Why do I need a key if the intervals are fixed?

You do not need one for the tool to run, but you need to know the song's key to choose an interval that fits. A major third above the melody is consonant over some chords and clashes over others, and no fixed interval is right everywhere. That is the honest limitation of a parallel harmoniser.

Why is the harmony quieter than the level suggests?

Because the copy is added at half the weight the control implies, so a setting of fifty percent contributes about a quarter of the mix. That keeps the harmony underneath the melody, which is how a supporting part is supposed to sit rather than competing with the lead.

Why does the duration stay the same?

Because the shifted copy is stretched back to its original length after being resampled. A plain pitch shift would shorten or lengthen the copy with the pitch, and the harmony would drift out of time with the melody within seconds.

Which interval is the safest?

An octave. It adds no new harmony at all, since every note is still a note of the same pitch class, so there is nothing to clash with the progression; what it adds is weight and brightness. A third is the most useful interval that actually harmonises, but it needs the key to be right.

Can I harmonise only part of the file?

The whole file is processed in one pass. To harmonise a section, keep that section with the trimmer, harmonise it, and join it back into place with the joiner, which is the usual way to apply any of these effects to part of a recording.

Harmonizer

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Harmonizer adds a second voice by shifting a copy of the recording by a musical interval and blending it under the original. The melody stays where it is and a parallel part appears above or below it.

What is Harmonizer?

Pick an interval, set the harmony level, drop a file, and process; the output is the original with the shifted copy mixed in at the chosen amount.

The intervals and the control:

Six choices, each a fixed musical distance: a minor third above, a major third above, a fifth above, an octave above, a fourth below, and an octave below.
A harmony level from nothing to everything, defaulting to fifty percent, with the report naming the interval, its frequency multiplier and the blend.
A report giving the interval, the frequency multiplier it produces, and the blend setting.
The duration is exactly preserved, because the shifted copy is stretched back to the original length after being resampled.

The chain is the same two-stage construction the pitch shifter uses, and the same reasoning applies. The copy is first resampled by the frequency ratio the interval implies, which moves its pitch by exactly that amount and shortens it by the same factor; it is then stretched back by that factor, which restores its original length without moving the pitch down again. That is what allows a harmony to sit underneath a melody for the whole duration of the file rather than drifting out of sync, which a plain resample would do immediately. The blend is weighted rather than a straight balance: the harmony is added at half the level the control suggests, so a setting of fifty percent contributes a quarter of the mix from the copy, which is why a harmony sits under the original rather than competing with it. That choice is deliberate, since a harmony at equal level with the melody does not read as support but as a second lead. The character of the harmony depends almost entirely on the interval chosen. A third above works with the melody's own key rather than against it only because a third is close enough to fit most diatonic progressions; a fifth is more strident and more obviously an interval; an octave is the safest of all because it adds no new harmony at all, only a higher or lower copy that thickens the line without harmonising it. The minus five semitones option is a fourth below, which sits in the same register as a bass and is often the most useful of the two downward choices.

What it does not do:

Fixed intervals only, with no chord tracking.
No scale awareness, so an interval that fits one chord may clash on another.
No independent level or tone control for the harmony voice.
Polyphonic material is shifted as a whole rather than analysed as separate voices.
Large shifts can sound smeared on transient-rich material.
Output is sixteen bit WAV.

Use it to thicken a single vocal line, to create a stacked guitar part from one take, or to add an octave doubling under a bass. When the material needs the harmony to follow the chords, that would require polyphonic pitch detection and is not something this tool attempts; the chord progression builder is the place to work out which notes belong together in a key. When the pitch itself should be corrected rather than doubled, the pitch correction does that instead.

How to use Harmonizer

Drop an audio file onto the panel.
Choose the interval, starting with a third or a fifth above for a melody.
Set the harmony level, which defaults to a supporting blend.
Process the file and download the harmonised WAV, which is the same length as the source.

When to use Harmonizer vs related tools

Harmonizer is the tool for adding a parallel part at a fixed interval, which is how a single take becomes a stack. Pitch Correction corrects the original rather than adding to it, and Auto-Tune Effect snaps the existing pitch to a scale. When two different performances rather than two copies should sound at once, Audio Merger layers separate files.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.