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This lesson will teach you about the musical interval of an octave, how composers use it, and introduce you to some examples of its use in different types of music.

What’s in a Name? What’s an Octave?

An octave is a type of musical interval, or measure of distance between two notes. Specifically, an octave is the distance between one note and another note with the same letter name.Take a look at the piano keyboard pictured here. (We’ll use piano for examples during this lesson, but octaves can be found on any instrument.)

An octave on a keyboard
An octave on a keyboard (original image by Scott Detweiler)

Count from the note labeled C to the next note labeled C — that’s an octave. You might have noticed that that distance spans eight of the keyboard’s white keys (including the two Cs themselves).

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The ‘oct-‘ in octave comes from a Latin prefix meaning ‘eight.’ That’s where the octave gets its name!There are other ways to think of an octave as well. In addition to being the distance between two similarly named notes, an octave spans twelve half steps — on the keyboard, any two adjacent keys are a half step apart. Check the keyboard again, not counting the first C this time – you’ll discover that the journey from one to the other takes us across twelve individual keys. Since two half steps combine to a single whole step (the distance between two adjacent white keys in most places on the piano), we can also say the octave spans six whole steps. Composers have manipulated all of these divisions in different ways, but let’s concentrate for now on how the octave itself has been used.

The Functions of the Octave

Each musical note represents a different sonic frequency, or speed at which the sound vibrates.

Two notes separated by an octave have a 2:1 frequency ratio; that is, the higher note has a frequency twice as high as the lower note. This relationship means that octaves can often be perceived as two of the same note, since their frequencies are so closely related.Because of an octave’s frequency relationship, it is said to be the most consonant, or pleasant-sounding interval. Consonance also indicates stability — that is, the octave does not have a tendency to move or resolve to any other type of interval. Because the octave is consonant and stable, composers since the 15th century have treated it as a point of resolution, or a destination for other intervals and melodic lines to resolve to.

Octaves often occur at the end of pieces, especially pieces from the Renaissance and Baroque eras which rely on counterpoint (the interaction of two or more melodies).When composing melodies, composers often manipulate the property of octave equivalency. Octave equivalency is the idea that, just as notes an octave apart share the same letter name, they share the same musical properties. Put differently, since all notes separated by octave are the same, they can all be treated as having the same musical function.

Look at the diagram provided: the note an octave away from C is also called C, and the note an octave away from G-flat is also G-flat. In fact, you’ll notice that both members of the octave are surrounded by the same array of black and white keys – their places on the piano are functionally the same. Both of these pairs of notes will share musical function with their partners an octave (or multiple octaves) away.

Octave equivalency: C=C, Gb=Gb
Somewhere Over the Rainbow
The opening bars of Somewhere Over the Rainbow by Harold Arlen

Just over a hundred years earlier, Ludwig van Beethoven composed his Ninth Symphony, the second movement of which starts with a few thundering octave leaps. Once again, the rule of octave equivalency allows Beethoven to change notes without changing function; so although the listener hears three D’s and three A’s, he/she hears the music charging downward, creating a sense of heaviness and tension.

Meanwhile, Beethoven doubles the melody at the octave, which reinforces the musical idea and makes it much more forceful.

Ninth Symphony
The opening bars of Beethovens 9th symphony, Mvmt. II

Summary

An octave is an interval (or a measure of distance between notes) that spans eight white keys on the piano keyboard. Notes that are separated by an octave are always referred to by the same letter name. The octave has two central properties: consonance, which leads composers to use it as a point of resolution in many compositions, and octave equivalence, which allows all notes of similar letter names (and therefore separated by octaves) to be treated as having the same function.

There are many uses of octaves in all sorts of music; two examples can be found in ‘Somewhere Over the Rainbow’ from The Wizard of Oz and in Beethoven’s Ninth Symphony.

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