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Chapter eleven: Acoustics
in the frequency of signals outside the spectrum of interfering noise, all serve to increase
the signal-to-noise ratio of the caller’s signal. In this regard, what limited evidence exists
suggests that marine mammals deal with environmental noise in the same manner that
terrestrial mammals do.
11.4 Tools and methods
The sound projector, microphone (for amphibious species), and hydrophone (underwater microphone) are mainstays in the arsenal of the marine mammal bioacoustician.
Projectors and hydrophones are transducers, that is, they either change an electrical
signal into motion (e.g., projector) or they change variations in pressure (i.e., the sound
pressure) into an electrical signal (e.g., hydrophone). Many underwater transducers can
do both, to some degree. There are many types of underwater projectors and receivers,
and each of these has different transmission capabilities or sensitivities at different
frequencies based upon their material composition and design. For this reason, proper
calibration of a transducer is critical to any study that desires to quantify sound or
project a desired level of sound. In line with the hydrophone is generally an amplifier for increasing the signal level and a filter. Filtering is often required in order to
eliminate unwanted signals or to prevent aliasing, which is a problem that occurs when
high-frequency signals are not digitally sampled at a high enough rate to adequately
characterize the signal.
In passive acoustic monitoring, which is common to the study of free-ranging marine
mammals, hydrophone arrays are used to record the sound pressure level of received
marine mammal sounds. Historically, as few as one hydrophone has been used to characterize sounds produced by marine mammals. However, arrays of hydrophones, appropriately placed in space, can be used for more advanced procedures such as localizing a
phonating animal (e.g., hyperbolic fixing). These techniques are useful for tracking vocal
animals at sea, monitoring their acoustic behavior, and possibly assisting with estimates
of population size (e.g., Marques et al. 2013; Stanistreet et al. 2013). In combination, this
type of information provides valuable insight into the animal’s ecology. With the increased
awareness of the potential for anthropogenic sound to impact marine mammals, the techniques of localizing, tracking, and monitoring acoustic behavior have become important
methods for relating particular types of sound exposure to changes in marine mammal
behavior. For example, work with bottom-mounted hydrophone arrays has demonstrated
that beaked whales cease the production of foraging-related echolocation signals when
exposed to certain levels of sonar (Moretti et al. 2014).
Underwater sound transducers are also a mainstay of work with animals under
human care. Indeed, almost all of the research on underwater hearing has involved the
use of these tools in psychophysical research protocols. Psychophysics is the field of study
related to how a physical quantity (e.g., sound frequency) translates to the subjective perception of the physical quantity. Psychophysical procedures involve training an animal
to perform a behavior in response to a given stimulus of which one or more of its physical properties are varied. The process is behavioral in that the animal is asked to make a
decision, with the question asked and the type of decision desired dictated by the experimental design. For example, until ~2005 when electrophysiological techniques became
more common for studying hearing in odontocetes (mentioned later), information about
the frequency range of hearing and hearing sensitivity were nearly always determined
through psychophysical means. A simple form of a psychophysical hearing test relates
the amplitude of a sound, typically measured as the sound pressure level, to an animal’s
Chapter eleven: Acoustics
in the frequency of signals outside the spectrum of interfering noise, all serve to increase
the signal-to-noise ratio of the caller’s signal. In this regard, what limited evidence exists
suggests that marine mammals deal with environmental noise in the same manner that
terrestrial mammals do.
11.4 Tools and methods
The sound projector, microphone (for amphibious species), and hydrophone (underwater microphone) are mainstays in the arsenal of the marine mammal bioacoustician.
Projectors and hydrophones are transducers, that is, they either change an electrical
signal into motion (e.g., projector) or they change variations in pressure (i.e., the sound
pressure) into an electrical signal (e.g., hydrophone). Many underwater transducers can
do both, to some degree. There are many types of underwater projectors and receivers,
and each of these has different transmission capabilities or sensitivities at different
frequencies based upon their material composition and design. For this reason, proper
calibration of a transducer is critical to any study that desires to quantify sound or
project a desired level of sound. In line with the hydrophone is generally an amplifier for increasing the signal level and a filter. Filtering is often required in order to
eliminate unwanted signals or to prevent aliasing, which is a problem that occurs when
high-frequency signals are not digitally sampled at a high enough rate to adequately
characterize the signal.
In passive acoustic monitoring, which is common to the study of free-ranging marine
mammals, hydrophone arrays are used to record the sound pressure level of received
marine mammal sounds. Historically, as few as one hydrophone has been used to characterize sounds produced by marine mammals. However, arrays of hydrophones, appropriately placed in space, can be used for more advanced procedures such as localizing a
phonating animal (e.g., hyperbolic fixing). These techniques are useful for tracking vocal
animals at sea, monitoring their acoustic behavior, and possibly assisting with estimates
of population size (e.g., Marques et al. 2013; Stanistreet et al. 2013). In combination, this
type of information provides valuable insight into the animal’s ecology. With the increased
awareness of the potential for anthropogenic sound to impact marine mammals, the techniques of localizing, tracking, and monitoring acoustic behavior have become important
methods for relating particular types of sound exposure to changes in marine mammal
behavior. For example, work with bottom-mounted hydrophone arrays has demonstrated
that beaked whales cease the production of foraging-related echolocation signals when
exposed to certain levels of sonar (Moretti et al. 2014).
Underwater sound transducers are also a mainstay of work with animals under
human care. Indeed, almost all of the research on underwater hearing has involved the
use of these tools in psychophysical research protocols. Psychophysics is the field of study
related to how a physical quantity (e.g., sound frequency) translates to the subjective perception of the physical quantity. Psychophysical procedures involve training an animal
to perform a behavior in response to a given stimulus of which one or more of its physical properties are varied. The process is behavioral in that the animal is asked to make a
decision, with the question asked and the type of decision desired dictated by the experimental design. For example, until ~2005 when electrophysiological techniques became
more common for studying hearing in odontocetes (mentioned later), information about
the frequency range of hearing and hearing sensitivity were nearly always determined
through psychophysical means. A simple form of a psychophysical hearing test relates
the amplitude of a sound, typically measured as the sound pressure level, to an animal’s