Octave
Twelve semitones, and the same note again. The upper frequency is exactly double the lower one, which is why your ear treats the two as the same note wearing different shoes.
- 12 semitones
- 2:1
- same note, higher
related pages
What it is
Twelve semitones. Double the frequency and you get the octave: 220 hertz and 440 hertz are both an A. This is the one interval that every musical culture on earth agrees about, which is why note names repeat every octave instead of running on forever.
How to recognise it
Two notes that fuse into one. An octave is so consonant that untrained listeners often cannot tell whether one note or two are playing.
Stretch it here, a few cents at a time, and the fusion breaks. That breaking point is octave equivalence becoming audible.
Where it shows up
In every scale, which spans exactly one octave before repeating. In basslines doubled an octave up. In the way a choir splits: the same melody, an octave apart, still counts as unison singing.
What it gets confused with
With the unison, if the two are close in timbre. The difference is that an octave still has a top and a bottom, and you can sing either one.