Unison

Play middle C twice at the same time from two sound sources. Both notes have the same pitch, C4, so the interval between them is a unison.

The sources can still sound different. A voice and violin on C4 share pitch while keeping their own timbre, attack and loudness. Your ear may hear one combined pitch with two sound identities.

A single instrument striking one note is not usually described as an interval by itself. Unison becomes useful as a relationship when two parts, voices or oscillators meet on the same pitch.

Interval names count written letter positions, including both ends. C to C uses one letter position, so its number is one: a unison or prime.

The semitone distance is zero because the second pitch has not moved. Number and distance answer different questions, just as C to E is called a third while spanning four semitones.

Its complete quality name is perfect unison. Unisons belong to the perfect family with fourths, fifths and octaves.

A frequency ratio compares vibration rates. If both sound sources vibrate at 440 hertz, the relationship is 440:440, which simplifies to 1:1.

At 440 hertz, both cycles repeat 440 times per second. When the waveforms align in phase, they reinforce each other. A phase difference can change the combined loudness and tone color while both sources remain on the same A.

Real instruments rarely produce perfectly identical waveforms. Breath, bowing, string movement and small tuning changes keep the combined sound alive.

Set one source to 440 hertz and the other to 441. Their peaks move in and out of alignment once per second, creating one audible beat per second.

Increase the difference to 443 hertz and the beating becomes roughly three times per second. The perceived pitch may still feel centered near A, but the sound now pulses more quickly.

Musicians listen to these beats while tuning sustained notes. Slower beating generally means the frequencies are closer. When the beating becomes too fast or the gap grows, your ear begins to hear a wider detuned sound or two separate pitches.

When several singers perform the same melody at the same pitch, they sing in unison. The written notes and rhythm match, although each voice contributes a different spectrum.

Small natural differences can make the group sound fuller than one singer. The goal is usually a shared pitch center, not electronically identical frequencies.

Instrument sections use unison similarly. Violins can play one line together; guitars can double a riff; horns can reinforce a melody. Balance and articulation then determine whether the parts fuse or remain individually noticeable.

A unison uses the same pitch in the same register: C4 with C4. Its frequency ratio is 1:1.

An octave uses the same pitch class in a different register: C4 with C5. The upper frequency is twice the lower one, giving a 2:1 ratio.

Both can blend strongly and share a letter name. The octave still has a clear lower and upper pitch, while the perfect unison has no pitch distance.

C to C sharp repeats the letter C, so it is written as an augmented unison. It spans one semitone.

C to D flat uses two letter names and is a minor second, even though C sharp and D flat share a piano key in equal temperament.

This distinction matters in notation and voice leading. An augmented unison describes one scale degree being altered; a minor second describes movement between neighboring letter names.

Record one guitar or vocal part twice and play both takes together. Their small differences in timing, tuning and tone keep the notes from fusing completely, which creates width and density.

A chorus effect imitates that process electronically. It copies the signal, delays it slightly and varies the copied pitch over time. The result moves around a unison instead of staying at an exact 1:1 relationship.

Synthesizers often detune two oscillators by a few cents for the same reason. Exact unison can sound focused; controlled near-unison can sound broader. Too much detuning changes the perceived chord or pitch center.

Two ideal waves at the same frequency can combine differently depending on phase.

When their peaks and troughs align, the amplitude increases. When one waveform's peak meets the other's trough, they can partially or completely cancel.

Acoustic instruments contain many harmonics and constantly changing phases, so complete cancellation is unusual. In recording and synthesis, duplicated signals can create clear phase reinforcement or cancellation. This is a timing relationship between waveforms, not a new interval.