There’s no arguing with this point. It’s science. The dot-com boom geeks who wanted you to have all the world’s music for free were clever enough to deploy basic concepts in psychoacoustics, or the study of how the brain interprets sound, in the architecture of the MP3 format. A quick-and-dirty summary of the compression processes at play includes the use of minimum audition thresholds, and simulated masking and bit/sample-rate management via the Nyquist-Shannon Theorem to discard any sonic information that doesn’t contribute meaningfully to a human’s listening experience.
In 12-TET, you don’t define your intervals by tidy ratios of whole numbers. Instead, you divide up the octave into twelve equally-sized semitones (the interval between two adjacent piano keys or guitar frets). You then add semitones together to make all the other intervals. To go up a semitone from any note, you multiply its frequency by the 12th root of two. To go down a semitone from any note, you divide its frequency by the 12th root of two. If you go up by an octave (12 semitones), you’re multiplying your frequency by the 12th root of two 12 times, which works out to two.