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Abstract:
This paper presents a quantitative assessment of the role of
the interaction between the airflow in the vocal tract and the
mechanical structures delimiting it (the "fluid-walls
interaction") in the shaping of complex articulatory paths,
called articulatory loops, that are observed during the
production of a velar consonant C, in VCV sequences. The work is
based on simulations made with a 2D biomechanical model of the
tongue coupled with a model of the airflow. The tongue model
includes the main muscles responsible for shaping and moving the
tongue in the midsagittal plane (posterior and anterior parts of
the genioglossus, styloglossus, hyoglossus, inferior and superior
longitudinalis and verticalis). Elastic properties of tissues are
accounted for by finite-element (FE) modeling of the tongue mesh
in 2D defined by 221 nodes and isoparametric elements. Muscles
are modeled as general force generators that (1) act on
anatomically specified sets of nodes of the FE structure, and (2)
modify the stiffness of specific elements of the model to account
for muscle insertions into tongue tissues. Collisions between
tongue surface and palatal or velar contours are also modeled.
The computation of the force generated during the contact is
based on "a penalty method", modeling a non-linear relationship
between contact force and position/velocity of points located on
the tongue surface. For the sake of simplicity, the flow model is
based on a simple 2D potential flow theory, accounting for
viscous losses as a perturbation of the inviscid solution. In
addition, flow separation effects within a vocal tract
constriction are taken into account. These effects are, indeed,
crucial to determine accurately the distribution of pressure, and
then of the hydrodynamical forces, within the constriction. [VCV]
sequences were simulated, where C was a velar consonant. Our
results suggest that for low to normal levels of subglottal
pressure the contribution of the "fluid-walls interaction" is
slight in comparison with the contribution of the biomechanics,
especially for back vowels. But in case of a strong subglottal
pressure this contribution could be significant essentially
because the Bernouilli effect is important. This could, in
particular, explain the forward loop observed for [k] in [ika].
[Work supported by CNRS (France), NSF and NIH (U.S.A.)]
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