From what I understand, one octave is the difference between two frequencies separated by a 2:1 ratio. So to go up one octave, you double the frequency (from 40Hz to 80Hz, or from 500Hz to 1,000Hz), and to go down one octave, you halve the frequency (from 40Hz to 20Hz, or 500Hz to 250Hz). Hence Kevin's

So what is an octave, really? The 2:1 definition doesn't really totally satisfy me, so the musical side of it may help some. Musical notes are broken into octaves, with the same "notes" repeating in each octave. The note "middle C" on a piano sounds similar (or complementary) to the "C" note that is one octave higher or lower.

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