Speech production

Speech production coordinates linguistic planning with movements of the respiratory and vocal apparatus. The vocal folds supply a sound source during phonation, while the vocal tract shapes it. Planning an utterance and executing its movements contribute different sources of variation.

The source-filter model of speech production represents a speech sound as an acoustic source shaped by a filter describing the vocal tract. During voiced speech, vibration of the vocal folds supplies a roughly periodic source; constrictions can also generate noise. The tract's resonances emphasize particular frequency regions called formants.^[ref]

Paul Boersma credits Gunnar Fant's 1960 account as the theoretical basis of this construction.^[ref]

In a simplified linear model, output spectrum $Y(f)$ is $S(f)H(f)$, where $S$ describes the source and $H$ the tract's transfer function. Keeping the source fixed while changing the filter can change vowel quality without changing the source's fundamental repetition rate. The Praat tutorial makes this test concrete by constructing a source signal and a formant filter separately, then combining them. The model's separation is an approximation; real sources and tracts can interact.^[ref]

A disorder of speech fluency involving disruptions such as repetitions, prolongations or blocks. Proposed timing and motor-control mechanisms must account for variability across conditions and development; brain differences measured after onset can include consequences or compensation.

Impaired planning or programming of the coordinated movements needed for speech, distinguished from a primary muscle weakness or a deficit in grammatical knowledge. It can arise in developmental or acquired conditions.

A developmental motor-speech condition affecting the planning or programming of speech movements. Language skills and nonverbal cognition require separate assessment rather than being inferred from the severity of speech output difficulty.