  During voiced speech, the human vocal folds interact with the vocal tract
acoustics. The resulting glottal source-resonator coupling has been observed
using mathematical and physical models as well as in in vivo phonation. We
propose a computational time-domain model of the full speech apparatus that, in
particular, contains a feedback mechanism from the vocal tract acoustics to the
vocal fold oscillations. It is based on numerical solution of ordinary and
partial differential equations defined on vocal tract geometries that have been
obtained by Magnetic Resonance Imaging. The model is used to simulate rising
and falling pitch glides of [a, i] in the fundamental frequency (f_o) interval
[150 Hz, 320 Hz]. The interval contains the first vocal tract resonance f_R1
and the first formant F1 of [i] as well as the fractions of the first resonance
f_R1/4 and fR1/3 of [a].
  The simulations reveal a locking pattern of the fo-trajectory at f_R1 of [i]
in falling and rising glides. The resonance fractions of [a] produce
perturbations in the pressure signal at the lips but no locking. All these
observations from the model behaviour are consistent and robust within a wide
range of feasible model parameter values and under exclusion of secondary model
components.
