Sonority hierarchy

The Sonority hierarchy is a hierarchy representing the sonority of classes of sounds.

Speech sounds are typically ranked according to their manner of articulation. Accordingly, in all sonority hierarchies, vowels are at the top of the hierarchy, consonants at the bottom. Most hierarchies are more finely graded:

greatest sonority > least sonority vowels > sonorant consonants > obstruents (Zec 1995), or vowels > glides > liquids > nasals > obstruents (Clements 1990) or vowels > glides > nasals > voiced obstruents   > voiceless obstruents (Katamba 1989) or vowels > liquids > nasals > voiced fricatives > voiceless fricatives = voiced plosives > voiceless plosives (Anderson & Ewen 1987)

Some hierarchies assign each individual sound to a rank of its own, thus ranking sounds also according to their place of articulation (Ladefoged 1993).

The sonority hierarchy can be used, to explain distributions of segments in syllables. The nucleus (i.e. vowel) of a syllable is the most sonorous element. The sonority of the surrounding consonants must decrease to the left and to the right starting from the vowel. Put differently: the more sonorous a segment, the closer to the nucleus of the syllable.

Example
in English the syllables matl, lkon are impossible since in matl the sonority in the sequence tl increases (must be: decreasing) and in lkon the sonority of the sequence lk decreases (must be increasing).

Links
Utrecht Lexicon of Linguistics

Other languages
German Sonoritätshierarchie