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Examination o f the characteristics o f stop voicing has been conducted from three different fields: physiology, acoustics and auditory science, and there have been language specific studies and cross-language studies.

Physiological investigations are concerned mainly with analysing the behaviour o f the larynx during the production o f stops in an attem pt to find

properties of stops in individual languages and the implications of the findings to universal phonetic and phonological properties of stops. These studies make use of techniques such as eletromyography (EMG) and fiberoptic observations. Hutters (1985) investigated vocal fold behaviour in aspirated and unaspirated stops in Danish using EMG and photo-electric glottography techniques. She concluded that these type of stops are distinguished by different vocal fold adjustments: with aspirated stops executed with the greater glottal width at the time of release in relation to their unaspirated cognates in which the glottal width is spindle shaped; and that variation in the timing dimension is a mere consequence of these laryngeal gestures. Dixit (1987) also used photo-electric glottographic techniques to examine the various mechanisms of vocal fold configurations in Hindi; in particular the aspect of timing of laryngeal gestures at the moment o f the burst in relation to events in the mouth, and the degree of glottal opening in the four types of Hindi stops: the voiced aspirated and unaspirated stops and the voiceless aspirated and unaspirated stops. He also conducted a comparative study with other similarly labelled stops in other languages, and concluded that laryngeal mechanisms used to generate the four types of stops in Hindi could also be relevant for the similarly labelled stops in other languages.

Yadav (1984) conducted a fiberoptic observation of Maithili stops. His results are somewhat similar to those of Hutters in that Maithili stops are also differentiated by various vocal fold adjustments, with VOT as a repercussion of this. Voiced stops in Maithili are described in terms of the abduction/adduction of the vocal folds and aspirated stops in terms of the amount of glottal adduction: with aspirated stops manifesting the greatest glottal opening at the moment of or immediately

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aspirated and glottalized. This study also concluded that these stops are differentiated by the degree of aspiration at the moment of release. Aspirated stops have the greatest degree of aspiration, followed by lax stops and lastly the so-called ‘tense’ (least aspirated) stops.^ But like Hutters, Kim a ^ e s that the crucial factor in distinguishing between the Korean stops is the glottal width at the time of release, with VOT being only a concomitant of this. Thus, the wider the glottal width, the heavier the aspiration which in turn means longer VOT, and vice versa.

® Authors have oflen reported an overlap in VOT values between lax and tense stops or between aspirated and lax stops in Korean. See, for instance, Kim (1965) and Han & Weitzman (1970).

Abberton (1972) conducted a non-invasive laryngographic study o f Korean stops and observed variation in vocal fold activity in the production o f these stops for tw o native speakers o f the Seoul dialect. The Lx signal showed gradual build-up o f the amplitude o f Lx traces (vocal fold contact waveform) for the slightly aspirated stops, ‘the so-called tense stops’ (for the male speaker), which is characteristically absent in the other stop types. There is inter-speaker variation though, because the female speaker differentiates between the stop types in term s o f VOT only.

Another laryngographic study is that o f Lindsey et at. (1992) (see also Haruna 1990) on Hausa, a Chadic (Affo-asiatic) language which contrasts ejectives with plain consonants. The main objectives o f this study were to investigate, amongst other things, w hether there was an Lx waveform type or types associated with laryngealized consonants in the language. The study also investigates waveform characteristics o f ejective consonants vis-à-vis plain consonants. At consonant onset, the Lx waveform for the intervocalic ejective affricate [ ts ’j in the word [ts a a ts ’aa] ‘rust’, was the type which the authors ‘associate with glottal closure, indicating that the glottal stricture lasts the full duration o f the con so n an t...’ (Lindsey, Hayward & Haruna 1992:519). This is shown in figure 3.1 (adapted from Lindsey, Hayward & H aruna 1992:519).

oo -100 «sa: T j f f f r V r

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^ Figure 3.1: The Lx shape forjthe intervocalic ejective ,^ ^ a te [ts’} in the ^

Tftsaats’aa] ‘rust^ (ad^ted fi'om Lindsey, HayWfd}arid Haruna (1 9 9 2:5^ . ' % e

number 1 points at the start of the consonaS chi the LÇ^veform, whicdi Mdic^t^

" c^aed vocal folds. The nunAer 2 shows decrease inJ>pen guckient ^ consonant onset f ^m^doamet of the suj^ouaA ièsæl ^

At vowel onset, the ejective affricate [ts’] also showed ‘markedly greater low frequency perturbation (slow up and dow n deflection) in the Lx w av efo rm ...’

