Why does /t/ after a consonant (ex: /ˈnaɪn.ti/) produce less air than /t/ at the beginning of word (ex: /ˈtaɪlænd/) or after a vowel (ex: /ˈraɪ.tər/)?

Solution 1:

The distinction between these two sounds, both generally represented by the letter "t" in English, is actually also made for "p" and "k." Each of these sounds is transcribed /t/ /p/ /k/ in "broad" phonetic transcriptions, but in a "narrow," more detailed transcription, there are two main types that are recognized by phoneticians.

We call one type "aspirated," and in transcriptions that distinguish the two, it's represented by the letter with a superscript "h" after it: [tʰ] [pʰ] [kʰ]. (The sound /tʰ/ exists in Vietnamese, where it is written "th"; the other two sounds do not.)

The second type is called "unaspirated": it's represented by plain [t] [p] [k]. These should correspond approximately to the Vietnamese sounds written with the letters "t," "p," and "k."

When to use one or the other is explained as follows on John Wells's phonetics blog:

English [p t k] are aspirated

• when they occur at the beginning of a syllable in which the vowel is strong.

They are unaspirated

• when preceded by s at the beginning of a syllable
• when followed by any FRICATIVE, as in lapse læps, depth depθ
• if immediately followed by another plosive as with the k in doctor ˈdɒktə || ˈdɑːktər. The release stage of the first plosive is then usually inaudible (‘masked’).

I'd add to this that it's safe to aspirate any "t," "p," or "k" at the very start of a word, no matter what sound comes after it.

The above rules apply to pretty much all varieties of English. As you can see, using these rules requires a fair amount of knowledge of the sound patterns of English: you'll need to know how a particular word is divided in syllables.

When /t/ comes after a vowel or after "r" and before a "weak" vowel like [ə] (writer) or [ɪ] (writing), or before a completely unstressed [oʊ] (grotto) or [i] (jetty), there may be differences in American and British pronunciation.

Many American varieties of English have a third pronunciation of "t" which is written in the International Phonetic Alphabet as [ɾ]. A "d" in this position is usually pronounced the same way, so this is less commonly transcribed as [d] by some people.

The pronunciation of "t" as [ɾ] usually does not occur before a strong vowel sound (any other vowel than [ə] or [ɪ]), as described in the following question: Why do photons and protons exhibit such anomalous behavior?