For instance, as shown in the sonority hierarchy above, vowels are considered [+syllabic], whereas all consonants (including stops, affricates, fricatives, etc.)
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]).
LUNA > /lua/ 'moon', DONARE > /doar/ 'donate', COLORE > /kor/ 'color'), but /r/ remains (CERA > /sera/ 'wax'), but Romanian has transformed the intervocalic non-geminate /l/ into /r/ (SOLEM > /so̯are/ 'sun') and reduced the geminate /ll/ to /l/ (OLLA > /o̯alə/ 'pot').
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").
[7] 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.
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.
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).
[10] Thermoregulation aims to ensure body temperature remains within a certain range of values, allowing for the proper functioning of cells.
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 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.