A sonority hierarchy or sonority scale is a hierarchical ranking of speech sounds (or phones). Sonority is loosely defined as the loudness of speech sounds relative to other sounds of the same pitch, length and stress, therefore sonority is often related to rankings for phones to their amplitude. For example, pronouncing the vowel [a] will produce a louder sound than the stop [t], so [a] would rank higher in the hierarchy. However, grounding sonority in amplitude is not universally accepted. Instead, many researchers refer to sonority as the resonance of speech sounds. This relates to the degree to which production of phones results in vibrations of air particles. Thus, sounds that are described as more sonorous are less subject to masking by ambient noises.
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, in which a less sonorous segment changes to copy a more sonorous adjacent segment (e.g. -tne- → -nne-).
Sonority hierarchies vary somewhat in which sounds are grouped together. The one below is fairly typical:
(glides and liquids)
Sound types are the most sonorous on the left side of the scale, and become progressively less sonorous towards the right (e.g., fricatives are less sonorous than nasals).
The labels on the left refer to distinctive features, and categories of sounds can be grouped together according to whether they share a feature. For instance, as shown in the sonority hierarchy above, vowels are considered [+syllabic], whereas all consonants (including stops, affricates, fricatives, etc.) are considered [−syllabic]. All sound categories falling under [+sonorant] are sonorants, whereas those falling under [−sonorant] are obstruents. In this way, any contiguous set of sound types may be grouped together on the basis of no more than two features (for instance, glides, liquids, and nasals are [−syllabic, +sonorant]).
|Most sonorous (weakest consonantality) to
least sonorous (strongest consonantality)
|low (open vowels)||[a]|
|mid vowels||[e o]|
|high vowels (close vowels) / glides (semivowels)||[i u j w] (first two are close vowels, last two are semivowels)|
|nasals||[m n ŋ]|
|voiced fricatives||[v ð z]|
|voiceless fricatives||[f θ s]|
|voiced plosives||[b d g]|
|voiceless plosives||[p t k]|
|complex plosives||Not found in English|
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] 
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.
Syllable structure tends to be highly influenced and motivated by the sonority scale, with the general rule that more sonorous elements are internal (i.e., close to the syllable nucleus) and less sonorant elements are external. For instance, the sequence /plant/ is permissible in many languages, while /lpatn/ is much less likely. (This is the sonority sequencing principle). This rule is applied with varying levels of strictness cross-linguistically, with many languages allowing exceptions: for example, in English, /s/ can be found external to stops even though it is more sonorous (e.g. "strong", "hats").
In many languages the presence of two non-adjacent highly-sonorous elements can be a reliable indication of how many syllables are in the word; /ata/ is most likely two syllables, and many languages would deal with the sequences like /mbe/ or /lpatn/ by pronouncing them as multiple syllables, with syllabic sonorants: [m̩.be] and [l̩.pat.n̩].
The sonority ranking of speech sounds plays an important role in developing phonological patterns in language, which allows for the intelligible transmission of speech between individuals in a society. Differences in the occurrence of particular sounds in languages around the world have been observed by numerous researchers. It has been suggested that these differences are as a result of ecological pressures.
This understanding was developed from the acoustic adaptation hypothesis, which was a theory initially used to understand differences in bird songs across varying habitats. However, the theory has been applied by researchers as a base for understanding why differences are shown in speech sounds within spoken languages around the world.
Maddieson and Coupe’s study on 633 languages worldwide observed that some of the variation in the sonority of speech sounds in languages can be accounted for by differences in climate. The pattern follows that in warmer climatic zones, language is more sonorous compared to languages in cooler climatic zones which favour the use of consonants. To explain these differences they emphasise the influence of atmospheric absorption and turbulence within warmer, ambient air, which may disrupt the integrity of acoustic signals. Therefore, employing more sonorous sounds in a language may reduce the distortion of soundwaves in warmer climates. Fought and Munroe instead argue that these disparities in speech sounds are as a result of differences in the daily activities of individuals in different climates. Proposing that throughout history individuals residing in warmer climates tend to spend more time outdoors (likely engaging in agricultural work or social activities), therefore speech requires effective propagation of sound through the air for acoustic signals to meet the recipient over these long distances. Unlike in cooler climates where people are communicating over shorter distances (spend more time indoors). Another explanation is that languages have adapted to maintain homeostasis. Thermoregulation aims to ensure body temperature remains within a certain range of values, allowing for the proper functioning of cells. Therefore, it has been argued that differences in the regularity of phones in a language are an adaptation which helps to regulate internal bodily temperatures. Employing the use of open vowels like /a/ which is highly sonorous, requires the opening of vocal articulators. This allows for air to flow out of the mouth and with it evaporating water which reduces internal bodily temperatures. In contrast, voiceless plosives like /t/ are more common in cooler climates. Producing this speech sound obstructs airflow out of the mouth due to the constriction of vocal articulators. Thus, reducing the transfer of heat out of the body, which is important for individuals residing in cooler climates.
A positive correlation exists, so that as temperature increases, so does the use of more sonorous speech sounds. However, the presence of dense vegetation coverage leads to the correlation occurring oppositely, so that less sonorous speech sounds are favoured by warmer climates when the area is covered by dense vegetation. This is said to be because in warmer climates with dense vegetation coverage individuals instead communicate over shorter distances, therefore favour speech sounds which are ranked lower in the sonority hierarchy.
Everett, (2013) suggested that in high elevation regions such as in the Andes, languages regularly employ the use of ejective plosives like /kʼ/. Everett argued that in high altitude areas, with reduced ambient air pressure, the use of ejectives allows for ease of articulation when producing speech. Moreover, as no air is flowing out of the vocal folds, water is conserved whilst communicating, thus reducing dehydration in individuals residing in high elevation regions.
A range of other additional factors have also been observed which affect the degree of sonority of a particular language such as precipitation and sexual restrictiveness. Inevitably, the patterns become more complex when considering a range of ecological factors simultaneously. Moreover, large amounts of variation are shown which may be due to patterns of migration.
The existence of these differences in speech sounds in modern day human language is said to be driven by cultural evolution. Language is an important part of culture. In particular, speech sounds in the sonority scale are more likely to be selected for in different environments as a language favours phonetic structures which allow for the successful transmission of messages in the presence of ecological conditions. Henrich highlights the role of dual inheritance, which propels changes in language that persist across generations. It follows that slight differences in language patterns may be selected for because they are advantageous for individuals in the given environment. Biased transmission then occurs which allows for speech pattern to be adopted by members of the society.