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TW. Cranford
first restrict the definition of the sounds whose generation we will consider.
For the purposes of this chapter, we can immediately exclude all sounds
produced by percussive activity upon the surface of the water by a part of
the body (flukes, flippers, torso, etc.). We will also not consider sounds that
might be produced as a by-product of bodily functions, like locomotion,
digestion, or respiration. For our purposes, only internally generated sounds
that function, or might function, in echolocation, prey capture, or social
facilitation will be considered.
All odontocetes apparently produce brief acoustic pulses or "clicks".
These sounds are generally characterized as broadband signals, although
specific signal characteristics vary with the recording circumstances, species,
and the individual in question. Clicks, when arranged in moderately rapid
trains of a few to several hundred per second, have been widely implicated
in odontocete echolocation behavior. A large body of literature now describes the acoustic characteristics and resolution capability of the odontocete echolocation system (Turner and Norris 1966; Evans and Powell 1967;
Nachtigall1980;Au 1990,1993). Most of this work has been conducted with
the bottlenose dolphin, Tursiops truncatus, but some work has been carried
out with other odontocete species (Norris 1975; Popper 1980; Au 1993).
1.1 Types of Sounds
Since the lives of most cetaceans are carried out in an oceanic world wholly
unfamiliar to most of us, we often know little of their behavior and natural
history. Gathering sound recordings from their underwater world is easy
relative to gathering other types of information, so it is commonly one of
the first pieces we have from the puzzling lives they lead. For pelagic species,
there is often little else. A good deal of the early literature catalogues the
first sounds recorded from free-ranging animals (Schevill, 1964). In most of
these instances, it was difficult or impossible to know the animal's orientation, direction, or distance from the recording hydrophone. Since signal
amplitude and frequency content usually varies with position in a sound
field (especially for the distinctly directional clicks), hydrophone placement
relative to the sound source will influence subsequent recordings (Au 1980,
1993; Au et al. 1986). In addition, recording techniques have not been standardized and few of the recordings made before 1970 extended very far into
the ultrasonic range. We now know that most of the dolphin's biosonar
output is ultrasonic. Consequently, the primary value in these early recordings has been the initial discovery of the widespread use of these sounds by
odontocetes. Another, perhaps unexpected, aspect of these early recordings
that may promote confusion in new students of this field but that may also
shed some light on our understanding of the click generation process will
be discussed later.
The scientific literature that discusses and attempts to define the various
types of sounds odontocetes produce has, at times, been based largely upon
subjective experiences and, as such, can be variable and inconsistent (see a
TW. Cranford
first restrict the definition of the sounds whose generation we will consider.
For the purposes of this chapter, we can immediately exclude all sounds
produced by percussive activity upon the surface of the water by a part of
the body (flukes, flippers, torso, etc.). We will also not consider sounds that
might be produced as a by-product of bodily functions, like locomotion,
digestion, or respiration. For our purposes, only internally generated sounds
that function, or might function, in echolocation, prey capture, or social
facilitation will be considered.
All odontocetes apparently produce brief acoustic pulses or "clicks".
These sounds are generally characterized as broadband signals, although
specific signal characteristics vary with the recording circumstances, species,
and the individual in question. Clicks, when arranged in moderately rapid
trains of a few to several hundred per second, have been widely implicated
in odontocete echolocation behavior. A large body of literature now describes the acoustic characteristics and resolution capability of the odontocete echolocation system (Turner and Norris 1966; Evans and Powell 1967;
Nachtigall1980;Au 1990,1993). Most of this work has been conducted with
the bottlenose dolphin, Tursiops truncatus, but some work has been carried
out with other odontocete species (Norris 1975; Popper 1980; Au 1993).
1.1 Types of Sounds
Since the lives of most cetaceans are carried out in an oceanic world wholly
unfamiliar to most of us, we often know little of their behavior and natural
history. Gathering sound recordings from their underwater world is easy
relative to gathering other types of information, so it is commonly one of
the first pieces we have from the puzzling lives they lead. For pelagic species,
there is often little else. A good deal of the early literature catalogues the
first sounds recorded from free-ranging animals (Schevill, 1964). In most of
these instances, it was difficult or impossible to know the animal's orientation, direction, or distance from the recording hydrophone. Since signal
amplitude and frequency content usually varies with position in a sound
field (especially for the distinctly directional clicks), hydrophone placement
relative to the sound source will influence subsequent recordings (Au 1980,
1993; Au et al. 1986). In addition, recording techniques have not been standardized and few of the recordings made before 1970 extended very far into
the ultrasonic range. We now know that most of the dolphin's biosonar
output is ultrasonic. Consequently, the primary value in these early recordings has been the initial discovery of the widespread use of these sounds by
odontocetes. Another, perhaps unexpected, aspect of these early recordings
that may promote confusion in new students of this field but that may also
shed some light on our understanding of the click generation process will
be discussed later.
The scientific literature that discusses and attempts to define the various
types of sounds odontocetes produce has, at times, been based largely upon
subjective experiences and, as such, can be variable and inconsistent (see a