Perfect unison
The perfect unison is two notes at the same pitch, with zero semitones and a ratio of 1:1, and it is the baseline for measuring every other interval.
From C
- Size
- 0 semitones
- Abbreviation
- P1
- Just ratio
- 1:1 (0.00 cents)
- Equal-tempered
- 0 cents
What it sounds like
The perfect unison is two sounds at one pitch, so it is a perfect consonance with no distance to measure. Its ratio, 1:1, is the simplest possible, and two identical tones share every partial, which is why they fuse into one sound. Melodically a unison is a repeated note that holds the line in place. Harmonically it is the strongest blend available, and two voices in unison sound like one voice with more body. The two sources only become audible as separate when they drift slightly apart.
How to recognise it by ear
A unison has no direction and no size, so the ear hears a single pitch. The cue is the absence of a second pitch: two instruments on the same note merge into one sound with more weight. When one of them drifts a few cents sharp, a slow pulse appears as the two pitches move in and out of phase. No reference tune is given, because reference songs vary for this interval. Practice: have a partner sing your note, then move one voice slightly sharp and listen for the pulse slowing as the voices come back into agreement.
Theory
The unison inverts to the perfect octave, and the octave inverts back to the unison. Both are perfect, so the quality does not change. A unison is not a chord interval, because a triad or seventh chord uses distinct pitch classes, but two instruments doubling one chord tone in unison is common. In strict counterpoint, a unison is allowed at the opening and closing of a line. Its just ratio is 1:1, which measures 0.00 cents. The equal-tempered unison is also 0 cents, so the two tunings agree exactly.
The perfect unison above every note
Spelled by letter name, so the interval’s number is always right: a perfect unison above E♭ is not the same spelling as above D♯.
| From | To | Hear it |
|---|---|---|
On the piano
Further reading
- Reading Intervals at Sight: Generic and SpecificFundamentals · 6 min
- Scientific Pitch Notation: What C4 Actually MeansFundamentals · 6 min
Questions
Is a unison really an interval?
Strictly, it is the zero case: two notes at the same pitch with no distance between them. Theory texts still list it as an interval, the perfect unison, because interval counting starts from it and it is the inversion partner of the octave. Its size is 0 semitones and its ratio is 1:1.
Why do two slightly mistuned voices beat on a unison?
Two sounds at nearly the same frequency interfere with each other. The combined sound swells and fades at a rate equal to the difference between their frequencies. This beating is the audible sign that two notes are not yet in tune, and it slows and disappears as the pitches converge.
Is a unison a consonance or a dissonance?
It is a perfect consonance, the most consonant interval available. Two identical pitches share every partial, so nothing rubs against anything. In practice a unison is judged by how well its two sources match: an ensemble that sounds like one voice has achieved it, and an ensemble that pulses has not.