First published online January 5, 2005
Journal of Experimental Biology 208, 297-308 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01378
Vocal tract filtering and sound radiation in a songbird
Brian S. Nelson1,*,
Gabriël J. L. Beckers2,3 and
Roderick A. Suthers1,3
1 Department of Biology, Indiana University, Bloomington, Indiana, 47405,
USA
2 Behavioural Biology, Institute of Biology, Leiden University, PO Box 9516,
2300RA, Leiden, The Netherlands
3 School of Medicine, Indiana University, Bloomington, Indiana, 47405,
USA
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Fig. 2. Illustration showing where a small speaker was inserted into the trachea,
how beak gape was varied, and how subjects were rotated with respect to
microphone positions. Beak gape was measured as the distance between the tips
of maxilla and mandible.
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Fig. 3. Representative spectra of frequency sweeps recorded from two subjects
(colored lines, 358 and 430) and spectra obtained after placing a silastic
plug into the glottis and closing the beak (0 mm, black lines). Spectra are
referenced to peak amplitude (1–10 kHz). Spectra obtained for subjects
390 and BB under these same conditions are similar (see Figs
4,
5,
6,
7).
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Fig. 4. Vocal tract resonances emphasize sound frequencies near  2 and 5.5 kHz.
(A) Vocal tract resonances estimated as the difference between a spectrum
obtained from the speaker mounted by itself (black line at zero) and spectra
recorded after inserting the port of the speaker into the trachea, setting
beak gape at 5 mm, and orienting each subject towards the microphone (0°,
colored lines). (B) Vocal tract resonances estimated from recordings obtained
at a distance of 15 cm from horizontally oriented subjects. The normal
condition represents a trachea length that was deemed as natural. The trachea
was then shortened or lengthened by gently pushing or pulling on the cut end
of the trachea. (C) Vocal tract resonances estimated as in B after removing 5
mm or 8 mm segments from the exposed portion of the trachea. Results for
subjects 390 and BB are similar. Spectra in (A) are noisy (rippled) because
these recordings were obtained at a distance of 90 cm. Spectra in (B) and (C)
are less noisy because these recordings were obtained at a distance of 15
cm.
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Fig. 7. We observed substantial changes in amplitude at 6.5 kHz as beak gape was
varied between 0 and 11 mm. Amplitude increases linearly with beak gape at 6.5
kHz but is plotted on a logarithmic scale (dB). Amplitude at this frequency
across all subjects can be approximated as A=–5+32 log(B), where A is
amplitude (dB) and B is beak gape (broken line). Amplitude values are plotted
relative to those obtained with a closed beak (0 mm).
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Fig. 8. Sound radiates from subjects in a relatively simple manner and a single
amplitude decrement of variable width (degrees) and depth (dB) near
130° explains most changes. Plotted are amplitudes at 2, 4 and 8 kHz
extracted from spectra of recordings obtained from subjects with a 5 mm beak
gape. Measurements are averaged across each hemifield (0–171°) and
are normalized relative to 0°.
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Fig. 9. Acoustic output becomes more directional as sound frequency increases and
as beak gape increases. Directionality calculated as S.D. across
360° (N=40) is plotted independently for each beak gape (colored
lines) and as a function of sound frequency. Broken lines at 3 dB are included
as references (see text).
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Fig. 10. Acoustic output became more directional when the microphone was lowered
20° but became less directional when the microphone was elevated 20°.
Directionality (S.D.) was calculated as in
Fig. 9.
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Fig. 11. (A) Second harmonics (2f0) in Florida `tow-hee' calls
have relatively low amplitudes. These results suggest that harmonics with
frequencies above 4 kHz may be strongly attenuated. Illustrated are the
mean relative amplitudes (thick lines) of harmonics in calls produced by 36
Florida subjects (N=1067). Thin lines represent the addition or
subtraction of one standard deviation. (B) Representative time-frequency
spectrograms of two Florida tow-hee calls, measurements of beak gape obtained
from video recordings, and amplitude traces for each call.
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Fig. 12. Towhees tend to produce low sound frequencies in `tow-hee' calls with a
small beak gape but produce high sound frequencies with a large beak gape.
Plotted are measurements of beak gape and measurements of peak sound frequency
obtained from audio frames corresponding with video frames. (A) Calls recorded
in Florida (362 frames from 50 calls produced by seven subjects). (B) Calls
recorded in Indiana (854 frames from 97 calls produced by 6 subjects). Sound
frequencies above the dashed line (3.75 kHz) seem to be attenuated as
towhees close their beaks (Fig.
5). Arrows depict directional pressures that are described in the
text.
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© The Company of Biologists Ltd 2005
