I As for final position, our impression of the literature on English phonemics is that three observable phonetic features, voicing, release and vocalic length, are attested for the contrast between /b d g/ and /p t k/. The occlusion of /b d gf, it is said, may be partially voiced or not voiced at all. If the final stops are audibly released, as they sometimes are,/p t k/ will have a stronger, more aspirated release. The single differentiating feature that all descriptions agree is regularly present is the greater length of vowels followed by /b d g/, but this is usually treated as a matter of vowel allophonics. (See, for example, the references cited in footnote 22.)

Recent work has led us to the conclusion that the English stop categories /bdg/ and /ptk/ are distinguished by the timing of changes in glottal aperture relative to supra-glottal ariculation. In word-initial position, the environment of current interest to us, this is manifested acoustically by voice onset time, that is, the time interval between the burst that marks release of the stop closure and the onset of quasi-periodicity which reflects laryngeal vibration. For citation forms of words this measure of voice onset time completely separates the two phonemic categories. In running speech, however, the separation is less sharp ; there is some overlap along the dimension of voice onset time. We have examined running speech in some detail to discover the extent to which certain contextual features are responsible for this overlap. It is clear that the presence of a voiceless stop in a stressed syllable makes for a greater lag in the onset of voicing. In unstressed syllables, an environment of high contextual redundancy as well as low functional yield for the phonemic contrast, there is considerable reduction of the distinction along the dimension. A further increase in voicing lag is noted in syllables bearing the final sentence stress. In addition, the farther such a syllable is from the end of the sentence, the less the effect is likely to be. The importance of voice onset time continues to be apparent, even in running speech, although other effects of context remain to be explored.

A series of thirteen two-formant vowels was synthesized and used as the basis of labelling and discrimination tests with a group of English-speaking listeners. The sounds varied only in F1/F2 plot and the resulting vowel qualities were such that listeners found no difficulty in assigning each sound to one of three phonemic categories, those of the vowels in bid, bed and bad. The results of the tests were compared with those previously obtained in experiments involving the consonant phonemes /b, d, g/. It appears from the data that the phoneme boundaries in the case of the three vowel phonemes are less sharply defined than in the case of the stop consonants. The labelling functions for the vowels show a gradual slope and the discrimination functions do not show any marked increase in sensitivity to change in the region of the phoneme boundaries. It is clear also that the listeners were able to discriminate differences very much smaller than would need to be distinguished simply in order to place vowels in the appropriate category. The results show further that the effect of sequence or acoustic context in the perception of vowels is very considerable. In all the aspects examined in these experiments, the perception of synthetic vowels is found to be different from that of synthetic stop consonants. These differences lend some support to the hypothesis that the degree of articulatory discontinuity between sounds may be correlated with the sharpness of the phonemic boundaries that separate them.

Just over fifty years ago, Lisker and Abramson proposed a straightforward measure of acoustic differences among stop consonants of different voicing categories, voice onset time (VOT). Since that time, hundreds of studies have used this method. Here, we review the original definition of VOT, propose some extensions to the definition, and discuss some problematic cases. We propose a set of terms for the most important aspects of VOT and a set of Praat labels that could provide some consistency for future cross-study analyses. Although additions of other aspects of realization of voicing distinctions (F0, amplitude, duration of voicelessness) could be considered, they are rejected as adding too much complexity for what has turned out to be one of the most frequently used metrics in phonetics and phonology.