rating formants from fundamental frequency or harmonics) rather than
relying on the catch-all acoustic term “dominant frequency.” Increased
precision of acoustic description will enable researchers interested in
the anatomy and physiology of sound production to more easily pin down
the mechanisms relevant in perception and communication, and thus
in the evolution of mate choice or speciation, in this important group of
vertebrates.
Another term that is confusing because it is used ambiguously is “pitch.”
Pitch is defined as “that attribute of auditory sensation in terms of which
sounds may be ordered on a musical scale.” Pitch is a subjective quality,
defined in human terms, that cannot be measured directly. It is, strictly
speaking, inappropriate to use the term in animal bioacoustics. However,
the term is convenient and its usage widespread, making it unlikely to
vanish from the technical literature. Thus, it is critical that bioacousticians
use the term consistently and precisely. For most periodic sounds, perceived
pitch corresponds to the physical variable fundamental frequency (or its
inverse, waveform period). Exceptions include periodic sounds that lack
energy at the fundamental frequency, so-called “missing fundamental”
stimuli. A sound with energy only at 200, 300, and 400 Hz will often have a
perceived pitch corresponding to a sine wave at 100 Hz, despite the lack of
physical energy at this frequency, due to perceptual processes that “restore”
the missing fundamental frequency. Although such phenomena may be
relevant in calls produced by birds or bats, where the fundamental frequency is suppressed (Hartley and Suthers 1988; Nowicki and Marler 1988),
in general there is a close correspondence between “pitch” and fundamental frequency. Thus, the use of “pitch” to refer to other acoustic parameters,
such as voice timbre or vocal tract resonances (e.g., Hausberger et al. 1991),
is to be discouraged.
2.1.4. Independence of Source and Filter
There is a superficial similarity between vertebrate vocal-production
systems and wind instruments such as the clarinet or trumpet, where the
reed or lips play the role of the source and the column of air contained by
the body of the instrument is analogous to the vocal tract. However, there
is an important difference in the physics of wind instruments and the vocal
tract. In wind instruments, the vibrating frequency of the source is largely
determined by the resonant frequencies of the instrument’s “vocal tract”;
that is, by the air column contained by the body. The instrumentalist manipulates the pitch of the instrument by changing the length of this air column
and thus the characteristic frequencies of the column’s vibratory modes. In
this case, it is appropriate to call the modes “resonances” because the source
vibrates in resonance with (at the same frequency as) the air column.
In contrast, the vibratory frequencies of the source and filter appear to
be independent in vertebrates, an independence that makes vocal acoustics
82
W.T. Fitch and M.D. Hauser
relying on the catch-all acoustic term “dominant frequency.” Increased
precision of acoustic description will enable researchers interested in
the anatomy and physiology of sound production to more easily pin down
the mechanisms relevant in perception and communication, and thus
in the evolution of mate choice or speciation, in this important group of
vertebrates.
Another term that is confusing because it is used ambiguously is “pitch.”
Pitch is defined as “that attribute of auditory sensation in terms of which
sounds may be ordered on a musical scale.” Pitch is a subjective quality,
defined in human terms, that cannot be measured directly. It is, strictly
speaking, inappropriate to use the term in animal bioacoustics. However,
the term is convenient and its usage widespread, making it unlikely to
vanish from the technical literature. Thus, it is critical that bioacousticians
use the term consistently and precisely. For most periodic sounds, perceived
pitch corresponds to the physical variable fundamental frequency (or its
inverse, waveform period). Exceptions include periodic sounds that lack
energy at the fundamental frequency, so-called “missing fundamental”
stimuli. A sound with energy only at 200, 300, and 400 Hz will often have a
perceived pitch corresponding to a sine wave at 100 Hz, despite the lack of
physical energy at this frequency, due to perceptual processes that “restore”
the missing fundamental frequency. Although such phenomena may be
relevant in calls produced by birds or bats, where the fundamental frequency is suppressed (Hartley and Suthers 1988; Nowicki and Marler 1988),
in general there is a close correspondence between “pitch” and fundamental frequency. Thus, the use of “pitch” to refer to other acoustic parameters,
such as voice timbre or vocal tract resonances (e.g., Hausberger et al. 1991),
is to be discouraged.
2.1.4. Independence of Source and Filter
There is a superficial similarity between vertebrate vocal-production
systems and wind instruments such as the clarinet or trumpet, where the
reed or lips play the role of the source and the column of air contained by
the body of the instrument is analogous to the vocal tract. However, there
is an important difference in the physics of wind instruments and the vocal
tract. In wind instruments, the vibrating frequency of the source is largely
determined by the resonant frequencies of the instrument’s “vocal tract”;
that is, by the air column contained by the body. The instrumentalist manipulates the pitch of the instrument by changing the length of this air column
and thus the characteristic frequencies of the column’s vibratory modes. In
this case, it is appropriate to call the modes “resonances” because the source
vibrates in resonance with (at the same frequency as) the air column.
In contrast, the vibratory frequencies of the source and filter appear to
be independent in vertebrates, an independence that makes vocal acoustics
82
W.T. Fitch and M.D. Hauser