How the numbers work
Modern tuning fixes A4 at 440 Hz and divides every octave into twelve equal semitones, each a frequency ratio of 2^(1/12) ≈ 1.0595. From those two facts every value on this page follows: octaves double, semitones multiply, and no note except the As lands on a round number. Convert freely with the note ↔ frequency converter, generate any in-between frequency with the tone generator, tune an instrument against your microphone with the chromatic tuner, or hear reference notes on the pitch pipe and online piano.