Vowel reduction is the historical process of weakening, shortening or disappearance of a vowel sound in an unstressed position. Stressed vowels cannot be reduced. This phenomenon is closely connected with the historical development of the language.
The neutral sound /ə/ represents the reduced form of almost any vowel in the unstressed position.
Example: to combine /tə kəmˈbaɪn/
The vowels and diphthongs /ɔː/, /aʊ/, and /ɔɪ/ are never reduced, and all vowels may occur in unstressed position without reduction, especially in compound words.
Nonetheless, it is true that some vowels, such as /ɪ/ and /ʌ/, reduce quite readily, so that there are not many English words which have them in unstressed positions.
Some English words alternate between having full but unstressed vowels and reduced vowels, depending on context. For example, the is typically /ðiː/ before a vowel-initial word (the apple) but /ðə/ before a consonant-initial word (the pear), though this distinction is being lost in the United States. Similarly with to: to America /tuː/ vs. to Britain /tə/. Most words, however, alternate depending on how much emphasis they are accorded. Some of these are:
- can: /kæn/, but also I can go /ˈaɪ kəŋ ɡoʊ/,
- and: /ænd/, but also you and me /ˌjuː ən ˈmiː/,
- he: /hiː/, but also will he go? /ˈwɪl hɪ goʊ/, and so on with a, at, would, that, has, etc.
There are also a number of English verb-adjective pairs that are distinguished solely by vowel reduction. For example, separate as a verb (as in 'what separates nation from nation') has a full final vowel, /ˈsɛpəreɪt/, whereas the corresponding adjective (as in 'they sleep in separate rooms') has a reduced vowel: /ˈsɛpərət/.
Types of reduction:
1. Quantitative (shortening of a long vowel sound).
2. Qualitative (both long and short vowels are shortened till [ə, i, u].
3. Elision (the omission of vowel).