9. Echolocation in Dolphins
379
source level was only 12dB. The maximum average peak-to-peak source
level was recorded at 227.6dB (case h). The largest single click measured
was 230 dB, emitted by Heptuna (case h). The largest single click measured
for D. leucas was approximately 225 dB (Au et al. 1987) and for P. crassidens, 228dB (Thomas and Turl 1990). M~hl et al. (1990) measured highintensity clicks from a narwhal (Monodon monoceros) that had similar
amplitUde levels as those emitted by odontocetes in Kaneohe Bay.
3. Echolocation Capabilities
The capabilities of any echolocation system are usually divided into two
general categories; target detection and target discrimination. The target
detection capabilities of an echolocating dolphin will be limited by ambient
or artificial noise, by reverberation, and by its own hearing sensitivity.
Echolocation experiments to determine the target detection capabilities of
dolphins in noise and reverberation have been performed by Au (1993).
However, target detection in a quiet environment in which a dolphin cannot
hear the ambient noise in the frequency range of its echolocation signals
has not been considered. Many different types of discrimination experiments have been performed (see Nachtigall1980 and Au 1993).
3.1 Target Detection in Noise
Three different types of target detection in noise experiments have been
performed with dolphins. The first type is the simple one in which a specific
target is moved progressively farther away from the position of an echolocating dolphin. Eventually, the target will be sufficiently far away that the
echoes from the target will have amplitudes that are similar to the amplitudes of the ambient noise, making the target progressively harder to detect.
The maximum detection range of two T truncatus was determined in
Kaneohe Bay by Murchison (1980) using a 2.54-cm diameter solid steel
sphere and by Au and Snyder (1980) using a 7.62-cm diameter water-filled
sphere. In both studies an overhead suspension system with a movable
trolley and pulleys was used to vary target range between two poles spaced
200m apart. 1be results of both experiments are displayed in Figure 9.10
with correct detection and false alarm rates plotted as a function of the
target range. The correct detection rate is the percentage of time the animal
correctly reported the presence of the target for target-present only trials.
The false alarm rate is the percentage of time the animal wrongly reported
that the target was present in target-absent trials. A common convention in
the sonar field is to use the 50% correct detection rate to define the detection threshold of a sonar (Urick 1983). The target detection threshold for
the 2.54-cm and 7.62-cm diameter spheres were 73 and 113m, respectively.
The animals' results for the two different targets are relatively consistent if
379
source level was only 12dB. The maximum average peak-to-peak source
level was recorded at 227.6dB (case h). The largest single click measured
was 230 dB, emitted by Heptuna (case h). The largest single click measured
for D. leucas was approximately 225 dB (Au et al. 1987) and for P. crassidens, 228dB (Thomas and Turl 1990). M~hl et al. (1990) measured highintensity clicks from a narwhal (Monodon monoceros) that had similar
amplitUde levels as those emitted by odontocetes in Kaneohe Bay.
3. Echolocation Capabilities
The capabilities of any echolocation system are usually divided into two
general categories; target detection and target discrimination. The target
detection capabilities of an echolocating dolphin will be limited by ambient
or artificial noise, by reverberation, and by its own hearing sensitivity.
Echolocation experiments to determine the target detection capabilities of
dolphins in noise and reverberation have been performed by Au (1993).
However, target detection in a quiet environment in which a dolphin cannot
hear the ambient noise in the frequency range of its echolocation signals
has not been considered. Many different types of discrimination experiments have been performed (see Nachtigall1980 and Au 1993).
3.1 Target Detection in Noise
Three different types of target detection in noise experiments have been
performed with dolphins. The first type is the simple one in which a specific
target is moved progressively farther away from the position of an echolocating dolphin. Eventually, the target will be sufficiently far away that the
echoes from the target will have amplitudes that are similar to the amplitudes of the ambient noise, making the target progressively harder to detect.
The maximum detection range of two T truncatus was determined in
Kaneohe Bay by Murchison (1980) using a 2.54-cm diameter solid steel
sphere and by Au and Snyder (1980) using a 7.62-cm diameter water-filled
sphere. In both studies an overhead suspension system with a movable
trolley and pulleys was used to vary target range between two poles spaced
200m apart. 1be results of both experiments are displayed in Figure 9.10
with correct detection and false alarm rates plotted as a function of the
target range. The correct detection rate is the percentage of time the animal
correctly reported the presence of the target for target-present only trials.
The false alarm rate is the percentage of time the animal wrongly reported
that the target was present in target-absent trials. A common convention in
the sonar field is to use the 50% correct detection rate to define the detection threshold of a sonar (Urick 1983). The target detection threshold for
the 2.54-cm and 7.62-cm diameter spheres were 73 and 113m, respectively.
The animals' results for the two different targets are relatively consistent if