Octave

Same pitch but different frequency

An octave is a specific interval or distance between two pitches.

In musicology, an octave is a distance between two pitches. In which, one pitch has a frequency that is twice the other (2:1). For example, a string that vibrates at 880 times per second produces a pitch that is an octave higher than a string that vibrates at 440 times per second.

Because of this "twice as fast" frequency relationship, when we hear two pitches that are an octave apart, we perceive them as being very similar, and therefore, they are given the same letter name. For example, a pitch produced by a string vibrating at 440 times per second is called "A", and another pitch produced by a string vibrating at 880 times per second is also called "A".

Since modern musical notation typically uses seven letters of the alphabet (and accidentals) to represent different pitches, the cycle starts again after the eighth. Therefore, pitches that are named with the same letter are part of the same "pitch class", or simply "octave".

Octaves are used in most of the music systems, such as tonal music (major and minor keys), pentatonic music, and serial music (twelve-tone technique).

Example of Octave

What is an Octave? | Music Theory | Video Lesson