Intervocalic Voicing in English

[petegray]

> Intervocalic <-t-, -tt-> in US English is not just voiced,

No, quite right.   There are not two, but three differences in English
between /t/ and /d/:
   voiceless  ~   voiced
   aspirated  ~  unaspirated
   tense    ~  lax  (I hope I've got those the right way round!)

Intervocalic /t/ can be voiced and unaspirated, but it remains tense,
whereas intervocalic /d/ is lax.   Thus some speakers report that they can
hear the difference.

NZ-speak also frequently has this intervocalic voiced /t/, so America and
Ireland are not linguistic isolates!

Peter