366
w.w.L. Au
discussed first, followed by a discussion of the second category of echolocation signals associated with dolphins that do not produce whistles. The
differences between these two classes of echolocation signals will be
obvious.
2.1 Echolocation Signals of Dolphins
Capable of Whistling
Most of the dolphin species are able to produce whistle signals. Among
some of the species in this category in which echolocation signals have been
measured are the bottlenose dolphin (Tursiops sp.), beluga whale (Delphinapterus leucas), killer whale (Orcinus orca), false killer whale (Pseudorca
crassidens), Pacific whitesided dolphin (Lagenorhynchus obliguidens),
Amazon River dolphin (Inia geoffrensis), Risso's dolphin (Grampus
griseus), tucuxi (Sotatia fiuviatilis), Atlantic spotted dolphin (Stene/la
frontalis), Pacific spotted dolphin (Stene/la attenuata), spinner dolphin
(Stene/la longirostris), pilot whale (Globicephala sp.), rough tooth dolphin
(Steno bredanensis), and Chinese river dolphin (Lipotes vexillifer) (Au
1993). However, most of the available data have been obtained for three
species, the bottlenose dolphin, beluga whale, and false killer whale.
Evans (1973) reviewed the state of knowledge of dolphin echolocation
and reported echolocation signals with peak frequency (frequency of
maximum energy) between 30 and 60kHz, and peak-to-peak source levels
(sound pressure level 1m from the animal) of about 170 to 180 dB for Tursiops truncatus. Prior to 1973, most echolocation signals of T. truncatus were
measured in relatively small tanks, and it was generally believed that peak
frequencies of the sonar signals were as reported by Evans (1973). It was
not until the study of Au et al. (1974) that certain features of biosonar
signals used by T. truncatus and other dolphins in open waters were discovered. We discovered that the signals had peak frequencies between 120
and 130 kHz, which were more than an octave higher than previously
reported peak frequencies between 30 and 60kHz (Evans 1973). We also
measured an average peak-to-peak click source level on the order of
220dB re 1~Pa at 1m, which represents a level 30 to 50dB higher than previously measured for T. truncatus. Examples of typical echolocation signals
emitted by T. truncatus are shown in Figure 9.1, for two situations. The top
signal is typical of signals used in the open waters of Kaneohe Bay, Oahu,
Hawaii, and the second signal represents typical signals for a T. truncatus
in a tank. Signals measured in Kaneohe Bay regularly have durations
between 40 and 70 ~s, 4 to 10 positive excursions, peak frequencies between
110 and 130kHz, and peak-to-peak source levels between 210 and 228dB
re 1~Pa. The signals in Figure 9.1 are not drawn to scale; if they were, the
tank signal would resemble a flat line. We attributed the use of highfrequency echolocation signals to the high ambient noise environment of
Kaneohe Bay caused by snapping shrimp.
w.w.L. Au
discussed first, followed by a discussion of the second category of echolocation signals associated with dolphins that do not produce whistles. The
differences between these two classes of echolocation signals will be
obvious.
2.1 Echolocation Signals of Dolphins
Capable of Whistling
Most of the dolphin species are able to produce whistle signals. Among
some of the species in this category in which echolocation signals have been
measured are the bottlenose dolphin (Tursiops sp.), beluga whale (Delphinapterus leucas), killer whale (Orcinus orca), false killer whale (Pseudorca
crassidens), Pacific whitesided dolphin (Lagenorhynchus obliguidens),
Amazon River dolphin (Inia geoffrensis), Risso's dolphin (Grampus
griseus), tucuxi (Sotatia fiuviatilis), Atlantic spotted dolphin (Stene/la
frontalis), Pacific spotted dolphin (Stene/la attenuata), spinner dolphin
(Stene/la longirostris), pilot whale (Globicephala sp.), rough tooth dolphin
(Steno bredanensis), and Chinese river dolphin (Lipotes vexillifer) (Au
1993). However, most of the available data have been obtained for three
species, the bottlenose dolphin, beluga whale, and false killer whale.
Evans (1973) reviewed the state of knowledge of dolphin echolocation
and reported echolocation signals with peak frequency (frequency of
maximum energy) between 30 and 60kHz, and peak-to-peak source levels
(sound pressure level 1m from the animal) of about 170 to 180 dB for Tursiops truncatus. Prior to 1973, most echolocation signals of T. truncatus were
measured in relatively small tanks, and it was generally believed that peak
frequencies of the sonar signals were as reported by Evans (1973). It was
not until the study of Au et al. (1974) that certain features of biosonar
signals used by T. truncatus and other dolphins in open waters were discovered. We discovered that the signals had peak frequencies between 120
and 130 kHz, which were more than an octave higher than previously
reported peak frequencies between 30 and 60kHz (Evans 1973). We also
measured an average peak-to-peak click source level on the order of
220dB re 1~Pa at 1m, which represents a level 30 to 50dB higher than previously measured for T. truncatus. Examples of typical echolocation signals
emitted by T. truncatus are shown in Figure 9.1, for two situations. The top
signal is typical of signals used in the open waters of Kaneohe Bay, Oahu,
Hawaii, and the second signal represents typical signals for a T. truncatus
in a tank. Signals measured in Kaneohe Bay regularly have durations
between 40 and 70 ~s, 4 to 10 positive excursions, peak frequencies between
110 and 130kHz, and peak-to-peak source levels between 210 and 228dB
re 1~Pa. The signals in Figure 9.1 are not drawn to scale; if they were, the
tank signal would resemble a flat line. We attributed the use of highfrequency echolocation signals to the high ambient noise environment of
Kaneohe Bay caused by snapping shrimp.