Table 1 Definitions of key terminology

Formant

A vocal tract resonance and/or a prominent spectral peak resulting from such a resonance. The relative positions of the first three formant frequencies (F1 to F3) determine the vowel quality in speech, whereas formant spacing among all measurable formants can be used to estimate apparent vocal tract length.

Fundamental frequency (f_o)

The lowest frequency at which a periodic signal is repeated. For voiced signals in most tetrapods, f_o corresponds to the rate at which the vocal folds are vibrating and is the perceptual correlate of pitch.

Nonlinear acoustic phenomena

Deviations from regular phonation such as frequency jumps (sudden changes in f_o), sidebands (amplitude or frequency modulation of the glottal source by additional oscillators), subharmonics (irregular vibration of the vocal folds, which produce weaker secondary frequencies at an integer fraction of f_o), and deterministic chaos (non-periodic vibration of the vocal folds).

Pitch

The perceived height or musical tone of a sound. In voiced signals, pitch is mainly determined by the fundamental frequency.

Resonances

In physics, the natural frequencies of an oscillator at which an external driving force produces maximum response. In voice science, this often refers to the frequencies that are preferentially transmitted by the air in the vocal tract.

Vocal control

The capacity to control the larynx (affecting the production of f_o) or the vocal tract (affecting the production of formants) in a flexible and/or voluntary manner, for example as a function of social context.

Vocal tract length (VTL)

The length of the airway from the sound-producing source to the aperture through which the sound is radiated into the environment (e.g., the mouth, nostrils, or beak). Formant frequencies scale inversely with VTL, so elongating the vocal tract by 10% on average lowers formants by 10%.

Voice modulation

Dynamic (time-varying) change of any property of the voice including but not limited to f_o and formant frequencies.

Vowel quality

Formant frequencies are equally spaced in a cylindrical vocal tract, which approximately corresponds to the neutral schwa vowel /ә/ (all phonetic symbols are taken from the International Phonetic Alphabet). When articulatory movements change the shape of the vocal tract, the lower formant frequencies are modified from these rest positions, and different vowels are produced. Because absolute formant frequencies depend on vocal tract length, and thus vary across speakers, vowel quality is best operationalized as speaker-normalized F1 and F2 relative to schwa.