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12 semitones · pure 2:1

The octave

Twelve semitones: the same note again, higher or lower. The upper note vibrates exactly twice as fast, so the two blend so well they share a name.

Why an octave sounds like the same note

A4 is 440 Hz and A5 is 880: every overtone of the upper note is one the lower note already has, so together they sound like one fuller note. When men and women sing the same tune together, they're usually singing in octaves.

Finding the octave by ear

Hum a note, then sing it again as high as feels like "the same note, up". That jump is the octave. It's also the easiest interval to check: two voices an octave apart that are in tune blend with no wobble at all. Take Me Out to the Ball Game, from 1908, opens with one going up, on "Take me", and so does Over the Rainbow, on "Some-where".

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Free tools by Stackapella

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Stackapella teaches you to sing harmony one part at a time, right in your browser.

Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up. Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up. Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up. Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up.
  • A guide sings your partHear your line over the others, slow it down, then take your turn.
  • Your notes light upEach note shows where you landed, and a hint when to go up or down.
  • Record every part yourselfStack your takes in time until you're the whole choir.
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Questions

What is an octave?

An interval of twelve semitones between a note and the next note with the same name: C4 to C5, or A4 to A5. The upper note has double the frequency.

How are octaves numbered?

From C: middle C is C4, so B3 is just below it and C5 an octave above. A4, the A above middle C, is the 440 Hz note an orchestra tunes to.

Is the octave the same pure and on a piano?

Yes: both are 2:1, 1200 cents (a cent is a hundredth of a semitone). It's the one interval equal temperament doesn't change.

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Ready to know them by ear? Try interval ear trainingInterval ear training · Name the gap between two notes