Biomedical Engineering Reference
In-Depth Information
(a)
1.0
(b)
1.0
-1.0
0.0
0.530
time (s)
0.533
0.0
frequency (Hz)
10000
Fig. 4.8.
The acoustic pressure (
a
) and Spectrum (
b
) for a note of a
Guira guira
.
Notice that the spectrum has many peaks, which do not monotonically decrease as
a function of the frequency. This is due to the effect of the filter (passive tract) on
the pressure fluctuations induced by labial oscillations
superimposed on the fundamental oscillation at 2333 Hz. It is important to
point out that the relative sizes of the harmonics provide the song with its
characteristic timbre.
This chapter has dealt with the problem of the generation of sound by
means of the induction of oscillations in syringeal labia. The airflow fluc-
tuations induced by these oscillations excite a tube that filters the signal,
reinforcing certain frequencies and damping others. We have also discussed
qualitatively the influence of the labial elasticity and bronchial pressure on
the acoustic features of the sound generated. In the following chapter, we
shall advance to the description of the instructions that the bird has to send
to its avian vocal organ in order to generate the various sounds that its song
is built from. We shall do so in the framework of simple models, similar to
those which we have discussed so far.
It is reasonable to keep in mind all the simplifications that we have made.
In the model that we have presented, the oscillating labia are symmetric.
In our model, the nonlinear dissipation is modeled as a simple polynomial
function. We do not take into account the internal structure of the oscillating
labia, although the difference in mass between the medial labium and the
medial tympaniform membrane might serve to induce the wave-like mode that
we are using in our description [Fee 2002]. We have also described the problem
of the generation of the sound separately from the problem of the linear
filtering of the sound, although the back-propagating waves (from partial
reflections at interfaces) could affect the oscillations of the labia, generating
extremely rich dynamics. Some of these issues will be revisited in Chap. 6.
However, it is important to know how successful the simplest models can be
before discussing these subtle effects. The exploration of the simplest models
is the focus of Chap. 5.
