6, which makes it easy to construct ratios that correspond closely to harmonious note combinations.
This article is an answer to the question (asked by Noel Gibson, Bath) 'Why are there 12 notes in an octave?'
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