About equal temperament
Equal temperament divides an octave into 12 equal semitones. The frequency of any note is calculated as f = 440 * 2^((n - 69) / 12), where n is the MIDI note number (A4 = 69). Each octave doubles the frequency: A4 = 440 Hz, A5 = 880 Hz, A3 = 220 Hz.
MIDI note numbers run from 0 (C-1) to 127 (G9). Middle C is C4, MIDI note 60, at approximately 261.63 Hz.