Biomedical Engineering Reference
In-Depth Information
(a)
10000
0.0
0.0
time (s)
0.5
(b)
6000
0.0
0.0
time (s)
0.9
Fig. 6.1.
In (
a
), we display the sonogram of a song produced by the eastern
slaty thrush (
Turdus subalaris
), which uses both sides of the syrinx to produce
a rather complex sound. In part (
b
), we find in the sonogram of the grassland
sparrow's song (
Ammodramus humeralis
) a series of paralell lines, which are a
signature of rather complex dynamics of the labia used to produce the sound. This
sonogram corresponds to a sound which cannot be reproduced with the simple
models described so far
this case, the lowest frequency before the transition is approximately 4000 Hz;
the first harmonic should therefore be at approximately 8000 Hz. However,
the new lines appear at 3800 Hz and 4200 Hz, etc.
Both examples imply a higher degree of complexity of the sound signal
than in the ones displayed in the previous chapters. However, there is an
important difference between them. In the first case, the bird uses both sides
of its bipartite vocal organ. The sound is generated by two sources that
each contribute their fluctuations (in principle, in an independent way), and
each contribution can be understood in terms of the elements that we have
presented so far. It is certainly true that to activate both sound sources with
a certain degree of coordination (instead of keeping one silent through the
active closing of the lumen) implies an important degree of complexity at the
level of neural the instructions sent to the vocal organ. However, the physics
involved is as simple as it was before.
The second case, in contrast, is more subtle: although the dynamics of
the lines appear to indicate that there is just one active source of sound,
