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Abstract:
Although humans normally speak with the head held upright,
intelligible speech may be produced from any orientation of the
vocal tract; in fact the earliest efforts to control the speech
organs ('babbling') typically precede control of physical
posture. It is well known that speech production is robust with
respect to external perturbation and so unsurprising that
articulator movements can be adjusted to compensate for
differences in gravitational loading. But because these
adjustments do not result in identical articulator trajectories
(which would be the case if perfect compensation occurred),
systematic comparison of trajectories across speaking postures
suggests a means of assessing the relative importance of
articulatory targets, in that the most significant of these for
intelligible communication are presumably the best controlled and
least variable. With this aim we have used the X-ray microbeam
system to observe two male Japanese speakers producing sustained
vowels,! vowels in a carrier context, CVCV sequences and running
speech repeated in sitting and supine phonation postures.
Subjects were fitted with tracking pellets on the tongue,
mandible, upper and lower lips, plus two for head correction
purposes. Results show the least variability in constriction
targets, some effect of linguistic structure, and a general
pattern of displacement consistent with gravitational direction
for articulators not under active control. Acoustics showed minor
effects in observed formant bandwidths. A perceptual study found
listeners able to distinguish between production types at a level
marginally better than chance. In general our results suggest
that while speakers adapt readily to the task of supine
articulation, their compensation strategies are directed towards
maintaining the consistency of their output acoustics rather than
rigid preservation of the articulatory trajectories that produce
them. While acoustically sensitive targets involving na! rrow
constrictions are produced with little variability between
postures, unnecessary effort opposing gravity is avoided when it
carries little acoustic consequence. Observed posture effects
were greatest for sustained vowels and least for running speech,
which indicates that the results of MR imaging of sustained
production in supine position should be interpreted with some
caution.
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