In phonetics, vowel reduction is any of various changes in the acoustic quality of vowels as a result of changes in stress, sonority, duration, loudness, articulation, or position in the word (e.g. for the Muscogee language), and which are perceived as "weakening". It most often makes the vowels shorter as well. Vowels which have undergone vowel reduction may be called reduced or weak. In contrast, an unreduced vowel may be described as full or strong. The prototypical reduced vowel in English is schwa. In Australian English, that is the only reduced vowel, though other dialects have additional ones.
Transcription
There are several ways to distinguish full and reduced vowels in transcription. Some English dictionaries indicate full vowels by marking them for secondary stress even when they are not stressed, so that e.g. ⟨ˌɪ⟩ is an unstressed full vowel while ⟨ɪ⟩ is a reduced schwi. Alternatively, the vowel quality may be portrayed as distinct, with reduced vowels centralized, such as full ⟨ʊ⟩ vs. reduced ⟨ʊ̈⟩ or ⟨ɵ⟩. Since the IPA only supplies letters for two reduced vowels, near-open ⟨ɐ⟩ and mid ⟨ə⟩, transcribers of languages such as RP English and Russian that have more than these two vary in their choice between an imprecise use of IPA letters such as ⟨ɨ⟩ and ⟨ɵ⟩, letters with centralized diacritics such as ⟨ɪ̈⟩ and ⟨ʊ̈⟩, or of para-IPA letters such as ⟨ᵻ⟩ and ⟨ᵿ⟩. The French reduced vowel is also rounded, and for a time was written ⟨ᴔ⟩ (turned ⟨œ⟩), but this was not adopted by the IPA and it is now generally written ⟨ə⟩ or occasionally ⟨ø⟩. Some languages also have a near-close compressed vowel /ʏ̈/ (para-IPA ⟨ʏ̵⟩), as opposed to a protruded vowel /ʊ̈/.
Weakening of vowel articulation
Phonetic reduction most often involves a mid-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 /ᵻ/ ranges from close [ï], [ɪ̈], [ë], to open-mid [ɛ̈]. The primary distinction is that /ᵻ/ 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 /ᵿ/ ranges through [ʊ̈] and [ö̜]; 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. Stress-related vowel reduction is a principal factor in the development of Indo-European ablaut, as well as other changes reconstructed by historical linguistics. 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].
Vowel inventory reduction 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).
Specific languages
English
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.
Latin
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