Voice and Formant

Changing a voice well means separating two things that naive pitch shifting smears together: the note being sung and the character of the voice.

That character lives in the formants.

This page grounds voice editing in libsonare's VoiceChanger and FormantWarp; for the vocabulary, read Editing Basics.

What a formant is

A formant is a peak of acoustic energy at a fixed frequency range. In a voice, those peaks come from resonances of the vocal tract: mouth, throat, and nasal cavity.

Formants define two things:

Formants affect	Example
Vowel identity	The difference between "ee" and "oh"
Perceived voice character	Whether a voice feels smaller, larger, brighter, or darker

The important point is that formants stay in roughly the same frequency regions regardless of the note. A soprano and a bass singing the same vowel have very different pitches, but their formants can sit in similar regions.

Put differently: pitch carries which note, formants carry whose voice and which vowel. An instrument keeps its timbre as you play up and down the keyboard, and in the same way a singer keeps the character their vocal tract gives them across different notes. Because the two are independent, it is worth being able to move them separately.

Why pitch shifting alone sounds wrong

A plain pitch shift moves everything up, formants included. Shift a voice up several semitones and the formants rise too, shrinking the apparent size of the singer — the "chipmunk" effect. Shift down and you get an unnatural, oversized voice. The problem is that pitch and formants were never meant to move together.

PARAM SWEEP · PITCH SHIFTIDLE

Pitch shift — moving the whole harmonic comb

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Pitch shifting transposes a sound without changing its length. Drag the semitones and watch the spectrum: every harmonic scales together and the fundamental marker tracks the new pitch. Because this shift is not formant-preserving, the formant bumps move too — the "chipmunk" effect. Press play to hear it.

Pitch

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Pitch and formant on separate controls

Good voice editing exposes the two independently:

• Change pitch alone → the same person sounds like they are singing a higher or lower note.
• Change formant alone → the voice sounds smaller/brighter or larger/darker at the same pitch.
• Change both → from subtle character tweaks to dramatic voice design.

To make a male voice read as female, for instance, raise the formants a little as well as the pitch. Leave the pitch where it is and lower the formants instead, and the same performance sounds fuller and calmer.

Having the two on separate controls is what makes natural transposition (transpose, then put the formants back) and creative voice design (warp formants freely) both possible from one tool. As always, small moves stay natural; large moves are an effect.

formantFactor stacks on top of the pitch shift

In voiceChange the pitch shift runs first, and a pitch shift moves the whole spectrum — formants included. FormantWarp then works on that already-shifted signal, so formantFactor multiplies the formant displacement the transpose has already caused. formantFactor: 1.0 means "no extra formant move", not "formants preserved": { pitchSemitones: 5, formantFactor: 1.0 } is a plain chipmunk shift, with the formants sitting 2^5/12 ≈ 1.33× too high.

To transpose while holding the formants where they were, pass the reciprocal of the pitch ratio:

const pitchSemitones = 5;
const formantFactor = 2 ** (-pitchSemitones / 12); // 0.749 for +5 semitones
const natural = voiceChange(vocal, sampleRate, { pitchSemitones, formantFactor });

Anything above that reciprocal brightens and shrinks the voice; anything below it darkens and enlarges it.

PARAM SWEEP · FORMANTIDLE

Formant shift — changing character, not pitch

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Formant shifting is pitch shift's counterpart: it moves the spectral envelope — the formant peaks that make a voice feel small and bright or large and dark — while leaving the note alone. Drag the factor and watch the harmonic comb and the fundamental marker hold their place as the envelope slides. Above 1.0 the voice brightens and shrinks; below 1.0 it darkens and enlarges; the pitch never changes. Press play to hear the character shift with no retuning.

Formant

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Compare this with the pitch shift above: there the harmonic comb — the evenly spaced stack of overtones above the sung note — slides and the formants ride along; here the comb and the fundamental (the lowest of those, the one that sets the note) hold still while only the envelope moves, meaning the formant shape riding on top of that comb. Those are the two independent axes voiceChange exposes.

How libsonare changes a voice

voiceChange takes pitchSemitones and formantFactor as separate arguments and applies them as two stages in that order. The pitch stage reuses the phase-vocoder/resampling backend from Time Stretch and Pitch Shift, and its resampling step scales the whole spectrum, envelope and all. The formant stage, FormantWarp (FormantWarpConfig), then does LPC analysis of the shifted signal and resamples that envelope along the frequency axis by formantFactor, leaving the harmonic structure — and therefore the note — untouched. A factor of exactly 1.0 skips the stage entirely, so the LPC round trip costs nothing on a pitch-only change. Lowering the factor moves the envelope down (larger, darker); raising it moves the envelope up (smaller, brighter). It operates on decoded mono samples like the other editing helpers.

Related: Editing Basics, Time Stretch and Pitch Shift, Pitch Correction, Editing DSP