Biomedical Engineering Reference
In-Depth Information
the distance between the cartilaginous rings on which the labia rest. There-
fore, a contraction of this muscle increases the stiffness of the labia. This
implies a higher restitution coe
cient, which in turn implies an increase in
the oscillation frequency.
In this way, the generation of a syllable in which the fundamental fre-
quency varies can be achieved through the construction of a slow movement
through the parameter space (of bronchial pressure and muscle tension). By
this we mean that instead of there being a constant tension and pressure, the
vocalization is performed while the parameters are slowly changed. “Slow”
has, in this context, a precise meaning: the rate at which the parameters are
changed is very small compared with the frequency of the oscillations that are
turned on. Let us suppose, for example, a trajectory in the parameter space
such as the one illustrated in Fig. 5.4a. As this path is traveled along, several
dynamical changes will take place. As we go from point 1 to point 2, the os-
cillations are turned on (the interlabial pressure overcomes the dissipation in
the system). As the pressure is further increased from point 2 to 3, the most
important change is an increase in sound intensity and a spectral enrichment
of the signal, without major changes in the fundamental frequency of the os-
cillations. As we go from point 3 towards point 4, as the restitution coe
cient
k
increases and the oscillations become faster (i.e., the fundamental frequency
increases). The path from point 4 to 5 is associated simply with a spectral
impoverishment of the signal, which becomes progressively more harmonic
and less intense, but without significant changes concerning its fundamental
(a)
(b)
k
(5)
F
4
5
4
3
12
3
(1)
2
p
T
Fig. 5.4.
The most remarkable feature of a syllable that has to be reproduced by
a model is the changing vibratory behavior over time within a syllable, i.e., the
changing fundamental frequency of the labial oscillations. For example, upsweeps
and downsweeps denote syllables in which the oscillations are turned on at a lower
and higher frequency, respectively, than that at which they are turned off. In order
to generate an upsweep, we have to enter the region of oscillations in the parameter
space with a
k
value smaller than the value at which we exit the region in which
oscillations take places. (
a
) Path in parameter space. (
b
) Sonogram associated with
thepathin(
a
)
