[/quote]The simplest contrast between 'can' stressed and 'can't' stressed is supposed to be the final [t] (I know I'm stating the obvious, but that's to get to the point, sorry for doing that). But the problem is that the final [t] becomes something more like a glottal plosive [t] (with some articulation of the [t] on the alveolar ridge). The most basic contrast in a sentence like 'I can do it' vs. 'I can't do it' , though, is that in the former the 'can' becomes unstressed (so, depending on your methodology, that can with a shwa, or [k] followed by a syllabic [n]). 'I c'n do it' vs. 'I can't do it' (with the vowel not being reduced in the latter).[quote="Metamorfose
How do the Americans who distinguish can (stressed) and can't do? Do they change vowels like BBC English?
How do you classify your American accent? Southern/New England...?
José
If I get into more technical detail, in the case of of stressed 'can' vs. 'can't', there is most likely more energy in the articulators in the initial /k/ of 'can't' because of a backward influence of the final /t/, a glottalized plosive [t]. There might be a reduction of the nasal feature as well, which spreads over the whole word/syllable once the initial [k-] transitions to the voicing (and nasalization), while in the case of stressed 'can', the voicing and nasalisation create the effect of a slightly longer internal vowel. The interesting part of this would be the timing and the visible facial signature when saying 'can' with stress vs. 'can't'.
I don't know of any Americans 'can't' with a standard reduced vowel like schwa. So the alternation of the internal /ae/ vowel of 'can' with the schwa helps to keep it rather distinct from 'can't' which, even if somewhat reduced in rapid speech, keeps the /ae/ vowel.
Overall, the visible facial gesture involved with the prepatory articulation and then articulated realization of 'can' (stressed) and 'can't' (unstressed) is actually quite distinct, even if not captured on slow motion video. So overall, if we map the articulatory gesture phonetically speaking onto the syllable (because they are one-syllable words in most accounts of pedagogical phonology) or onto the word, they are quite distinct.
I would say my dialect of US English is North Midlands, but of a type right on the Mason-Dixon line. One word used to differentiate speakers north from south is how to say the word 'greasy'. The internal [s] becomes a [z] if you are South Midlands--at least that is how a sociolinguist explained it to me. Some vocabulary choices are also a giveaway. Since coming to Japan, I've learned to neutralize my accent to something rather like Broadcast American (one of the things I might pursue if I ever re-patriate). OTOH, I worry that I neutralize too far--beyond neutralization--in creating my 'motherese' pronunciation of EFL for Japan. I've actually heard some long-term foreigners here whose accent re-develops to the point where their accent is considered rather odd by native speakers of anglophone countries--not just neutralized and internationalized, but odd.
I want to avoid that.