Biomedical Engineering Reference
In-Depth Information
which allows us to predict qualitative changes in the spectral features of
syllables produced in these atmospheres. Performing numerical simulations
of the models defined in the previous sections, we have obtained the syllables
displayed in Fig. 6.7.
In Fig. 6.7 we display syllables obtained with models that take into ac-
count the coupling between the source and the filter (i.e. the acoustic-feedback
model of Sect. 6.2) (Fig. 6.7a), and the spatial structure of the oscillating labia
(i.e., the two-mass model of Sect. 6.3) (Fig. 6.7b). In each case, we compare
the syllables that are generated in two atmospheres of different density (air
and heliox). In Fig. 6.7b, the syllable is generated while the bronchial pressure
is raised, in order to display the appearence of subharmonic behavior. The
main conclusion that we can obtain from this numerical experiment is that
the subharmonic behavior due to source-filter coupling can be suppressed if
the density of the atmosphere is lowered. On the other hand, the changes
(a)
20000
20000
0.0
0.0
0.0
time (s)
.2
0.0
time (s)
.2
(b)
20000
20000
0.0
0.0
0.0
time (s)
.1
0.0
time (s)
.1
Fig. 6.7.
Predicted sonograms in heliox experiments. (
a
) Acoustic-feedback model
(Sect. 6.2).
Left
: a subharmonic sonogram in ordinary air.
Right
: when ordinary air
is replaced by heliox, which has a density one-third that of air, the coupling between
the source and tract decreases according to (6.20) and (6.21), and the subharmonic
behavior disappears. (
b
) Two-mass model (Sect. 6.3). The results of the two-mass
model do not depend on the air density, as long as the coupling between the source
and tract is negligible. A syllable is displayed which shows a transition to a period-
three solution as the sublabial pressure is slightly raised. This transition is due to the
appereance of higher-order modes of labial vibration. The only difference between
ordinary air (
left
) and heliox (
right
) is a change in the vocal-tract resonances
