An octave is a pitch interval in which frequency doubles when ascending or halves when descending. In 12-tone equal temperament, it spans 12 semitones. section 10.4.1
Checking octave calculations
Using the semitone representation, concert A is 440 Hz: 12 semitones higher is 880 Hz, and 12 semitones lower is 220 Hz. A note 12 semitones above another retains the same frequency ratio. section 10.4.1
Figure 10.8 Formula to compute the frequency (f) from a given semitone distance to concert A (s) in 12-tone equal temperament
The frequency formula connects a semitone distance from concert A with octave doubling in 12-tone equal temperament. section 10.4.1

