The sonority hierarchy ranks speech sounds by amplitude. The sonority of speech sounds is related to their amplitude. The speech sounds with the greatest amplitude also have the greatest sonority.
In the syllable, the nucleus is the peak. Thus in the word "cat" the vowel "a" has the greatest sonority and the plosives in the onset and the coda have the least. The sonority sequencing principle aims to outline the structure of the syllable in terms of sonority. Certain languages have strict principles regarding syllable structure. Japanese, for example, does not allow plosives in the syllable coda.
Sonority hierarchies can vary in the way that they group sounds together. A typical one groups plosives, affricates and fricatives at the bottom of the sonority hierarchy. They form a class of obstruents. The other sounds form a class of sonorants. They consist of nasals, liquids, high vowels and finally mid and low vowels.
In English, the voiceless plosives are at the bottom of the sonority hierarchy. They are followed by the voiced plosives, the voiceless fricatives, the voiced fricatives, the nasals, the liquids, the high front vowel, the high back vowel, the mid front vowels, the mid back vowels, the low front vowel and the low back vowel.
The sonority hierarchy groups sounds according to their level of sonority. At the bottom of the sonority hierarchy are the obstruents and at the top are the sonorants. Sonorant hierarchies can vary from one language to another but they are usually quite similar.