Biomedical Engineering Reference
In-Depth Information
in the spectral features of the vocalization produced by structured labia are
more subtle: the complexity of the oscillations at the source is not decreased,
but changes are observed in the resonances, which grow by up to 75% in the
heliox atmosphere.
6.4.1 Signatures of Interaction Between Sources
There is yet another surprise behind the bipartite structure of the syrinx.
In the sonogram in Fig. 6.1a, we saw the signature of two sources that were
simultaneously active. The sonogram consisted of two lines with indepen-
dent time evolutions. There are no major di
culties in understanding this
phenomenon: the sources act independently, and both inject pressure fluctu-
ations that are added at the tracheal entrance [Nowicki 1997]. The tract will
simply filter the harmonic components as we have analyzed so far, in order
to achieve the final vocalization. However, when we analyzed the behavior
of one source, we noticed that in some circumstances, the resulting pressure
at the input of the trachea could show fluctuations so large that the labial
dynamics were affected. It is natural, then, to expect that it could be possible
that the fluctuations induced by an active source could influence the behavior
of the second source, if this were also to become active. Since it is not likely
that both sources will show oscillations at exactly the same frequency, the
resulting behavior in the case of coupling can be easily recognized.
To give us an idea of what we can expect to find in the case of two cou-
pled sources, let us revisit the simple model of the oscillating labia. Remem-
ber that the key for establishing oscillations in one source is the interlabial
pressure, which is responsible for the force that overcomes the system's dis-
sipation and sustains the oscillations. However, the existence of a coupling
changes this scenario somewhat. Imagine that one of the sources is “on”,
and generates important pressure fluctuations at the input of the trachea.
The second source, as it begins to oscillate (for example, as the activity of
the dorsal muscle keeping the labia together is decreased), finds a complex
scenario. Each time that the labia have a divergent profile (i.e. open to the
trachea), the source will experience a fluctuating pressure that oscillates at
the frequency of the first source. Therefore, the second source will oscillate
with an amplitude modulated at the first source's frequency. In Fig. 6.8a, we
show a harmonic signal and its spectrum (composed of a unique frequency).
In Fig. 6.8b, we show the resulting signal if we modulate the amplitude of the
first source with a different frequency. The spectrum of this signal now shows
new frequencies, with the signature of the coupling being frequencies equal
to the sum and difference of the original frequencies. Coupling between two
avian sound sources has been experimentally demonstrated in a characteristic
syllable of a black-capped chickadee (
Parus atricapillus
), a sort of internal
duet between the two sound sources [Nowicki and Capranica 1986].
An amazing fact is that some species have the capacity to choose between
using the two sources independently or making them interact. This seems to
