the sonority scale

In English, the sonority scale, from highest to lowest, is the following:

[a] > [e o] > [i u j w] > [ɾ] > [l] > [m n ŋ] > [z v ð] > [f θ s] > [b d ɡ] > [p t k] [3][5][6][7]

In layman terms this scale, in which members of the same group hold the same sonority, represents from greatest to least the presence of vibrations in the vocal folds. Vowels have the most vibration, whereas consonants are characterized as such in part due to the lack of vibration or a break in vibration.

The top of the scale, open vowels, has the greatest amount of air being used for vibration where as the bottom of the scale has the least amount of air being used for vibration of the vocal folds. This can be demonstrated by putting a few fingers on one's throat and pronouncing an open vowel such as the vowel [a], and then pronouncing one of the plosives (also known as stop consonants) of the [p t k] class.

In the vowel case, there is a consistent level pressure generated from the lungs and diaphragm, as well as the pressure difference in one's body and outside the mouth being minimal. In the plosive case, the pressure generated from the lungs and diaphragm changes significantly, as well as the pressure difference in one's body and outside the mouth being maximal before release (no air is flowing and vocal folds are not in resistance to the air flow).

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.