In phonetics, articulation may be divided into two large classes, obstruents and sonorants. An obstruent is a consonant sound formed by obstructing outward airflow, causing increased air pressure in the vocal tract.
Obstruents are those articulations in which there is a total closure or a stricture causing friction, both groups being associated with a noise component; in this class there is a distinctive opposition between voiceless and voiced types.
Obstruents are subdivided into stops, fricatives, and affricates. Obstruents are prototypically voiceless, though voiced obstruents are common. This contrasts with sonorants, which are rarely voiceless.