Strong vowels are all the monophthongs, except the /ə/ (schwa) and the short /i/ and /u/. So, here is the list of strong English monophthongs: / iː ɪ ʊ uː e ɜː ɔː ɒ æ ʌ ɑː /
We already know that strong vowels tend to appear in stressed syllables most of the time, but it’s also important to remember that they can be found in unstressed syllables too. There are plenty of examples: massage /ˈmæsɑːʒ/, ambassador /æmˈbæsədə/, colleague /ˈkɒliːɡ/, torment /ˈtɔːment/, alternative /ɔːlˈtɜːnətɪv/. All the phonemes in red are strong vowels in unstressed syllables.
Listen to the word ambassadors in this recording and notice how the first a (unstressed) has the same strong vowel quality as the second a (stressed): /æmˈbæsədəz/:
This actually seems to contradict a conclusion we reached in a previous article, that whenever you have an unstressed syllable it’s likely to contain a weak vowel. But remember that we are just talking about tendencies. So, that conclusion is still valid: in un unstressed syllable you’ll normally have a weak vowel, but in a few cases you can find a strong one as well.
This doesn’t only affect monophthongs, but also diphthongs: ideal /aɪˈdiːəl/, regulate /ˈreɡjuleɪt/, blackout /ˈblækaʊt/ obese /əʊˈbiːs/
And, within a clause, we have many examples of diphthongs occupying unstressed positions.
We’re very proud of our daughter. ǀ wɪə ˈveri ˈpraʊd əv ɑː ˈdɔːtə ǀ (as you will probably have noticed, in this last example my pronunciation of the possessive our is not /aʊə/ but /ɑː/ because of smoothing).