Octave

Pitches an octave apart have a 1:2 (“one-to-two”) frequency ratio. If one pitch is at 200 Hz, a pitch an octave higher is at 400 Hz. A pitch an octave lower is at 100 Hz.

In 12-tone equal temperament, the octave is an interval which spans twelve semitones.

One special property of this interval, known as octave equivalence, is that pitches an octave apart are often heard as being very similar. If a melody is played in multiple octaves at the same time (sometimes called “doubling at the octave”), the two lines will tend to “fuse” into one.

If you sing, sometimes you'll notice that certain pitches are too high or low to sing comfortably. When this happens, we will often naturally sing the pitch in a different octave, and it usually still sounds good.

Most (but not all!) tuning systems involve a pattern of intervals that repeats at the octave. And many tuning systems use the same name for pitches separated by octaves. For example, every F# in a DAW or on a piano is an octave apart.

References

• Wikipedia entry‍

Context

Introduction to EDO tunings, 5-EDO, 7-EDO, 7-EDO + 5-EDO, 11-EDO, 12-TET(EDO), 13-EDO, 19-EDO, 22-EDO, 24-EDO, 31-EDO, 35-EDO, 36-EDO, 41-EDO, 43-EDO, 53-EDO, 55-EDO, 72-EDO, Make your own tuning