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2-to-1 Ratio: Interval of an Octave
Your brain distinguishes all other frequency ratios from the 2-to-1 ratio, the octave interval, in two ways:
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By associating a “different-from-octave” qualitative sound with each note representing each “non - 2-to-1" frequency ratio, within the context of the octave interval.
For example, as you play the white keys on the piano from C up to the next C, you hear the notes D, E, F, G, A, and B all sounding qualitatively different from the C you started the scale with.
But when you get to the C at the top of the scale, even though it’s a different note, it sounds qualitatively the same as the C you started with. Yes, it’s higher in pitch, but it still sounds to your brain like the identical note you started with, C.
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By associating a feeling of imbalance or unrest with each non-octave note. This feeling of unrest or tension increases in intensity as frequency ratios become more complex with respect to the octave interval.