Consonants are classified as fricatives (generated by moving air turbulently through some part of the vocal apparatus), unvoiced plosives (bursts of air resulting from opening a passage previously closed), and voiced plosives (fast changes in spectrum brought about by opening or closing a passage).  Some phonemes considered as consonants (‘l’, ‘w’) are better described as dipthongs, continuous changes in vowel formation.  Fricatives and dipthongs may be prolonged at will, but plosives have short, characteristic durations in the tens of milliseconds.  The sound quality of plosives (as measured, for example, in a short-time spectrum) depends strongly on the adjoining vowel(s).