9. Echolocation in Dolphins
371
The signals of the species depicted in Figure 9.4 often have multiple components as shown in (c) for the Dall's porpoise and (d) for the Commerson's dolphin. Kamminga and Wiersma (1981) also presented some P
phocoena clicks that consisted of multiple pulses. Ripples in the spectrum
of the Dall's porpoise and the Commerson's dolphins come from the multiple component nature of the signals. Double pulses of equal amplitudes
were also often recorded from the Commerson's (Evans et al. 1988) and
Hector's dolphin, Cephalorhynchus hectori (Dawson 1988). However, these
double pulses may be the result of a surface-reflected component, which is
often difficult to discern. One reason for the use of long-duration, narrow
bandwidth signals may be related to the relatively small size of the porpoises in the family Phocoenidae and the dolphins in the genus
Cephalorhynchus, since for a given amplitude the energy in a signal is
directly proportional to duration.
2.3 Some Properties of Echolocation Signals
2.3.1 Transmission Beam Pattern
The outgoing echolocation signals of dolphins and other odontocetes are
projected in a directional beam that has been measured in the vertical and
horizontal planes for the bottlenose dolphin (Au et al. 1978, 1980, 1986),
beluga whale (Au et al. 1987) and the false killer whale (Au et al. 1995).
The composite beam pattern from the three measurements on T. truncatus
along with the averaged waveform from a single trial as measured by five
hydrophones, are shown in Figure 9.5. The beam patterns for the beluga and
false killer whales are shown in Figure 9.6.
The beamwidth, defined as the angle between the -3dB points, for the
bottlenose dolphin was approximately 10.2° in the vertical plane and 9.7°
in the horizontal plane. The beamwidth for the beluga whale was approximately 6.5° in both planes. Four beam patterns corresponding to the four
signal types described in Figure 9.2b are shown for the false killer whale
in the lower panels of Figure 9.6. For the highest frequency (type-IV signal)
the 3-dB beamwidth in the vertical plane was approximately 10° and 7° in
the horizontal plane. The beam axis in the vertical plane for the bottlenose
dolphin and the beluga whale was +5° above the horizontal plane. For the
false killer whale, 49% of the beam axis was at 0° and 32% at _5°.
The waveforms detected by the various hydrophones in Figure 9.5 indicate that signals measured away from the beam axis will be distorted in
comparison to the signals measured on the beam axis. The further away
from the beam axis, the more distorted the signals will be. The frequencies
shown with each waveform are the frequencies of local maxima in the spectrum. At some angles the averaged signal had multiple peaks in the spectrum and these peaks are listed in an order of decreasing amplitude. The
presence of multipaths is apparent for the signals measured away from the
371
The signals of the species depicted in Figure 9.4 often have multiple components as shown in (c) for the Dall's porpoise and (d) for the Commerson's dolphin. Kamminga and Wiersma (1981) also presented some P
phocoena clicks that consisted of multiple pulses. Ripples in the spectrum
of the Dall's porpoise and the Commerson's dolphins come from the multiple component nature of the signals. Double pulses of equal amplitudes
were also often recorded from the Commerson's (Evans et al. 1988) and
Hector's dolphin, Cephalorhynchus hectori (Dawson 1988). However, these
double pulses may be the result of a surface-reflected component, which is
often difficult to discern. One reason for the use of long-duration, narrow
bandwidth signals may be related to the relatively small size of the porpoises in the family Phocoenidae and the dolphins in the genus
Cephalorhynchus, since for a given amplitude the energy in a signal is
directly proportional to duration.
2.3 Some Properties of Echolocation Signals
2.3.1 Transmission Beam Pattern
The outgoing echolocation signals of dolphins and other odontocetes are
projected in a directional beam that has been measured in the vertical and
horizontal planes for the bottlenose dolphin (Au et al. 1978, 1980, 1986),
beluga whale (Au et al. 1987) and the false killer whale (Au et al. 1995).
The composite beam pattern from the three measurements on T. truncatus
along with the averaged waveform from a single trial as measured by five
hydrophones, are shown in Figure 9.5. The beam patterns for the beluga and
false killer whales are shown in Figure 9.6.
The beamwidth, defined as the angle between the -3dB points, for the
bottlenose dolphin was approximately 10.2° in the vertical plane and 9.7°
in the horizontal plane. The beamwidth for the beluga whale was approximately 6.5° in both planes. Four beam patterns corresponding to the four
signal types described in Figure 9.2b are shown for the false killer whale
in the lower panels of Figure 9.6. For the highest frequency (type-IV signal)
the 3-dB beamwidth in the vertical plane was approximately 10° and 7° in
the horizontal plane. The beam axis in the vertical plane for the bottlenose
dolphin and the beluga whale was +5° above the horizontal plane. For the
false killer whale, 49% of the beam axis was at 0° and 32% at _5°.
The waveforms detected by the various hydrophones in Figure 9.5 indicate that signals measured away from the beam axis will be distorted in
comparison to the signals measured on the beam axis. The further away
from the beam axis, the more distorted the signals will be. The frequencies
shown with each waveform are the frequencies of local maxima in the spectrum. At some angles the averaged signal had multiple peaks in the spectrum and these peaks are listed in an order of decreasing amplitude. The
presence of multipaths is apparent for the signals measured away from the