370
w.w.L. Au
center frequency plotted against source level. The solid line in the figure is
a linear regression curve fit of the data and has a correlation coefficient
of 0.80.
The bimodal property of the echolocation signals in Figure 9.2 seems to
suggest that the response of the sound generator may be determined by the
intensity of the driving force that eventually causes an echolocation signal
to be produced. When the intensity of the driving force is low, only signals
with low amplitudes and low-frequency peaks are produced. Therefore,
in small tanks, the signals resemble the tank signal of Figure 9.1, and the
bimodal feature is suppressed since the high-frequency portion of the
source cannot "kick in" for a low driving force. As the driving force
increases to a moderate level, the low-frequency peak also increases in
amplitude, and the high-frequency portion of the signal begins to kick in.
As the driving force increases further, the amplitude of the high-frequency
peak becomes larger than that of the low-frequency peak, resulting in type
III signals. As the driving force continues to increase to a high level, the
amplitude of the high-frequency peak becomes much greater than the
amplitude of the low-frequency peak and completely dominates the lowfrequency peak, causing the bimodal feature to be suppressed.
2.2 Echolocation Signals of Dolphins That
Do Not Whistle
The second class of echolocation signals are produced by dolphins and porpoises that have not been shown to emit any whistle signals. Their sound
production is restricted to click signals that are high frequency, narrow
bandwidth, and low intensity as can be seen in the example of Figure 9.4
showing echolocation signals used by five of these species. The duration of
the signals is much longer, with many oscillations, and the spectra much narrower (bandwidths are less than half as wide) than the echolocation signals
used by whistle-producing dolphins. The signals of the harbor porpoise,
Phocoena phocoena, (MlIlhl and Andersen 1973; Kamminga and Wiersma
1981; Hatakeyama et al. 1988) and the finless porpoise, Neophocaena phocaenoides, (Kamminga 1988) are similar in shape and time duration with
peak frequencies between 120 and 140kHz. Echolocation signals with peak
frequencies between 90 and 115 kHz have been measured for a Dall's
porpoise, Phocoenoides dalli, in a tank (Hatakeyama and Soeda 1990)
and with peak frequencies of 120 to 160kHz in the open sea (Awbrey
et al. 1979; Hatakeyama and Soeda 1990). Kamminga and Wiersma (1981)
measured peak frequencies of about 124kHz for a Commerson's dolphin,
Cephalorhynchus commersonii, in a tank and Evans et al. (1988) measured
peak frequencies of 130 to 140kHz in the open sea. The pygmy sperm
whale (Kogia sp.) also produce high-frequency (peak frequencies around
130kHz), narrow bandwidth click signals similar to the other nonwhistling
odontocetes (Carder et al. 1995).
w.w.L. Au
center frequency plotted against source level. The solid line in the figure is
a linear regression curve fit of the data and has a correlation coefficient
of 0.80.
The bimodal property of the echolocation signals in Figure 9.2 seems to
suggest that the response of the sound generator may be determined by the
intensity of the driving force that eventually causes an echolocation signal
to be produced. When the intensity of the driving force is low, only signals
with low amplitudes and low-frequency peaks are produced. Therefore,
in small tanks, the signals resemble the tank signal of Figure 9.1, and the
bimodal feature is suppressed since the high-frequency portion of the
source cannot "kick in" for a low driving force. As the driving force
increases to a moderate level, the low-frequency peak also increases in
amplitude, and the high-frequency portion of the signal begins to kick in.
As the driving force increases further, the amplitude of the high-frequency
peak becomes larger than that of the low-frequency peak, resulting in type
III signals. As the driving force continues to increase to a high level, the
amplitude of the high-frequency peak becomes much greater than the
amplitude of the low-frequency peak and completely dominates the lowfrequency peak, causing the bimodal feature to be suppressed.
2.2 Echolocation Signals of Dolphins That
Do Not Whistle
The second class of echolocation signals are produced by dolphins and porpoises that have not been shown to emit any whistle signals. Their sound
production is restricted to click signals that are high frequency, narrow
bandwidth, and low intensity as can be seen in the example of Figure 9.4
showing echolocation signals used by five of these species. The duration of
the signals is much longer, with many oscillations, and the spectra much narrower (bandwidths are less than half as wide) than the echolocation signals
used by whistle-producing dolphins. The signals of the harbor porpoise,
Phocoena phocoena, (MlIlhl and Andersen 1973; Kamminga and Wiersma
1981; Hatakeyama et al. 1988) and the finless porpoise, Neophocaena phocaenoides, (Kamminga 1988) are similar in shape and time duration with
peak frequencies between 120 and 140kHz. Echolocation signals with peak
frequencies between 90 and 115 kHz have been measured for a Dall's
porpoise, Phocoenoides dalli, in a tank (Hatakeyama and Soeda 1990)
and with peak frequencies of 120 to 160kHz in the open sea (Awbrey
et al. 1979; Hatakeyama and Soeda 1990). Kamminga and Wiersma (1981)
measured peak frequencies of about 124kHz for a Commerson's dolphin,
Cephalorhynchus commersonii, in a tank and Evans et al. (1988) measured
peak frequencies of 130 to 140kHz in the open sea. The pygmy sperm
whale (Kogia sp.) also produce high-frequency (peak frequencies around
130kHz), narrow bandwidth click signals similar to the other nonwhistling
odontocetes (Carder et al. 1995).