>>Looks like a moderately diphthonged vowel: [eig].<<
You had to analyse it visually to detect these minutiae though. As short and sharp as it is, the listener (myself included) wouldn't be able to tell it was diphthongised, audially. It would be interpreted the same as [Eig] if [Eig] underwent the same relative diphthongisation. Otherwise, logically, we would hear "ayg" and not "egg"; would you agree?
* [Eig] might not be a good representation. So instead, let's say the same _relative_ diphthongisation starting from the [E] vowel position.
Anyway, I want to conclude that [ei] or [e] are allophones of [E], at least in light of the above recording of "egg".
Listeners could easily detect the diphthong.
It's not that these features cannot be detected, it's that each of us learns to ignore phonetic features that are not phonemic, while carefully picking out the features that are phonemic. If we have not learned the difference between a diphthong and a monophthong in this position to be phonemic, we normally won't hear it, even though are ears and brain still register it.
''Of course there are different accents out there but if you want some back up''.
I don't need any back up to tell me that there are different accents out there. I already know that. I know that there are different accents out there, but you think that other accents are just phantasies.
I'm not some person that thinks my accent is all there is.
As for ''egg''. I do detect a sound similar to a diphthong in it, in my accent. I'm not sure if I'd actually say [eig]. It's close to [eig] but not exactly.
>>Listeners could easily detect the diphthong.<<
As I've said before, this is negligible because relative to the other major diphthongs it is slight; [E] in "there" as you say is pure, not absolutely, but only relatively so.
It is a minor side effect, perhaps because of the terminal consonant 'g' (think of "dark L"), and other speakers would minimise the amount of diphthongisation anyway. As Paul said, it isn't exactly [eig] either. And [Eg] from this speaker would be interpreted the same way.
As Paul brought up before, a similar thing happens with /a:/ where vowel position is not fixed. In "art", it can be very closed or open like in a posh RP pronunciation; these differences are negligible.
With my ears I can't detect any diphthongisation in the "egg" but Mxsmanic's hypothesis that we've learnt to ignore such phonetic features sounds plausible enough.
It is quite normal, though, for monophthongs to have on-glides and off-glides. The Aussie and Kiwi /i:/ have a very significant on-glide: it's almost a diphthong and you don't need any spectrogram to detect it.
The link that Mick gave us a few weeks ago goes into more detail about this (in the context of /hVd/ in a Sydney accent). According to Cox the stressed short vowels tended to have significant off-glides in the pronunciation of her subjects.
http://www.ling.mq.edu.au/home/cox_felicity/vowel%20description22.pdf
Would you say, Mxsmanic, that your spectrogram is consistant with those findings? Could this slight diphthongisation be an off-glide? I certainly detect no first and second target with a transition in between as you find in true diphthongs.
Whether it be [e], [ei] or [e] plus an off-glide towards [i] it is significant that the "e" in this "egg" is not [E]. However what we don't hear is anything like "aig". It's obvious that this phone (whatever it may be exactly) is an allophone of /e/ (the vowel in "egg") and not an allophone of /ei/ (the vowel in "age").
Mxsmanic is insisting that [e] is an allophone of /ei/. This I don't deny. [e] is an allophone of /ei/ in certain accents (and not only Californian). However, in other accents it is not. In AusE and NZE [e] is an allophone of /e/. To us [e] only sounds like an allophone of /ei/ in case we recognise the speaker to have a non-Aussie/Kiwi accent.
If Mxsmanic's spectrogram had shown the /e/ in "egg" to be an [e], then this would be good evidence supporting the claim. We don't hear /ei/ inspite of the phone's being [e], an allophone (according to Mxsmanic) of /ei/. That the spectrogram actually showed a slight diphthongisation towards [ei] could only strengthen the case.
I admit, however, that context might also be playing its part. There is not such word as "aig", one must admit. One must also acknowledge that "egg on his face" is a well worn metaphore. Indeed if a speaker were to say "aig on his fess", I must confess, one could easily still hear that long dead metaphore.
A strange thing though, I only detect that difference in ''egg''. In ''beg'', I don't here it at all.
* Edit: Both /a:/ and /^/ re: vowel positions.
Now,
I think what Jim is saying is true. The "egg" is not a [eig] diphthong but an "offglide", "onglide" or something or other (This isn't my domain so I don't know the terms). The exact same thing happens with the "ee" vowel in "meet": it's not a diphthong at all.
I'm not sufficiently interested in Australian accents to further investigate the matter. I prefer to limit myself to GAE and RP, one of which I speak, and both of which I consider useful standards for ESL learners. Other accents are curiosities that I usually manage to understand, but I don't see any utility in studying or emulating them, or in explaining them to my students.
There are several Australian teachers at my school. If a student were to explicitly request instruction in Australian pronunciation, I'm sure they could handle it. As far as I know, nobody has ever asked for such instruction, though. Indeed, students rarely express a preference for any specific pronunciation at all. A few Asian students want GAE, but that's all I can recall offhand. Some students have requested listening practice for accents such as that of Glasgow, because of a professional requirement to deal with people from that city, but they've never asked to learn to pronounce that way themselves.
GAE is popular in North or South America,
but British accents seem more common in the rest of the world.
More Native GAE speakers, perhaps.