A sonority scale is a ranking of sounds by how how much 'sound' they produce. For example, if you say the vowel [a], you will produce much more sound than if you say the stop [t]. Sonority scales are especially important when analyzing syllable structure; many rules about what segments may appear in onsets or codas together are formulated in terms of the difference of their sonority values.

Sonority scales vary somewhat in which sounds are grouped together. The one below is fairly typical (1 indicates lowest sonority):

  1. stops
  2. fricatives
  3. nasals
  4. liquids
  5. high vowels
  6. non-high vowels

(copied from Everything2 and posted here by the original author, Ryan Gabbard (elwethingol of Everything2))