Marc Verha
Fri, Mar-21-08, 06:18
Thanks a lot, m3d.
The joy of sacs
Bart de Boer
1. Introduction This paper investigates an idea that was put
forward (and hinted at in Fitch
2) by Tecumseh Fitch at the Cradle of Language conference in
Stellenbosch, South Africa. The idea is that air sacs may
have played an important role in early hominid
vocalizations. Many other primates have air sacs, notably
chimpanzees, gorillas and orangutans. It is therefore
likely that our latest common ancestor also had air sacs,
and the shape of a recently discovered Australopithecine
hyoid bone (Alemseged et al.2006) also points in this
direction. Many functions have been proposed for air sacs,
among them resonance chambers, sound radiators, CO2 buffers
to prevent hyperventilation and means to help exaggerate
size. Whatever their function in other primates, the fact
that humans are the only apes that do not have air sacs
might be related to the fact that we have speech. Here I
investigate the influence of the presence of an air sac on
the set of (vowel) signals that can be produced.
3. Preliminary results An articulatory model (based on
Mermelstein 1973) of the female vocal tract was extended
with an air sac (figure 1 left). As all existing
articulatory models were designed to model humans, no
efforts were ever made to model air sacs. The model is
therefore still under development. The present model
consists of a simple side tube, with anatomically
plausible dimensions, attached above the larynx of the
standard articulatory model. A more sophisticated model,
based partly on models of bird calls (Fletcher et
al.2004) is under development. Both models (with and
without air sacs) produced 10 000 random articulations,
and the first and second formants of these articulations
were measured. These are presented in the right part of
figure 1. A logarithmic scale is used, as this
corresponds to humans¹ logarithmic perception of
frequencies. It can be observed that the articulations
made with the model with an air sac have lower formant
frequencies and cover a smaller area of acoustic space
than those of the model without an air sac (m = 0.151, s
= .006 versus m = 0.324, s = 0.009).
4. Discussion The observed lowering of formant frequencies
supports the theory that air sacs help to exaggerate
size, an evolutionary useful function. However, the price
is a reduced ability to produce distinctive speech. It is
therefore possible that humans lost air sacs because they
evolved speech.
Alemseged, Z., Spoor, F., Kimbel, W. H., Bobe, R., Geraads,
D., Reed, D., et al. (2006). A juvenile early hominin skeleton
from dikika, ethiopia. Nature,
4(1), 296-301. Fitch, W. T. (2000). The evolution of speech: A
comparative review. Trends in cognitive sciences, 4(7),
258-267. Fletcher, N. H., Riede, T., Beckers, G. J. L., &
Suthers, R. A. (2004). Vocal tract filtering and the coo¹
of doves. Journal of the Acoustical Society of America,
116(6), 3750-3756. Mermelstein, P. (1973). Articulatory
model for the study of speech production. Journal of the
Acoustical Society of America, 53(4), 1070-1082.
The joy of sacs
Bart de Boer
1. Introduction This paper investigates an idea that was put
forward (and hinted at in Fitch
2) by Tecumseh Fitch at the Cradle of Language conference in
Stellenbosch, South Africa. The idea is that air sacs may
have played an important role in early hominid
vocalizations. Many other primates have air sacs, notably
chimpanzees, gorillas and orangutans. It is therefore
likely that our latest common ancestor also had air sacs,
and the shape of a recently discovered Australopithecine
hyoid bone (Alemseged et al.2006) also points in this
direction. Many functions have been proposed for air sacs,
among them resonance chambers, sound radiators, CO2 buffers
to prevent hyperventilation and means to help exaggerate
size. Whatever their function in other primates, the fact
that humans are the only apes that do not have air sacs
might be related to the fact that we have speech. Here I
investigate the influence of the presence of an air sac on
the set of (vowel) signals that can be produced.
3. Preliminary results An articulatory model (based on
Mermelstein 1973) of the female vocal tract was extended
with an air sac (figure 1 left). As all existing
articulatory models were designed to model humans, no
efforts were ever made to model air sacs. The model is
therefore still under development. The present model
consists of a simple side tube, with anatomically
plausible dimensions, attached above the larynx of the
standard articulatory model. A more sophisticated model,
based partly on models of bird calls (Fletcher et
al.2004) is under development. Both models (with and
without air sacs) produced 10 000 random articulations,
and the first and second formants of these articulations
were measured. These are presented in the right part of
figure 1. A logarithmic scale is used, as this
corresponds to humans¹ logarithmic perception of
frequencies. It can be observed that the articulations
made with the model with an air sac have lower formant
frequencies and cover a smaller area of acoustic space
than those of the model without an air sac (m = 0.151, s
= .006 versus m = 0.324, s = 0.009).
4. Discussion The observed lowering of formant frequencies
supports the theory that air sacs help to exaggerate
size, an evolutionary useful function. However, the price
is a reduced ability to produce distinctive speech. It is
therefore possible that humans lost air sacs because they
evolved speech.
Alemseged, Z., Spoor, F., Kimbel, W. H., Bobe, R., Geraads,
D., Reed, D., et al. (2006). A juvenile early hominin skeleton
from dikika, ethiopia. Nature,
4(1), 296-301. Fitch, W. T. (2000). The evolution of speech: A
comparative review. Trends in cognitive sciences, 4(7),
258-267. Fletcher, N. H., Riede, T., Beckers, G. J. L., &
Suthers, R. A. (2004). Vocal tract filtering and the coo¹
of doves. Journal of the Acoustical Society of America,
116(6), 3750-3756. Mermelstein, P. (1973). Articulatory
model for the study of speech production. Journal of the
Acoustical Society of America, 53(4), 1070-1082.