Biomedical Engineering Reference
In-Depth Information
source 1
10000
source 2
0.0
1.8
2.1
time (s)
Fig. 3.2.
Sonogram of the song of an eastern slaty thrush (
Turdus subalaris
)
[Straneck 1990b]. While many birds use only one pair of labia to sing (by keep-
ing the other side silent as a result of active muscular work in order to close it),
this bird is singing with
both
sides of the syrinx independently, at the same time.
Notice the two sources of sound evolving independently
human voiced sounds is due mainly to reconfigurations of the vocal tract (by
tongue position, labial shape, and other configurational measures). The size
of the vocal tract, the aperture of the beak, etc., do affect the quality of the
vocalization in birdsong. Birds might make such changes in their vocal tract
in order to actively coordinate its filter characteristics with the output of
the syrinx [Nowicki 1987]. But sound quality and the main properties of the
song are primarily defined at the level of the syrinx. In order to understand
this point, let us observe the difference between the sonograms in Fig. 3.3.
(a)
(b)
"a"
"i"
8000
3000
0.0
0.0
0.5
time (s)
1.5
0.7
time (s)
1.9
Fig. 3.3.
Difference between birdsong and human speech, at the level of spec-
tral and temporal features. (
a
) Three syllables of the chingolo sparrow's song
(
Zonotrichia capensis
). Notice the marked time evolution of the fundamental fre-
quency, spanning 3000 Hz in the third syllable. Although it is not evident at this
scale, the syllables of the chingolo have almost no harmonic content. (
b
) Sonogram
of an utterance of the word “taxi”. In contrast to the bird's sonogram shown on the
left
, human vowels have a very rich spectral content and the fundamental frequency
(the stroke at the lowest frequency) remains practically constant
