Biomedical Engineering Reference
In-Depth Information
4.3.3 Controlling the Oscillations
One of the parameters that a bird has to control in order to establish an
oscillation in the labia is the bronchial pressure. The bronchial pressure has
to be larger than a certain critical value in order to set up the oscillations,
which periodically obstruct the airflow. (This periodic obstruction is, as seen
in previous chapters, the physical origin of the sound.)
It is important to point out that the labial elasticity
k
is also controlled
by the bird. In the case of the oscine birds, the activity in the siringealis
ventralis muscle determines the stretching of the labia (and therefore their
stiffness). In Fig. 4.7, we show the spectral content of four different oscillations
(obtained from numerical simulations like the ones performed in order to
generate Fig. 4.2) for different values of the bronchial pressure
p
and labial
elasticity
k
.
There are two salient features to note. First, as the labial elasticity
k
increases, the frequency of the oscillations increases. Second, the bronchial
pressure not only determines whether or not there will be oscillations, but
also determines the spectral content of the sound. The greater the bronchial
pressure, the more violent the collisions between the labia and against the
high
k
low
k
frequency
frequency
low
p
high
p
Fig. 4.7.
The features of the oscillations that this model generates depend on two
parameters: the restitution constant of the labia (
k
) and the pressure (
p
). The higher
the restitution constant, the higher the fundamental frequency of the oscillations
(compare, for the same pressure, the
upper
panels with the
lower
panels). In each
of the panels, the spectrum of the resulting oscillations is displayed. On the other
hand, as the pressure becomes higher, the oscillations become more “violent”. This
results in a higher spectral content of the signal, which can be seen from the spectra
by comparing how many harmonics are needed to represent the sound. Compare,
for the same restitution constant, the panels on the
left
with the panels on the
right
