Diphthongize, verb.
Definition (with object): Change (a vowel) into a diphthong.
Examples:
1. The short vowel was not lengthened or diphthongized.
2. But at normal conversational speed its ghastly sequence of four diphthongized long vowels (in the nicely symmetrical but un-IPA Trager-Smith transcription, / ay ey iy ey /) sounds something like ah-ee-yay-ee-yee-yay-ee.
3. Stealth has a long, tense, diphthongized high vowel [i], whereas stealth has a short, lax, mid-vowel.
4. In particular, the vowel of the last syllable was a pretty good IPA [e], not diphthongized like the vowel in English babe but also not laxed and shortened like the vowel in English bed.
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@cambridge_dic
