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Vowel reduction is the term in phonetics that refers to various changes in the acoustic quality of vowels related to changes in stress, sonority, duration, loudness, articulation, or position in the word (e.g. for Creek language^[1]), which are generally perceived as weakening.

In phonology, vowel reduction refers to a reduction of the number of distinct vowels, rather than their quality, such as over time or between related dialects. In some cases these two concepts may be related. For example, when vowels are phonetically reduced in English, there is also a reduction it the number of vowel contrasts. In other cases, however, phonemic reduction is due to historical vowel mergers (such as the merger of the a vowels in Mary, merry, mary in much of the the United States) and has nothing to do with 'weakening'.

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.

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.

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.

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: clipping (phonetics). 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.

Some languages, such as Finnish, Hindi, and classical Spanish, are claimed to lack vowel reduction.^[2] At the other end of the spectrum, Slovene^[3] has a stressed reduced vowel: /e/ appears as schwa [ə] in some reducing environments (such as /er/ when no other vowel is adjacent), even when the syllable is stressed.

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 in English
• Vowel reduction in Russian

In phonology, however, vowel reduction generally refers to changes in the number of vowels during the evolution of a language, or across genetically related languages, rather than weakening of individual vowels.

An example is provided by Japonic languages. Proto-Japanese had 8 vowels; this has been reduced to 5 in modern Japanese language, but in Yaeyama language the vowel reduction has progressed further, to 3 vowels.

• Clipping (phonetics)
• Elision
• Relaxed pronunciation
• Silent letter
• Schwa
• Unstressed vowel

1. ^ Acoustic vowel reduction in Creek: Effects of distinctive length and position in the word (pdf)
2. ^ R. M. Dauer. "Stress-timing and syllable-timing reanalysed". Journal of Phonetics. 11:51-62 (1983).
3. ^ Comparison between humans and machines on the task of accentuation of Slovene Words

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