(Ibid. 1992:520). By contrast in the plain intervocalic fricative [s] in the word [taasaa] ‘metal bowl’ the open quotient at the start o f the consonant increases, ‘anticipating abduction rather than adduction of the vocal folds, and showed faster up and down deflections on the Lx traces’ (Ibid. 519 - 520). Ejectives thus generally show decreased open quotient than plain consonants. Other characteristics of ejectives at vowel onset observed in Hausa included irregular mode of vocal fold vibration with period-by-period fluctuation in amplitude, and this was observed for the ejective stop [k’j in the word [shaak’aa] ‘smell, sniff* (Haruna 1990:505).

Acoustic examination of stops incorporates the major contribution of Lisker and Abramson (1964). This work examined how effective the aspect of timing is in distinguishing stops with different voicing configurations in eleven different languages. VOT is shown to be effective in distinguishing the voicing contrasts of stops in these languages and it manages to classify languages into different groups depending on the number of stops established per language: two- way contrasting languages, three-way contrasting languages and four-way contrasting languages (although the dimension of timing does not satisfactorily distinguish the stop types in this category of languages). Stops are effectively divided into those with voicing lead, those with short voicing lag (sometimes the release of the burst is simultaneous with the onset of glottal pulsing) and those with long voicing lag. These are roughly associated with the phonological categories of ‘voiced stops’, ‘voiceless unaspirated stops’ and ‘voiceless aspirated stops’.^ Other similar studies have since supported Lisker and Abramson’s findings (e.g. Flege 1979 and Keating et al. 1983). Stops exhibiting breathy voice, and the so-called ‘tense’ stops in Korean are some of the few that VOT does not satisfactorily distinguish.

Haag (1979)* investigated the relationship between VOT and place of articulation in German stops, and came to the conclusion that a correlation exists between the magnitude of VOT and place of articulation. Generally, VOT tended to increase as the place of articulation moved to the back o f the mouth. Other works include those of Keating et al. (1983) who examined allophonic variation for voiced and voiceless stops in fifly-one languages. They observed that the languages under investigation tend to aspirate the /p,t,k/ series before stressed

^ The ‘voiced aspirated stops’ also fall within this category, but VOT does not effectively distinguish these stc^s from the voiceless aspirated stops and from the voiced stops.

^ I was dependent on the literature for the informaticai in this article since it was writtm in German.

vowels, maintain voicing for /b,d,g/ in intervocalic environments and have some type o f voiceless unaspirated stop in all contexts.

With respect to auditory observation, an example could be that of Williams (1977) who conducted a perceptual study on initial stops in some Spanish dialects with an attempt to identify acoustic cues which distinguish between the voicing categories of Spanish stops in this context. VOT is identified as the main cue in identifying contrast between the dialects. Lisker & Abramson (1970) conducted a speech perception experiment using synthesised speech to study the identification modes of speakers of English, Spanish and Thai by systematically varying the time range (VOT) for the stops. They observed that the informants tended abruptly to shift from hearing a sound as ‘voiced’ to hearing it as ‘voiceless’, and that this was dependent on the timing (VOT) boundary point of each language. This is known as categorical perception. Shimizu (1977) also performed a perception experiment on the identification and distinguishing of initial stops in Japanese by native speakers using synthetic speech. He also observed results which were similar to those of Lisker & Abramson discussed above. The Japanese informants tended to abruptly stop hearing a sound.‘voiceqto hearing it as ‘voiceless’ at a certain VOT perceptual boundary. '