The model word Tench uses for these vowel symbols is <lack>. He writes:
Lack is transcribed either as l æ k or l a kI downloaded one of the sound files from the web pages of CUP to listen to the "Southern England Standard pronunciation" of the word <lack>. My impression is that the vowel the speaker uses is best represented by the ash symbol. Listen for yourself, please:
You choose! The first one is traditional and is also handy to represent American accents, the second one represents most modern British accents, especially of the younger generation. By having both symbols available, you can begin to see how we can exploit them for transcribing different acccents. Get used to using one of them. (13)
I must say I'm not particularly happy with Tench's suggestion to choose either symbol. This is too confusing for beginners. What's more: "Get used to using one of them" - what does this request entail? Toss a coin and whatever the result will be, use that symbol for the rest of your transcription career? And: If learners are granted this freedom in the case of "æ/a", why not with "e/ɛ" as well? Tench writes:
So, for comparative purposes, when, for instance, comparing the vowels of English and another language, or the vowels of two different accents of English, we need to keep the ordinary Roman letter <e>as the IPA symbol for the /e/ sound, and rely on the Greek letter epsilon, <ε>, as the IPA symbol for the /ε/ sound. (10)The same arguments could be put forward to keep the symbols <æ> and <a> apart.
(There is more to come on this book in a future blog entry)