I'm having some practice with American English sounds and I don't understand why some native speakers seem to add a schwa after vowel sounds. For example the word cat is pronounced in this lesson on pronuncian.com(about short vowel sounds) as /kæət/ and tip as /tɪəp/.
Then there's another issue: while describing the different sounds they pronounce the æ sound as /æə/ and so it sounds pretty like a dipthong. Could someone help me make everything clear please?
This is a dialect feature which is becoming more general in American English. You probably won't find it as much in Minnesota and North Dakota, compared to the South, and it's more frequent with younger speakers. It is probably related to general vocalic bulking-up, where "long" vowels are all phonetically diphthong ("seat" [sɪjt], "slake" [slɛjk].
In her pronunciations of citation vowels, "ɪ" etc, the diphthongization is very noticeable. This is because the only monophthong in final position is [ə] and possible [ɑ] for some speakers – it takes a trained phonetician to say [ɪ] and not [ɪə]. I also think that she hyperarticulates a bit when she says e.g. "cat" as an example, which makes diphthongization more pronounced (compared to her pronunciation of words in connected speech).
Consider the difference in tongue position between the /æ/ of "cat" and the following /t/. The /t/ phoneme of English has neutral vowel color -- it's not palatalized, velarized, or rounded. In pronunciation, how do you get from the palatal vowel /æ/ to the neutral /t/, at the end of very same syllable? You have to (1) neutralize the /æ/ to a nonpalatal vowel, (2) palatalize the /t/, or (3) insert a glide reflecting the change in tongue position going from palatal to nonpalatal position. It's just mechanical.
Various languages or dialects of English make different choices among (1)-(3). You're hearing (3). I think other English dialects choose (2).
