Resonances and Formants

A resonance is a frequency at which a system naturally vibrates with high amplitude. The vocal tract has multiple resonances simultaneously, each of which amplifies sound near its own frequency. The vocal tract is the air-filled space above the vocal folds — throat, mouth, and (when open to it) nasal cavity — and its shape determines where these resonance frequencies fall.

A formant is the peak that appears in the spectrum when one or more harmonics happen to fall near a resonance. The literature on the voice does not always sharply distinguish formants from resonances, and the words formant, resonance, and overtone are sometimes used interchangeably. The useful distinction:

  • A resonance is a property of the vocal tract — it exists whether or not anyone is making a sound.

  • A harmonic is a frequency component of the source signal — what the vocal folds (or another source) actually produce.

  • A formant is what you see in the spectrum: a peak where harmonic energy and a resonance line up.

Source and Filter

A common framing for voice acoustics is the source–filter model:

  • The source is the vocal-fold vibration, which produces a buzz rich in harmonics. The fundamental frequency of that buzz determines the perceived pitch.

  • The filter is the vocal tract, whose resonances amplify some frequencies and attenuate others.

The same source can produce different vowels by reshaping the filter; the same vowel can be sung at different pitches by changing the source while keeping the filter shape. This separation is what makes it possible to recognise a vowel independently of its pitch, and to recognise a singer’s voice across the range.

Formant Numbering and Vowels

Vocal-tract resonances are numbered upward from the lowest: F1, F2, F3, F4, … The first two are the most important for distinguishing vowels:

  • F1 reflects the openness of the mouth — roughly, how far the jaw is dropped.

  • F2 reflects the front-back position of the tongue.

Each vowel corresponds to a characteristic (F1, F2) pair. The Vowel Chart in VoceVista is exactly this: a two-dimensional plot of F2 against F1, with the standard IPA vowel symbols positioned at their typical formant pairs.

F3 contributes to the perception of "rhotacised" sounds (such as the English /r/) and to brightness. F4 and F5 are involved in voice timbre and individual voice identity.

The Singer’s Formant

In trained classical voices, F3, F4, and F5 cluster together into a strong peak in the 2.5–3.5 kHz region, known as the singer’s formant. Because orchestral instruments produce relatively little energy in that band, the singer’s formant lets a voice project over an orchestra without amplification. It is one of the most studied features of vocal acoustics and shows up clearly on the LTAS of an operatic voice.

Reading Resonances in the Display

In a Spectrum, resonances appear as broad humps in the curve, with one or more harmonics riding on top of each hump. In a Spectrogram, they appear as bright horizontal bands that are wider than the harmonic lines and slowly drift up or down as the singer changes vowel.

In VoceVista

The Vowel Chart shows the F1/F2 pair for the current cursor position, mapped against IPA vowel symbols. A Note Slider can be drawn to mark a specific frequency in the Spectrogram, useful for tracking a single formant. For visualising the formant envelope as a smoothed curve, see Smoothed Curves and the Spectral Envelope.