Key & harmony
Octave
An octave is a doubling of frequency — twelve semitones, and the interval at which a note repeats itself with the same name. It is the one interval every listener hears as "the same note, higher".
In practice
A doubling. It is why 174 and 87 mix, and why the frequency spectrum is described in octaves rather than hertz.
The practical reading is that anything a factor of two apart is closely related and will sit together — tempos, frequencies, and notes alike. When a detector reports a tempo that looks wrong, check the octave before anything else: half and double are by far the most common errors, and they are the only wrong answers that are also arithmetically defensible.
12
Semitones in an octave
2×
The frequency ratio
½ / 2×
The tempo errors it explains
Where these come from: The definition of the interval and its consequence for tempo detection — arithmetic, not a catalog figure. For what we can measure, see the tempo case.
Why it matters
It is why the half-time and double-time confusion exists at all. A pulse at half a track’s tempo is an octave down in the rhythmic sense, and it explains the record just as well arithmetically, which is exactly why a detector picks the wrong one and why 174 mixes cleanly with 87.
It is also the unit the whole frequency spectrum is organised in. A sub stack covers the bottom two octaves, a high-pass filter sweeps upward in them, and the bands on a three-band EQ are octave-wide regions rather than arbitrary slices.
Frequently asked
Why do 174 and 87 BPM mix?+
They are an octave apart rhythmically — one beat of the slower track spans two of the faster. The grids agree even though the numbers look far apart.
How many octaves does a sub stack cover?+
Broadly the bottom two, below where the main speakers take over. The exact crossover is set in the venue’s processor.
Is an octave the same note?+
The same note name at double the frequency, and the ear hears it as such — which is why octaves never clash and why an octave-apart bassline sounds like reinforcement rather than a second part.
Related terms
Semitone
A semitone is the smallest step in Western music — one fret, one piano key including the black ones. Twelve of them make an octave, and everything a pitch fader does is measured in fractions of one.
Perfect fifth
A perfect fifth is the interval of seven semitones — the most consonant interval after the octave, and the one the Camelot wheel is built from. Moving one step round the wheel is moving a fifth.
Frequency range
The audible spectrum runs roughly 20 Hz to 20 kHz, and DJ mixers divide it into three: lows carry the kick and bass, mids carry vocals and most instruments, highs carry hats and air.
Half-time vs double-time
Half-time keeps the tempo and moves the drums to half the rate, so the track feels slower without being slower. Double-time does the reverse. Neither changes the BPM — both change what the BPM feels like.
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