The INTERVAL between the first and eighth notes in a major scale, and the interval common to practically all SCALEs ever evolved. It is characterized by the FREQUENCY ratio 2:1, such as that produced when the length of a vibrating string is halved.

A NOTE whose PITCH is an octave above or below a given note gives the subjective impression of duplicating that note, such as when men and women sing in UNISON, but actually are producing notes an octave apart. With SINE TONEs, however, the exact 2:1 ratio appears flat to most listeners, and a sharpening or stretching of the interval is regarded as preferable. With complex tones, octaves appear more exactly tuned when BEATS among the upper partials are minimized.

The interval of the octave is that found between the first and second HARMONIC in a harmonic SPECTRUM; therefore, two spectra with FUNDAMENTALs an octave apart will tend to blend together into a single perceived spectrum. See INTERVAL for comparative representations. The musical abbreviation for octave is 8va.

Sound Example: Two notes an octave apart, heard sequentially, then together.

Sound Example: Men and women singing in unison, but actually an octave apart.