A useful website for letting students hear the glide required for intelligible diphthong pronunciation is Eric Armstrong’s voice & speech source. A useful tool for letting students see the glide required for diphthongs is the University of Iowa’s “American English Sounds.”
Students may nevertheless get stuck on one or more minimal pairs they cannot seem to make any progress in perceiving. It can be helpful at this point to train them in physically articulating the difference between the two problem sounds – e.g., via Nilsen & Nilsen’s (2010) Pronunciation Contrasts for monophthongs and the University of Iowa’s “American English Sources” tool for diphthongs – so they get a sense of what exactly the difference is that they are listening for (e.g., When I pronounce /æ/, does my mouth look more open in a mirror as well as sound more open to my ears than my pronunciation of /ɛ/?)
Encouraging students to pronounce the problem vowel for as long a time as they need in order to check that they really have produced its distinguishing features can be useful.
It can also be beneficial to encourage students to temporarily exaggerate other articulatory differences between problem phonemes. For example, making tense vowels as tense as physically possible to most clearly distinguish them from lax vowels or opening the mouth as widely as possible for low vowels to most clearly distinguish them from mid vowels, etc., since their necessarily unstable pronunciation of new phonemes will naturally gravitate back toward standard norms over time.
Finally, for long-term change in students’ L2 pronunciation to occur, teachers must follow segmental training giving students conscious awareness of how a particular clear vowel is physically articulated with homework activities aimed at developing students’ new L2 pronunciation habits, as described in the subsection above on “Understanding segmental pronunciation as a habit.” After all, it is unreasonable to expect students to have the mental resources necessary to be able both to consciously apply their new knowledge of how the various clear vowels of English are pronounced while simultaneously needing to engage in the higher-level cognitive processes of:
- Comprehending what others are saying;
- Identifying connections between what others are saying and what they already know;
- Figuring out semantically what they want to say next; and
- Figuring out how to say it (in terms of information structure, politeness, grammar, etc.)