source: a large anatomical literature records a huge variety of air sacs, diverticula, elongated snouts or trachea, or other resonating structures among
vertebrates (see next section). However, most of these papers are old and
many are in German, and these morphological features have received little
attention from modern bioacousticians. Furthermore, the functional importance of these features has received almost no study despite the fact that
such variation may play a significant role in shaping the communication
systems of different species.
The column of air contained in the vocal tract, like any column of air, has
elasticity and mass and thus will vibrate preferentially at certain frequencies, called normal modes or “resonances.” As the sound energy generated
by the source passes through this air column, it may set one or more of
these modes into vibration.The presence of the vocal tract will thus enhance
the transmission of these frequencies while damping or attenuating others;
it acts as a spectral filter on the source signal. In speech science, these filtering frequencies are called “formants,” from the Latin formare, meaning
“to shape,” because they sculpt the vocal signal on its way from the source
out to the environment. This term is preferable to the term “vocal tract resonances” both due to its brevity and because it highlights the independence
of source and filter, which is indicated by most available work on the subject
(see Section 2.1.4). Thus, the most basic acoustic model of the vocalproduction system has two components: the sound-generating source (e.g.,
syrinx, larynx) and the filter (the air column contained by the vocal tract).
The function of the filter varies among species. Human speech uses changes
in formant frequencies to code meaning directly: formants and their movements are the most important acoustic cue in speech, which typically provides external reference. In both humans and other species, formants can
also be internally referential, providing cues to identity, body size, age, or
sex (Rendall 1996; Fitch 1997; Fitch and Giedd 1999; Riede and Fitch 1999).
A different use of the vocal tract filter is to suppress certain frequencies,
typically to enhance the salience of some particular source components
(e.g., the second harmonic). This is the case in some birds (Nowicki 1987)
and bats (Hartley and Suthers 1988). In all cases, it is extremely important
to recognize that formants are an acoustic entity independent from the
source (the fundamental frequency and its harmonics) in terms of production, acoustic analysis, and perception. Formants can vary independently
of the source, and formants have little or no influence on pitch perception (which is determined by source characteristics), at least in humans
(Lieberman and Blumstein 1988; Titze 1994).
All terrestrial vertebrates possess a vocal tract that can be predicted from
basic physics to have a substantial acoustic effect on production of many
call types. In both birds and mammals, the evidence for formant frequencies is abundant based on even a cursory examination of spectrograms.
Despite this, there has been little attention to formants, or research on the
anatomy, physiology, or acoustics of the nonhuman vocal tract, compared
80
W.T. Fitch and M.D. Hauser
vertebrates (see next section). However, most of these papers are old and
many are in German, and these morphological features have received little
attention from modern bioacousticians. Furthermore, the functional importance of these features has received almost no study despite the fact that
such variation may play a significant role in shaping the communication
systems of different species.
The column of air contained in the vocal tract, like any column of air, has
elasticity and mass and thus will vibrate preferentially at certain frequencies, called normal modes or “resonances.” As the sound energy generated
by the source passes through this air column, it may set one or more of
these modes into vibration.The presence of the vocal tract will thus enhance
the transmission of these frequencies while damping or attenuating others;
it acts as a spectral filter on the source signal. In speech science, these filtering frequencies are called “formants,” from the Latin formare, meaning
“to shape,” because they sculpt the vocal signal on its way from the source
out to the environment. This term is preferable to the term “vocal tract resonances” both due to its brevity and because it highlights the independence
of source and filter, which is indicated by most available work on the subject
(see Section 2.1.4). Thus, the most basic acoustic model of the vocalproduction system has two components: the sound-generating source (e.g.,
syrinx, larynx) and the filter (the air column contained by the vocal tract).
The function of the filter varies among species. Human speech uses changes
in formant frequencies to code meaning directly: formants and their movements are the most important acoustic cue in speech, which typically provides external reference. In both humans and other species, formants can
also be internally referential, providing cues to identity, body size, age, or
sex (Rendall 1996; Fitch 1997; Fitch and Giedd 1999; Riede and Fitch 1999).
A different use of the vocal tract filter is to suppress certain frequencies,
typically to enhance the salience of some particular source components
(e.g., the second harmonic). This is the case in some birds (Nowicki 1987)
and bats (Hartley and Suthers 1988). In all cases, it is extremely important
to recognize that formants are an acoustic entity independent from the
source (the fundamental frequency and its harmonics) in terms of production, acoustic analysis, and perception. Formants can vary independently
of the source, and formants have little or no influence on pitch perception (which is determined by source characteristics), at least in humans
(Lieberman and Blumstein 1988; Titze 1994).
All terrestrial vertebrates possess a vocal tract that can be predicted from
basic physics to have a substantial acoustic effect on production of many
call types. In both birds and mammals, the evidence for formant frequencies is abundant based on even a cursory examination of spectrograms.
Despite this, there has been little attention to formants, or research on the
anatomy, physiology, or acoustics of the nonhuman vocal tract, compared
80
W.T. Fitch and M.D. Hauser