1.4 · Chromatic Notes
Octaves
Walk twelve semitones up from any key and you land on the same letter again. That distance is an octave. The higher note vibrates exactly twice as fast: double the frequency, same letter.
The tower of A
Each A on the piano vibrates at double the frequency of the one below it: 110, 220, 440, 880 Hz. The number in a note name identifies its octave, which is how we tell one A from another. A full piano spans A0 (lowest note) to C8 (highest note), with octaves numbered starting from C.
Same note, higher up
An octave doesn't sound like a new note. It sounds like the same note, lifted. Compare: C up to the next C blends; C up to G sounds like a new place.
Quiz
1 / 4A4 vibrates at 440 Hz. How fast does A5 vibrate?