The octave is the interval between two notes that are of a higher or lower pitch level but which are designated by the same letter name. In acoustic terms, two pitches are separated by an octave when the frequencies of the two pitches are respectively twice, and half the frequency of each other. Notes that lie an octave apart are heard and understood to be in some fundamental respect equivalent: middle C and high C are separated by an octave, but the “C-ness” of both notes is preserved. Apart from the interval of a unison (which maintains the same pitch level between two notes; e.g., if I hum middle C while playing the note on the piano I am humming in unison with the piano), the octave is the only interval that has this property.
Rainbow Melody Hear Audio File
In this example, the melody on the opening word “Some-where” ascends by an octave; the two notes sung to the two syllables of “somewhere” are an octave apart. This ascending octave is answered by a descending octave between the second and third measures.