156
such as Franciscana dolphins. Finally, the analog voltage is digitized by the converter to create a discrete digital time series.
There is a variety of different devices that can be used to obtain sound recordings
to study the acoustic repertoire of cetaceans: dipping hydrophones, towed hydrophone arrays, sonobuoys, bottom-moored autonomous recorders and acoustic tags.
Dipping Hydrophones are commonly submerged from a fixed platform or a vessel with the engine off. They can consist of a single hydrophone or of several hydrophones which allow characterization of source levels and directionality. The ideal
situation when using this kind of devices for characterization is that researchers
make the recordings when the species of interest is visually detected within a reasonable distance (a few meters up to a few kilometers depending on the species), in
absence of any other odontocete species, and so the recorded sounds can be assigned
to that species with a high level of confidence. Some advantages of this setup are
that they are easily handled and can be used in shallow waters such as inshore bays
or rivers. A disadvantage is the requirement of at least one person that should be at
the site of deployment all the time while recording, which also limits the use of
these gears in remote areas where navigation using small vessels is not suitable.
Additionally, with the single hydrophone arrangement no accurate localization is
possible, and source level estimates cannot be accurate.
Towed Hydrophone Arrays are usually towed from a moving vessel and used to
localize vocalizing individuals through real-time calculations using the time difference of arrival of an acoustic signal at various hydrophones in the array. The key
using this methodology is to obtain a visual match of the recorded signal to identify
the species producing that particular sound. These systems have the advantage that
they allow passive acoustic monitoring of species during the track line of large vessels. With this system localization is possible. Some cons are that they need to be
towed a long distance far away from the moving vessel in order to avoid signals
getting saturated with the vessel noise and so large vessels are required and they are
not suitable for inshore or shallow waters.
Sonobuoys are designed as expendable devices to be dropped from a vessel and
the signals detected by the hydrophone are transmitted by a radio transmitter to a
receiver on the vessel. A specialized type of sonobuoys called DIFAR also allows to
determine the direction of the sounds relative to the sonobuoys. The benefit of this
is that using 3 or more of these instruments it is possible to determine the exact location of the sound source and then correlate the recorded signal to visual observations of cetacean species (mostly mysticetes, due to the limitations of sonobuoys to
record high frequencies) at that location. This will allow us to associate which species produce certain sounds and also calculate the source levels of the signals for
which the distance of the vocalizing animal to the hydrophone is required at a certain background noise level. Knowing the source level of the signals helps to optimize detection and classification routines frequently used for PAM, since sounds
from different species may not only be distinguishable by their frequency content,
bandwidth and durations but also by different source levels of the emitted sounds
M.L. Melcón et al.
such as Franciscana dolphins. Finally, the analog voltage is digitized by the converter to create a discrete digital time series.
There is a variety of different devices that can be used to obtain sound recordings
to study the acoustic repertoire of cetaceans: dipping hydrophones, towed hydrophone arrays, sonobuoys, bottom-moored autonomous recorders and acoustic tags.
Dipping Hydrophones are commonly submerged from a fixed platform or a vessel with the engine off. They can consist of a single hydrophone or of several hydrophones which allow characterization of source levels and directionality. The ideal
situation when using this kind of devices for characterization is that researchers
make the recordings when the species of interest is visually detected within a reasonable distance (a few meters up to a few kilometers depending on the species), in
absence of any other odontocete species, and so the recorded sounds can be assigned
to that species with a high level of confidence. Some advantages of this setup are
that they are easily handled and can be used in shallow waters such as inshore bays
or rivers. A disadvantage is the requirement of at least one person that should be at
the site of deployment all the time while recording, which also limits the use of
these gears in remote areas where navigation using small vessels is not suitable.
Additionally, with the single hydrophone arrangement no accurate localization is
possible, and source level estimates cannot be accurate.
Towed Hydrophone Arrays are usually towed from a moving vessel and used to
localize vocalizing individuals through real-time calculations using the time difference of arrival of an acoustic signal at various hydrophones in the array. The key
using this methodology is to obtain a visual match of the recorded signal to identify
the species producing that particular sound. These systems have the advantage that
they allow passive acoustic monitoring of species during the track line of large vessels. With this system localization is possible. Some cons are that they need to be
towed a long distance far away from the moving vessel in order to avoid signals
getting saturated with the vessel noise and so large vessels are required and they are
not suitable for inshore or shallow waters.
Sonobuoys are designed as expendable devices to be dropped from a vessel and
the signals detected by the hydrophone are transmitted by a radio transmitter to a
receiver on the vessel. A specialized type of sonobuoys called DIFAR also allows to
determine the direction of the sounds relative to the sonobuoys. The benefit of this
is that using 3 or more of these instruments it is possible to determine the exact location of the sound source and then correlate the recorded signal to visual observations of cetacean species (mostly mysticetes, due to the limitations of sonobuoys to
record high frequencies) at that location. This will allow us to associate which species produce certain sounds and also calculate the source levels of the signals for
which the distance of the vocalizing animal to the hydrophone is required at a certain background noise level. Knowing the source level of the signals helps to optimize detection and classification routines frequently used for PAM, since sounds
from different species may not only be distinguishable by their frequency content,
bandwidth and durations but also by different source levels of the emitted sounds
M.L. Melcón et al.