A common phonological rule of North American English is to change /t,d/ to a “flap” transcribed as either quasi Americanist [D] or IPA [ɾ] (indicating that this sound is a type of <r>).
Note: I tend to use [D] for the North American English flap since Americans think this sound is either /t,d/ and rarely confuse it with /r/ …unless they think the word is Spanish. Similarly the North American flap sounds an awful like like a “d” to other non-American English speakers (like the editors of the Oxford English Dictionary who feel that Americans pronounce atom with a “d”).
BUT, If you’re an IPA purist just close your eyes and change [D] to [ɾ].
Is that [D] a /t/ or a /d/?
In phonology class, I present cases where a flapped /t,d/ could cause potential ambiguities (see below).
- a[D]om bomb, A[D]am’s rib
- Swe[D]ish ‘from Sweden’, swee[D]ish ‘slightly sweet’
- ri[D]er, wri[D]er
- Toy Yo[D]a, Toyo[D]a as seen in this prize lawsuit
- Penn State Policy AD 20 (“an AD-20 rule”) vs. the “80-20 rule”.
As it happens, in many North American dialects (including mine), the vowel before original (underlying) /d/ is slightly longer phonetically than original /t/. Thus the vowel of “toy Y[o:D]a” is slightly, but perceptibly longer than “Toy[oD]a” in many dialects of American English. That’s why there isn’t more confusion in conversational North American English.
FYI – When the Oxford English Dictionary started transcribing the U.S. pronunciation of atom with a /d/, a lot of U.S. English speaking linguists objected that it was still a /t/. The lengthening rule again partly explains this U.S. native speaker intuition. Not that linguists always pay as much attention to native speaker intuition as much as they should.
A Rare Confusions with <r>
Technically speaking, the North American flap IS a type of <r>, specifically a rhotic tap. It’s the Spanish single “r” as in pero /peɾo/. As I mentioned before, few English speakers will perceive the North American as a rhotic. This is probably because English <r> is not a tap, but an very marked alveolar approximant /ɹ/.
But if sometimes the confusion can happen if Spanish or Spanish like phonology is introduced into the word. For example, when I watched the Roswell sci-fi show (set in Roswell, NM), there was an alien character apparently known from local Native American tribal memory. For years, I thought it was a Spanish style name Nasero /naseɾo/, but actually it was Nasedo [naseɾo]. On the flip side, I had a friend with a Mexican American sister in law named Yerida /jeɾida/, but my Anglo friend thought the name was Yedida [jeɾiɾa].
I would be curious to see if there is research comparing the acoustics and perception of the North American flap vs. other languages.