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This study examined the distribution of allophonic variants t, I or U, of medial /t/ using a corpus of conversational speech. We utilized a large conversational speech database to generate statistics about the frequency of occurrence of medial flaps. Results confirmed the prevalence of flapping in American English. Low frequency words showed a redistribution of variants from U to t compared to dominant flapping in high frequency words. A similar redistribution of variant production occurred for morphologically complex words (e.g. dirty) compared with morphologically simple words (e.g. water). A second analysis examining vowel length prior to medial /t/ and medial /d/ showed that, while both stops were pronounced as a flap, the vowel preceding medial /d/ tended to the longer. These results were discussed in reference to potential processes in recognizing spoken words.
Patterson et al. (Sun,) studied this question.