§ Octaves are double frequency apart (TODO)

As I am reading BasicSynth, I learnt that:
Each half-step between notes in the musical scale represents a frequency ratio of 21/121.0592^{1/12} \simeq 1.059.
The ()1/12(\cdot)^{1/12} is reasonable, because an octave contains 12 semitones. The base 22 is quite mysterious! I wanted to know why this is. a multiple oCochlea and why frequencies are 1:2