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In phonetics, vowel reduction is any of various changes in the acoustic quality of vowels, which are related to changes in stress, sonority, duration, loudness, articulation, or position in the word (e.g. for the Creek language), and which are perceived as "weakening". It most often makes the vowels shorter as well.
Such a vowel may be called reduced or weak. In contrast, an unreduced vowel may be described as full or strong.
Phonetic reduction most often involves a centralization of the vowel, that is, a reduction in the amount of movement of the tongue in pronouncing the vowel, as with the characteristic change of many unstressed vowels at the ends of English words to something approaching schwa. A well-researched type of reduction is that of the neutralization of acoustic distinctions in unstressed vowels, which occurs in many languages. The most common reduced vowel is schwa.
Whereas full vowels are distinguished by height, backness, and roundness, according to Bolinger (1986), reduced unstressed vowels are largely unconcerned with height or roundness. English /ə/, for example, may range phonetically from mid [ə] to [ɐ] to open [a]; English /ɨ/ ranges from close [i], [ɪ], [e], to open-mid [ɛ]. The primary distinction is that /ɨ/ is further front than /ə/, contrasted in the numerous English words ending in unstressed -ia. That is, the jaw, which to a large extent controls vowel height, tends to be relaxed when pronouncing reduced vowels. Similarly, English /ɵ/ ranges through [ʊ] and [o]; although it may be labialized to varying degrees, the lips are relaxed in comparison to /uː/, /oʊ/, or /ɔː/. The primary distinction in words like folio is again one of backness. However, the backness distinction is not as great as that of full vowels; reduced vowels are also centralized, and are sometimes referred to by that term. They may also be called obscure, as there is no one-to-one correspondence between full and reduced vowels.
Sound duration is a common factor in reduction: In fast speech, vowels are reduced due to physical limitations of the articulatory organs, e.g., the tongue cannot move to a prototypical position fast or completely enough to produce a full-quality vowel (compare with clipping). Different languages have different types of vowel reduction, and this is one of the difficulties in language acquisition; see, e.g., "Non-native pronunciations of English" and "Anglophone pronunciation of foreign languages". Vowel reduction of second language speakers is a separate study.
There are several ways to distinguish full and reduced vowels in transcription. Some English dictionaries mark full vowels for secondary stress, so that e.g. ⟨ˌɪ⟩ is a full unstressed /ɪ/ while ⟨ɪ⟩ is a reduced unstressed schwi.[a] The vowel quality may be considered distinct, with reduced vowels centralized, e.g. with full ⟨ʊ⟩ vs reduced ⟨ɵ⟩. Some transcriptions of English and Russian use non-IPA letters for reduced vowels, such as ⟨ᵻ⟩ and ⟨ᵿ⟩ in English.
Such vowel reduction is one of the sources of distinction between a spoken language and its written counterpart. Vernacular and formal speech often have different levels of vowel reduction, and so the term "vowel reduction" is also applied to differences in a language variety with respect to, e.g., the language standard.
Some languages, such as Finnish, Hindi, and classical Spanish, are claimed to lack vowel reduction. Such languages are often called syllable-timed languages. At the other end of the spectrum, Mexican Spanish is characterized by the reduction or loss of the unstressed vowels, mainly when they are in contact with the sound /s/. It can be the case that the words pesos, pesas, and peces are pronounced the same: [ˈpesə̥s].
In some cases phonetic vowel reduction may contribute to phonemic (phonological) reduction, which means merger of phonemes, induced by indistinguishable pronunciation. This sense of vowel reduction may occur by means other than vowel centralisation, however. Many Germanic languages, in their early stages, reduced the number of vowels that could occur in unstressed syllables, without (or before) clearly showing centralisation. Proto-Germanic and its early descendant Gothic still allowed more or less the full complement of vowels and diphthongs to appear in unstressed syllables, except notably short /e/, which merged with /i/. In early Old High German and Old Saxon, this had been reduced to five vowels (i, e, a, o, u, some with length distinction), later reduced further to just three short vowels (i/e, a, o/u). In Old Norse, likewise, only three vowels were written in unstressed syllables: a, i and u (their exact phonetic quality is unknown). Old English, meanwhile, distinguished only e, a, and u (again the exact phonetic quality is unknown). Catalan, a Romance language, also shows reduction, but in differing degrees depending on dialect. The Valencian dialect reduces the number of possible vowels from seven to five in unstressed environments, merging [ɛ] into [e] and [ɔ] into [o]. The central Catalan dialect goes even further, distinguishing only [i], [u] and [ə ~ ɐ], with [ɛ], [e] and [a] becoming [ə ~ ɐ] and [ɔ] and [o] merging into [u]. In Italian, the vowels noted e and o in spelling are pronounced [e] and [o] in unstressed syllables.
Stress is a prominent feature of the English language, both at the level of the word (lexical stress) and at the level of the phrase or sentence (prosodic stress). Absence of stress on a syllable, or on a word in some cases, is frequently associated in English with vowel reduction – many such syllables are pronounced with a centralized vowel (schwa) or with certain other vowels that are described as being "reduced" (or sometimes with a syllabic consonant as the syllable nucleus rather than a vowel). Various phonological analyses exist for these phenomena.
Old Latin had initial stress, and short vowels in non-initial syllables were frequently reduced. Long vowels were usually not reduced.
Vowels reduced in different ways depending on the phonological environment. For instance, in most cases they reduced to /i/. Before l pinguis, an /l/ not followed by /i iː l/, they became Old Latin /o/ and Classical Latin /u/. Before /r/ and some consonant clusters, they became /e/.
In the Bulgarian language the vowels а, о and е can be reduced when unstressed to ъ, у and и, respectively. The most prevalent is а > ъ, while о > у is considered incorrect in literary speech. The last reduction е > и is prevalent in the more Eastern dialects of the language.
There are five vowel phonemes in Standard Russian. Vowels tend to merge when they are unstressed. The vowels /a/ and /o/ have the same unstressed allophones for a number of dialects and reduce to a schwa. Unstressed /e/ may become more central if it does not merge with /i/.
Other types of reduction are phonetic, such as that of high vowels (/i/ and /u/), which become near-close so этап ('stage') is pronounced [ɪˈtap], and мужчина ('man') is pronounced [mʊˈɕːinə].
The Proto-Slavic language possessed two short high vowels known as yers: a short high front vowel denoted as ĭ or ь, and a short back vowel denoted as ŭ or ъ. These vowels underwent reduction and were eventually deleted in certain positions in a word in the early Slavic languages, beginning from the late dialects of Proto-Slavic. This process is known as Havlík's law.