In phonology, voicing (or sonorization) is a sound change where a voiceless consonant becomes voiced due to the influence of its phonological environment; shift in the opposite direction is referred to as devoicing or desonorization. Most commonly, the change is a result of sound assimilation with an adjacent sound of opposite voicing, but it can also occur word-finally or in contact with a specific vowel.
For example, the English suffix -s is pronounced [s] when it follows a voiceless phoneme (cats), and [z] when it follows a voiced phoneme (dogs). This type of assimilation is called progressive, where the second consonant assimilates to the first; regressive assimilation goes in the opposite direction, as can be seen in have to [hæft?].
English no longer has a productive process of voicing stem-final fricatives when forming noun-verb pairs or plural nouns, but there are still examples of voicing from earlier in the history of English:
The voicing alternation found in plural formation is losing ground in the modern language,. Of the alternations listed below many speakers retain only the [f-v] pattern, which is supported by the orthography. This voicing of /f/ is a relic of Old English, at a time when the unvoiced consonants between voiced vowels were 'colored' by an allophonic voicing (lenition) rule /f/ -> [v]. As the language became more analytic and less inflectional, final vowels or syllables stopped being pronounced. For example, modern knives is a one syllable word instead of a two syllable word, with the vowel e not pronounced and no longer part of the word's structure. The voicing alternation between [f] and [v] occurs now as realizations of separate phonemes /f/ and /v/. The alternation pattern is well maintained for the items listed immediately below, but its loss as a productive allophonic rule permits its abandonment for new usages of even well-established terms: while leaf~leaves in reference to 'outgrowth of plant stem' remains vigorous, the Toronto ice hockey team is uncontroversially named the Maple Leafs.
Sonorants (/l r w j/) following aspirated fortis plosives (that is, /p t k/ in the onsets of stressed syllables unless preceded by /s/) are devoiced such as in please, crack, twin, and pewter.
In many languages including Polish and Russian, there is anticipatory assimilation of unvoiced obstruents immediately before voiced obstruents. For example, Russian 'request' is pronounced /'proz?b?/ (instead of */'pros?b?/) and Polish pro?ba 'request' is pronounced /'prba/ (instead of */'prba/). This process can cross word boundaries as well, for example Russian ? /'dod b?/ 'daughter would'. The opposite type of anticipatory assimilation happens to voiced obstruents before unvoiced ones: ?? /?p'sp?t?/.
Final devoicing is a systematic phonological process occurring in languages such as German, Dutch, Polish, and Russian, among others. In these languages, voiced obstruents in the syllable coda or at the end of a word become voiceless.
Initial voicing is a process of historical sound change where voiceless consonants become voiced at the beginning of a word. For example, modern German sagen ['za:?n?], Yiddish ['zn?], and Dutch zeggen ['z] (all "say") all begin with [z], which derives from [s] in an earlier stage of Germanic, as still attested in English say, Swedish säga ['s?j:a], and Icelandic segja ['sei:ja]. Some English dialects were affected by this as well, but it is rare in Modern English. One example is fox (with the original consonant) compared to vixen (with a voiced consonant).