An octave is the relationship between two notes that share a note name at different heights. From a low C to the next C above is one octave, which on a keyboard is 12 semitones counting the white and black keys together. Because they are perceived as the same kind of note despite the difference in height, octaves are the basis of extending range.
Why do they sound like the same note?
The higher C vibrates at twice the frequency of the lower one, yet we hear it as belonging to the same C family. Because of this relationship, moving a melody up an octave keeps its shape while making it brighter and more prominent.
Relationship to unison
In unison, where the same note is doubled, doubling the same melody an octave apart is common, not only at exactly the same pitch. A low note adds weight and a high note adds definition, emphasizing the main melody.
Using octaves in arrangement
Putting the bass in a low octave and the melody in a high one keeps their roles from colliding. Doubling the melody an octave up in the chorus only, or playing a riff in two octaves, are also ways to add variety to an arrangement.
Working in your DAW
In most DAWs, moving MIDI notes 12 semitones up or down shifts them by one octave. After changing octave, check that the part’s range does not collide with other instruments and that the low end has not become muddy.
Relationship to semitones
Counted in semitones and whole tones, one octave is 12 semitones. Confirming on a keyboard or piano roll that the same note name appears every 12 keys helps you get a feel for range.
