A sonority hierarchy or sonority scale is a ranking of speech sounds (or phones) by amplitude. For example, if you say the vowel [a], you will produce much louder sound than if you say the plosive [t]. Sonority hierarchies are especially important when analyzing syllable structure; rules about what segments may appear in onsets or codas together, such as SSP, are formulated in terms of the difference of their sonority values. Some languages also have assimilation rules based on sonority hierarchy, for example, the Finnish potential mood (e.g. -tne- → -nne-).
Sonority hierarchies vary somewhat in which sounds are grouped together. The one below is fairly typical:
In English, the sonority scale, from lowest to highest, is the following:
([p t k] < [b d ɡ] < [s f θ] < [z v ð] < [m n] < [l] < [r] < [i u] < [e o] < [a]) 
More finely nuanced hierarchies often exist within classes whose members cannot be said to be distinguished by relative sonority. In North American English, for example, of the set /p t k/, /t/ is by far the most subject to weakening when before an unstressed vowel (v. the usual American pronunciation of /t/ as a flap in later, but normally no weakening of /p/ in caper or of /k/ in faker).
In Portuguese, intervocalic /n/ and /l/ are typically lost historically (e.g. Lat. LUNA > /lua/ 'moon', DONARE > /doar/ 'donate', COLORE > /kor/ 'color'), but /r/ remains (CERA > /sera/ 'wax'), whereas Romanian transformed the intervocalic non-geminate /l/ into /r/ (SOLEM > /so̯are/ 'sun') and reduced the geminate /ll/ to /l/ (OLLA > /o̯alə/ 'pot'), but kept unchanged /n/ (LUNA > /lunə/ 'moon') and /r/ (PIRA > /parə/ 'pear'). Similarly, Romance languages often show geminate /mm/ to be weaker than /nn/, and Romance geminate /rr/ is often stronger than other geminates, including /pp tt kk/. In such cases, many phonologists refer not to sonority, but to a more abstract notion of relative strength, which, while once posited as universal in its arrangement, is now known to be language-specific.
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