What is the temporal, or sequential, structure of Reference American English vowels and vowel sequences? On the surface level, RA English has various kinds of temporally complex, or dynamic, vowels. It has long vowels (as in eat [iyt]) and short vowels (as in it [It]). It has seven kinds of glides: on-glides (wad [wɑd]) and off-glides (Dow [dɑo]); up-glides and in-glides (both exemplified in idea [aydiyə]), back-glides and front-glides, and even r-glides, which go out into another dimension entirely. It has an indefinite number of ``phthongs'': monophthongs (it), diphthongs (out [aUt]), triphthongs (our [aUɚ]), and n-phthongs (wire [wayɚ], wiry [wayɚi], wirier [wayɚiɚ], you are wirier [yuaɚwayɚiɚ]...). In this section we reduce all these dynamic vowels or vowel-sequences to a simple underlying phonological structure.
We may begin this task by first considering a basic phonological unit: the syllable. Minimal pairs like layer:lair, mower:more, see-er:sear3.7may be phonologically distinguished as two syllables versus one.3.8 This fundamental category allows us to divide many classes of n-phthongs into sequences of syllables, each one of which contains vocalic segments from a much smaller set of n-phthongs, where n10#103. If we exclude the supra-syllabic phenomena of stress, rhythm, etc., then the temporal structure of an n-phthong simplifies to the bare sequence of syllables which makes it up, and our question can be focused more sharply: ``What is the temporal structure of RA English vowels within the syllable?'' As in this first step, so later: temporal phonological structure is simply the unidirectional sequencing of abstract units.
This simplified view of time does not apply to phonetics. In phonetics, time is continuous rather than discrete, as it is in phonology -- this is discussed at length in Appendix 1. Thus the temporal patterning of phonetic events is on a continuous scale. Temporal patterning is of two kinds: duration and ``complexity''. Duration concerns how long things stay the same across time, and complexity concerns the patterns of change across time.
Duration has an absolute limit of zero; the shortest possible class of vowels are those that reach or approach the absolute limit of zero duration, or complete deletion. In fact, there is a significant proportion of vowels that are realized by so little phonetic substance that there is nothing to be measured (for example, 7.6% of 4821 vowels for a Chicago speaker). There are also a number of vowels whose acoustic manifestation is very, very short -- as little as a single pitch pulse. There is in fact a continuum of duration reduction for vowels, governed by several factors including stress, position in the utterance, vowel height, and adjacent phonological environments (for recent studies, see Crystal & House 1990; van Santen & Olive 1990). Absence of stress correlates with the shortening of vowels. Utterance-initial syllables may be nearly or entirely deleted.3.9 Pre-boundary syllables are lengthened according to the strength of the boundary, so that even unstressed vowels may be quite long before pause. The more open the mouth, the longer the vowel. Between voiceless consonants, vowels often devoice: no acoustic vowel (that is, a voiced vocalic acoustic segment) may remain. Other effects on duration or length than these may also exist. These factors are evident in results, presented in Appendix 3, derived from acoustic vowel duration measurements done for each of the speakers examined in this thesis.
As for complexity, it is clear that such short vowels have little time to be complex: measurable gliding can hardly occur within a single pitch pulse. When the short vowels discussed below occur in lengthening environments in some U.S. dialects (stereotypically in Southern dialects), such as in stressed monosyllables, they develop phonetic inglides, so that they are phonetically more complex than the same vowel classes in non-lengthening environments.