GA /ɛ/ is a half-open, front, unrounded vowel. The jaw is slightly lower than for the first vowel in the diphthong /eɪ/. The tip of the tongue may touch the back of the front bottom teeth. Relax your lips. Recently, this vowel has tended to become more open and central.
/æ/ is an open, front, unrounded vowel. Arch the blade of your tongue forward and let the tip rest against the base of the front bottom teeth. Draw your lips toward the corners of your mouth (as you do when you smile), with the muscles slightly tensed.
A noticeable change in the pronunciation of GA /æ/ has been taking place in the urban speech of the Eastern States. It involves the tensing of lax /æ/. This means that the vowel is becoming longer, and tends to diphthongize. The first element is very front, while the second element is central. So instead of a centralized [æ] we get [æə]. At the same time, there is a tendency to make the vowel more close, as a result of which we get pronunciations like [ɛə] or even [eə]. Because tensing and raising of the vowel usually goes hand in hand in cities like Philadelphia, Buffalo, New York City, Syracuse, Rochester, etc., we may use the term /æ/-TENSING to refer to both phenomena.
The process is most likely to apply before voiceless fricatives and /m, n/, provided these consonants close the syllable concerned. For other speakers, also voiced plosives may trigger the rule. Notice that in these contexts, it remains distinct from GA /ɛ/, which may be more open than some realizations of /æ/, but would still be more central and lack the diphthongization (in these dialects).
ban Ben gas guess
can’t Kent flash flesh
bad bed tan ten
laugh deaf ham hem
path Beth man men
/æ/-TENSING is least common before voiceless stops: rap, fat, sack [ræp, fæt, sæk].