The Sources of Sound in Birdsong - The Physics of Birdsong - page 57

walls. The increase in the strength of the collisions is responsible for the richer
spectral content of the sound produced [Gardner et al. 2001].
There is still one more element to discuss. According to our description
of the syrinx in Chap. 3, this is a bipartite device [Goller and Suthers 1995],
with a pair of labia in the junction between the bronchi and the trachea. In
some species, both sources are used simultaneously, creating rich and com-
plex sounds. However, it is usually the case that one finds a high degree of
laterality, i.e., the use of only one source of sound, on the left or right de-
pending on the species [Goller and Suthers 1996a]. In order to achieve this,
birds often actively close one of the conduits, forcing the labia on one side to
stay pressed against each other. This is done through the activation of dorsal
muscles (specifically, the siringealis dorsalis and the tracheobronchialis dor-
salis), and in terms of our model of a labium, this corresponds to a force
f that does not depend on either x or y . If this term is large enough, the
oscillations can be stopped, by forcing the labia to stay pressed against each
other.
It is by means of the control of these parameters (control of the bronchial
pressure with respiratory muscles, labial tension with ventral muscles and
active adduction with dorsal muscles) that the bird is capable of establishing
oscillatory dynamics in the labia, periodically obstructing the airflow that
feeds the vocal tract [Laje et al. 2002].
The oscine birds have a large set of muscles to control the acoustic fea-
tures of their vocalizations. Nonoscine birds, however, also have some control
[Suthers 2001]. Doves, for example, can alter the tension of the oscillating
membranes with the same muscles as used to gate the airflow. On the other
hand, the high pressure in the interclavicular air sac inflates it, affecting
the tension of the oscillating membranes. These processes contribute in a
complex way to determining the acoustic features of nonoscine vocalizations
[Beckers et al. 2003b].
4.4 Filtering the Signal
An element lies between the source of sound and what we ultimately hear: the
vocal tract, which is the “tube” that runs from the syrinx to the beak. The
airflow fluctuations produced in the syrinx (at the entry to the vocal tract)
generate, as we have seen in Chap. 2, sound waves that travel through the
tract. In regions where the tract alters its shape (for example, by becoming
narrower or bending), part of the incident wave will be reflected and part
of it will be transmitted. The same happens at the beak. Part travels to
the exterior and part of the incoming wave is reflected back into the tract.
The consequences of this were discussed in Chap. 2. The basic result is that
some spectral components will be reinforced while others will be damped. In
Fig. 4.8, we display the song of a pirincho, ( Guira guira ) [Straneck 1990a],
in which the presence of a filter is clear in the fluctuations (of half a period)