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Articulatory phonetics studies how the moving structures of the vocal tract produce speech. Airflow supplies energy; vocal fold vibration can provide a repeating source; the tongue, lips and other articulators shape or interrupt the flow. Producing voice and shaping recognizable speech are related but distinguishable processes.^[ref]

A simple observation compares a sustained “mmm” with a sustained “sss”. Lightly touching the outside of the throat can help detect the vibration accompanying voicing, while a mirror shows the different lip configurations. This is a qualitative demonstration: the external sensation alone does not measure the detailed motion of the vocal folds.

Descriptions of consonants commonly distinguish place of articulation, where a constriction is formed, from manner of articulation, how the airflow is modified. These descriptions explain why changing the tongue or lips can change the recorded sound without changing the source of airflow. Acoustic phonetics measures the resulting signal, while articulation describes its physical production.

Phonetics studies the physical production, transmission and perception of speech. Articulatory phonetics examines the moving structures that shape the airflow; acoustic phonetics examines the resulting sound waves; auditory phonetics examines how a listener perceives them. These are different measurements along the same communication chain.

During voiced speech, airflow from the lungs drives vocal fold vibration. Movements of the tongue, lips and other parts of the vocal tract shape the emerging sound. This separates the production of a vibrating source from the articulation of recognizable speech, and both from the language expressed by it.^[ref]

A simple acoustic comparison records the same spoken phrase twice, once as a statement and once as a question. The sound pressure time series and its spectrogram allow changes in fundamental frequency, segment duration and pauses to be compared. The comparison investigates physical differences in prosody; the written words alone do not specify those measurements.

The vocal tract is the air passage above the sound-producing region of the larynx, including the pharyngeal, oral and, when acoustically coupled, nasal cavities. Movements of the tongue, lips, jaw and soft palate change its shape. These changes help turn a laryngeal sound source or a turbulent air stream into distinguishable speech sounds.^[ref]

A simple acoustic model treats an unbranched tract of length $L$ as a uniform tube closed at one end and open at the other. A pressure maximum at the closed end and pressure minimum at the open end allow odd quarter wavelengths, givingWith $L=0.17$ m and sound speed $c=340$ m/s, these resonances are $500$, $1500$ and $2500$ Hz. A real vocal tract is neither uniform nor permanently closed at the glottis; its varying cross-section and branching shift the resonances. These spectral resonances, associated with formants, differ from the repetition frequency of vocal-fold vibration. This is why changing articulation can change a vowel without requiring the same change in voice pitch.