In recent years there has been a marked tendency among younger speakers of American English for the alveolar flap [D], which is the sound that appears as the contextual variant of the phonemes /t/ and /d/ before unstressed syllables, to be replaced with a full stop [t] and [d]. Thus, the word student, which in standard/traditional American English is pronounced with an alveolar flap preceding the unstressed vowel, is heard in the speech of adolescents and young adults with a fully plosive [d] instead of [D]. Concomitantly, in this speech variant the unstressed vowel in student has a lesser degree of both quantitative and qualitative reduction, meaning that it approximates to its stressed variant [ɛ] as in tent, instead of the normative [ə] or [ɨ] in position after primary stress.
The probable reason for the eclipse of the alveolar flap in this position is not difficult to find. It has to do with the decline of fully reduced unstressed vowels throughout contemporary English pronunciation, a tendency spearheaded by younger speakers, possibly due the influence of the printed/digitized word. There is, more specifically, a symmetry or parallelism between the semiotic value of reduction in the consonant and reduction in the (post-tonic) vowel. The alveolar flap is, after all, a reduced variant of the basic plosive sound, in the sense that flapping is an attenuation of the acoustic and articulatory force that characterizes the unflapped, fully plosive basic variant t or d. Similarly, an unstressed vowel is a reduced contextual variant of the basic vowel. In both cases, therefore, it is a reduction that occurs, illustrating the linguistic principle which dictates that units and contexts (and their variants) are always governed by PATTERNS OF COOCCURRENCE.