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Rate effects on aerodynamics of intervocalic stops : evidence from real speech data and model data
(2008)
This paper is a first attempt towards a better understanding of the aerodynamic properties during speech production and their potential control. In recent years, studies on intraoral pressure in speech have been rather rare, and more studies concern the air flow development. However, the intraoral pressure is a crucial factor for analysing the production of various sounds.
In this paper, we focus on the intraoral pressure development during the production of intervocalic stops.
Two experimental methodologies are presented and confronted with each other: real speech data recorded for four German native speakers, and model data, obtained by a mechanical replica which allows reproducing the main physical mechanisms occurring during phonation. The two methods are presented and applied to a study on the influence of speech rate on aerodynamic properties.
The present article illustrates that the specific articulatory and aerodynamic requirements for voiced but not voiceless alveolar or dental stops can cause tongue tip retraction and tongue mid lowering and thus retroflexion of front coronals. This retroflexion is shown to have occurred diachronically in the three typologically unrelated languages Dhao (Malayo-Polynesian), Thulung (Sino-Tibetan), and Afar (East-Cushitic). In addition to the diachronic cases, we provide synchronic data for retroflexion from an articulatory study with four speakers of German, a language usually described as having alveolar stops. With these combined data we supply evidence that voiced retroflex stops (as the only retroflex segments in a language) did not necessarily emerge from implosives, as argued by Haudricourt (1950), Greenberg (1970), Bhat (1973), and Ohala (1983). Instead, we propose that the voiced front coronal plosive /d/ is generally articulated in a way that favours retroflexion, that is, with a smaller and more retracted place of articulation and a lower tongue and jaw position than /t/.
Studying kinematic behavior in speech production is an indispensable and fruitful methodology in order to describe for instance phonemic contrasts, allophonic variations, prosodic effects in articulatory movements. More intriguingly, it is also interpreted with respect to its underlying control mechanisms. Several interpretations have been borrowed from motor control studies of arm, eye, and limb movements. They do either explain kinematics with respect to a fine tuned control by the Central Nervous System (CNS) or they take into account a combination of influences arising from motor control strategies at the CNS level and from the complex physical properties of the peripheral speech apparatus. We assume that the latter is more realistic and ecological. The aims of this article are: first, to show, via a literature review related to the so called '1/3 power law' in human arm motor control, that this debate is of first importance in human motor control research in general. Second, to study a number of speech specific examples offering a fruitful framework to address this issue. However, it is also suggested that speech motor control differs from general motor control principles in the sense that it uses specific physical properties such as vocal tract limitations, aerodynamics and biomechanics in order to produce the relevant sounds. Third, experimental and modelling results are described supporting the idea that the three properties are crucial in shaping speech kinematics for selected speech phenomena. Hence, caution should be taken when interpreting kinematic results based on experimental data alone.
Articulatory token-to-token variability not only depends on linguistic aspects like the phoneme inventory of a given language but also on speaker specific morphological and motor constraints. As has been noted previously (Perkell (1997), Mooshammer et al. (2004)), speakers with coronally high "domeshaped" palates exhibit more articulatory variability than speakers with coronally low "flat" palates. One explanation for that is based on perception oriented control by the speaker. The influence of articulatory variation on the cross sectional area and consequently on the acoustics should be greater for flat palates than for domeshaped ones. This should force speakers with flat palates to place their tongue very precisely whereas speakers with domeshaped palates might tolerate a greater variability. A second explanation could be a greater amount of lateral linguo-palatal contact for flat palates holding the tongue in position. In this study both hypotheses were tested.
In order to investigate the influence of the palate shape on the variability of the acoustic output a modelling study was carried out. Parallely, an EPG experiment was conducted in order to investigate the relationship between palate shape, articulatory variability and linguo-palatal contact.
Results from the modelling study suggest that the acoustic variability resulting from a certain amount of articulatory variability is higher for flat palates than for domeshaped ones. Results from the EPG experiment with 20 speakers show that (1.) speakers with a flat palate exhibit a very low articulatory variability whereas speakers with a domeshaped palate vary, (2.) there is less articulatory variability if there is lots of linguo-palatal contact and (3.) there is no relationship between the amount of lateral linguo-palatal contact and palate shape. The results suggest that there is a relationship between token-to-token variability and palate shape, however, it is not that the two parameters correlate, but that speakers with a flat palate always have a low variability because of constraints of the variability range of the acoustic output whereas speakers with a domeshaped palate may choose the degree of variability. Since linguo-palatal contact and variability correlate it is assumed that linguo-palatal contact is a means for reducing the articulatory variability.