Tampuan phonology

Similar to many Mon-Khmer languages, Tampuan employs clear (modal) vowels and lax (breathy) vowels. However the existence of relatively few minimal pairs in which difference in register or phonation is the sole difference in two words led Huffman to categorize Tampuan as a “transitional language” rather than a register language. Crowley, on the other hand, cites extensive diphthongization, especially in the Eastern Dialect, as a sign that Tampuan has crossed the threshold into the category of a register language and is possibly in the process of evolving to Huffman's final phase, namely, a “restructured” language exemplified by modern Khmer.

Consonants
The 28 consonant phonemes of the Tampuan language are laid out in the table below as reported by Crowley. All may occur as initial consonants while only the phonemes in the colored cells may occur as a syllable coda. *[s] sometimes occurs as an allophonic variant of syllable-initial [ç]

Vowels
The vowels of Tampuan show a two-way register contrast between lax and modal voicings as well as length (duration) contrast. As with other Bahnaric languages, tense vowels occur significantly more often than lax vowels. Seventy-five percent of dictionary words use tense vowels. As can be seen in the chart below, the vowels are unevenly distributed. For example, the short lax close vowels have no tense equivalents. Also, there are more close lax vowels than open. Crowley notes that the tense vowels show a trend toward diphthongization in the close range while in the open vowels, it is the lax sounds that are diphthongs, a pattern well documented in historical stages of Khmer and Brao that indicates the language is possibly in an evolutionary stage of restructuring away from a register language.

In addition to the vowels above, the tense diphthong can be found in Lao borrowings and personal names. The lax vowels and  have the allophones  and, especially in the southern Western dialect. Short also has a slightly diphthongized allophone. The tense vowel varies to  or, depending on dialect.

Syllable structure
Tampuan words can either be monosyllabic or exhibit the typical Mon-Khmer “sesquisyllabic” pattern of a main syllable preceded by an unstressed “pre-syllable”. The maximal word is represented by C(R)v(N)-C(C)V(C) where “C” is a consonant, “R” is, “v” is an unstressed vowel, “N” is a nasal, or , and “V” can be any of the vowel nuclei listed above. The pre-syllable and the components in parentheses are optional (not necessary for proper word formation) and the final “C” is limited to the phonemes noted above. In many words the pre-syllable, being unstressed, is further reduced to a syllabic nasal or, in Crowley's terms, a “nasal presyllable” represented as a glottal stop followed by a nasal as in “pestle” or  “stone”.