HearmonyWhat Is an Octave? Definition, Notes and How It Sounds
An octave is the interval between one note and the next note with the same name, such as C up to the next C. It spans 12 half steps, and the upper note vibrates exactly twice as fast as the lower one: A4 is 440 Hz, A5 is 880 Hz. That 2:1 ratio is why the two notes blend so completely that they sound like the same note, only higher.
What is an octave in music?
An octave is the distance from a note to its nearest namesake above or below. On a piano, start on any C and count up to the next C: you pass seven white keys and five black keys, twelve steps in all.
The name comes from the Latin octavus, "eighth." Count the letter names of a scale from C to C, including both ends, and you get eight: C D E F G A B C. Interval charts write the octave as P8, a perfect interval with the number eight.
The octave is also the frame for every other interval. Seconds, thirds, fifths and sevenths all fit inside it, and anything larger (a ninth, a tenth) is an octave plus a smaller interval.
How many notes are in an octave?
An octave contains 12 different notes in Western music, which means 12 half steps (semitones) from bottom to top. The other numbers you may have heard depend on what you count:
| What you count | Number | From C to C |
|---|---|---|
| Different pitches | 12 | C, C♯, D, D♯, E, F, F♯, G, G♯, A, A♯, B |
| Half steps (semitones) | 12 | Each key to the very next key |
| Piano keys, both Cs included | 13 | C up to and including the next C |
| Letter names in a major scale | 7 | C D E F G A B |
| Scale notes, top C included | 8 | C D E F G A B C (the "oct-" in octave) |
| Cents | 1,200 | 100 cents per half step |
So "how many semitones are in an octave" has one answer: 12. If half steps are new to you, half steps and whole steps shows where they sit on the keyboard and how scales use them.
Why do notes an octave apart sound the same?
Notes an octave apart sound alike because the higher note's frequency is exactly double the lower note's. Every vibration of the low note lines up with two vibrations of the high one, so the waves reinforce each other instead of clashing.
Overtones explain the rest. A single piano or voice note contains fainter higher pitches at 2, 3, 4 and more times its base frequency, called the harmonic series. The first overtone of A3 (220 Hz) is 440 Hz, which is A4 itself, and every overtone of A4 is also an overtone of A3. The upper note adds nothing the lower note didn't already contain. Musicians call this octave equivalence, and it's why note names repeat every 12 steps.
Going down works the same way: halve the frequency and you drop an octave. A3 is 220 Hz, A2 is 110 Hz, A1 is 55 Hz.
Octave numbers: what C4 and middle C mean
Octave numbers tell you which C, D or A you mean. In scientific pitch notation, the system most American textbooks use, middle C is C4 and the number goes up by one at every C. So the B just below middle C is B3, and the D just above it is D4.
| Note | Frequency (A4 = 440 Hz) | Where it is |
|---|---|---|
| A0 | 27.50 Hz | Lowest key on an 88-key piano |
| C2 | 65.41 Hz | Two octaves below middle C |
| C3 | 130.81 Hz | One octave below middle C |
| C4 | 261.63 Hz | Middle C |
| C5 | 523.25 Hz | One octave above middle C |
| C6 | 1,046.50 Hz | Two octaves above middle C |
| C8 | 4,186.01 Hz | Highest key on an 88-key piano |
Each C doubles the one below it. The piano covers a little more than seven octaves, from A0 to C8. Some keyboards and music programs label middle C as C3 instead, so check which convention a manual uses before you compare numbers.
What does an octave sound like?
An octave sounds like the same note repeated higher or lower: open, hollow and completely settled. The best-known reference is the start of "Somewhere Over the Rainbow," where "Some-where" leaps up exactly one octave. For a falling octave, sing that leap backwards. The interval songs chart has references for every other interval too.
You hear octaves every time a mixed group sings "Happy Birthday." Men usually sing about an octave below women and children, yet everyone seems to be singing the same tune. That's octave equivalence at work, and it's why a man can match a woman's note an octave lower without feeling out of tune.
Played together, an octave can blend so well that it sounds like one richer note rather than two. These are the intervals people most often confuse with it:
- Perfect fifth (7 half steps). Also open and stable, but you clearly hear two different notes. If you can hum both notes as "the same note," it's an octave.
- Major seventh (11 half steps). One half step short of the octave. It sounds like an octave that missed, with a sharp rub.
- Minor ninth (13 half steps). One half step past the octave, with the same kind of clash.
Octaves on piano, guitar and in sheet music
Every instrument has a quick way to find an octave, and sheet music has its own shorthand for moving music up or down by one.
- Piano: an octave is the span from thumb to little finger on most adult hands, eight white keys from C to C. Left-hand octaves are a standard way to thicken a bass line.
- Guitar: the 12th fret of any string plays the octave of the open string. A movable octave shape: a note on the low E string (3rd fret, G) has its octave two strings higher and two frets up (D string, 5th fret, G). From the D string to the B string, or from the G string to the high E string, add one more fret because of how the B string is tuned.
- Sheet music: "8va" over a passage means play it an octave higher; "8vb" means an octave lower; "15ma" means two octaves higher. A treble clef with a small 8 underneath sounds an octave lower than written, which is how guitar and tenor parts are usually notated. Our guide to treble and bass clef notes covers reading the staff itself.
On a piano, play the same note in three octaves (C3, C4, C5) one after another, then all together. Listen for how the three fuse into a single, fuller sound. Then swap the top C for a B and hear the fusion break.
Self-test: five octave questions
- A4 is 440 Hz. What frequency is A3?
- How many half steps are there from G2 to G4?
- You play the B just below middle C. What is its octave number?
- Is E4 up to F5 an octave?
- On guitar, which fret plays the octave of an open string?
Answers: 1. 220 Hz (half of 440). 2. 24, two octaves of 12. 3. B3, because the number changes at C. 4. No: it's 13 half steps, a minor ninth. 5. The 12th fret.
Practice hearing octaves in Hearmony
Hearmony, the iPhone ear-training app the Hearmony team builds, uses the octave as a first landmark. First Notes, the free first adventure, opens with higher-or-lower questions that start with leaps as wide as an octave and get narrower as you improve. Leap Lagoon's first unit pairs the octave with the perfect fifth, the two most stable leaps, and Interval Peaks later mixes the octave with every other interval, going up, going down and played together.
Tuning Lab works from the other end: it starts from the fact that a half step is 100 cents and asks you to hear smaller and smaller pitch differences, down to about 6 cents. Hearmony is on the App Store for iPhone, and the ear training for beginners guide shows where octaves fit in the order of skills.