nadiatims Wrote:What do you mean by "pureness of each vowel quality"?
By pureness, I mean how a monophthong doesn't change its quality from its onset to the end. English monophthongs can sound like diphthongs to native Japanese speakers because Japanese vowels are all monophthongs that are completely pure (i.e., they keep the exact same quality throughout utterance) while some English monophthongs tend to make slight gliding sound changes. In fact, some English vowels that are perceived as monophthongs by native English speakers are actually diphthongs in the linguistic sense.
For example, as others said, え and お tend to be realized as diphthongs "eh" and "oh" in open syllables, which are obviously not pure vowels. (If you don't know what open syllables are, they're syllables that end with vowels. Every Japanese mora except ん is an open syllable in a sense.) So these two vowels sound like えい and おう respectively. If you actually want to say えい because, for example, you want to say 映画 (えいが), it should take 2 moras (3 moras for the entire word). But native English speakers tend to cram the two distinct sounds into one mora as you would into one syllable when pronouncing "say," "pay" and so on.
It's quite difficult to explain these things in detail without linguistic jargon, especially when people who do need detailed explanations can't even hear the difference. But the point is that you're supposed to speak Japanese sort of like Spanish.
As for American English vs. other dialects, I don't think it matters much as far as how native Japanese speakers perceive your accent. In fact, your average Japanese person can't even hear the difference between typical British and American accents. They sound pretty much the same to the untrained Japanese ear. As I said in the previous post, vowel qualities just don't matter as long as you use equivalent monophthongs. I can't stress this enough, but they don't contribute much to how your accent sounds. The wrong pitch accent and wrong use of moras are the culprit. Accent doesn't work the English way in Japanese.
Edit: If you didn't know the term "pure vowel" and are a native English speaker, I strongly recommend you learn a little bit of phonetics and phonology first. "Impure" vowels in your Japanese is a dead giveaway that you're not a native speaker while lots of non-English speakers have no problem.
Edit 2: Neither English nor Japanese is a tonal language. But this doesn't mean each syllable is completely monotonous or perfectly flat pitch-wise. "Non-tonal language" simply means that tones are not used to distinguish phonemes. There are tonal pitch shits in English too, though whatever tone you use it doesn't change the fact that "ah" is "ah" and "ee" is "ee." But you're certainly using a particular set of tones in a semi-systematic way; phonological and prosodic context dictates which tone you tend to use. So, if you use the kind of tone you always use in English (and follow the English rules for tones) when speaking Japanese, your Japanese will sound accented.
Edited: 2012-03-21, 1:33 pm