The vocal tract: how the voice is shaped
The vocal tract is the tube between the vocal folds and the lips. The folds make a buzz; the shape of this tube decides which vowel you hear and how bright or dark the voice sounds.
What the vocal tract is
The vocal tract is the air-filled space above the glottis: the lower throat (laryngeal part), the pharynx behind the tongue, the mouth, and the lips at the end. A side branch, the nasal cavity, opens or closes with the soft palate (velum). In an adult man the tract is about 17 cm long; in an adult woman it is usually somewhat shorter, around 14–15 cm.
Source and filter
The source–filter model splits voice production into two parts. The source is the vibrating vocal folds, which produce a buzz made of a pitch (the fundamental frequency, F0) and many harmonics above it. The filter is the vocal tract, which strengthens the harmonics that fall near its resonances and weakens the others. These resonances are called formants.
This is why the same note can be sung as different vowels: the pitch comes from the folds, but the vowel comes from the shape of the tract.
What changes the shape, and what it does to the sound
Jaw opening: opening the mouth wider usually raises the first formant (F1). Open vowels such as “ah” have a high F1, closed vowels such as “ee” and “oo” a low one.
Tongue position: moving the tongue body forward and up (“ee”) raises the second formant (F2); pulling it back and down (“oo”, “aw”) lowers it.
Lips: rounding and protruding the lips lengthens the tube and lowers all the formants; spreading them as in a smile shortens it and raises them.
Larynx height: a lower larynx lengthens the tract and lowers the formants (a darker, rounder sound); a higher larynx shortens it and brightens the sound.
Soft palate: lowering the velum opens the nasal branch, which adds nasal resonances and damps the vowel.
Try it in the Voice Lab
The Voice Lab draws the tract as a 3D and a 2D model of an adult male head and neck. Move the jaw, tongue, lip, soft-palate and larynx sliders and watch the predicted formants F1–F4 change. Hover any part to see its name and role. The model is a teaching simplification, not a scan, and the formants are computed from the same tube shape that is drawn.
Questions and answers
How long is the human vocal tract?
About 17 cm in an adult man and roughly 14–15 cm in an adult woman, measured from the glottis to the lips. Lowering the larynx or protruding the lips makes it longer.
Does the vocal tract make the sound?
No. The sound source is the vibration of the vocal folds. The tract only filters it, strengthening some frequencies (the formants) and weakening others.
Why do different vowels sound different on the same note?
Because each vowel has its own tract shape and therefore its own formant frequencies, while the pitch stays the same.
Go deeper: lessons
- How a Voice Is Made: the Whole Chain — Power, source, filter, radiation: the four-stage model that organises everything about singing technique.
- Jaw, Tongue, Lips and Soft Palate — The articulators shape vowels — and tension in them travels straight to the larynx.
- Resonance, Formants and Larynx Position — Why a low, stable larynx gives a rich tone, what the singer's formant is, and why the larynx tends to climb.