This chapter describes the utilization of underwater acoustic models for the evaluation of marine-system noise impacts associated with the installation and operation of marine-hydrokinetic energy (MHK) devices, particularly in coastal oceans. Coastal environments are generally characterized by high spatial and temporal variabilities, which make them very complex acoustic environments. Underwater acoustic models serve as enabling tools for assessing noise impacts on marine systems by generating analytical metrics useful in managing coastal resources. This review is set in the context of an underwater soundscape, which is a combination of sounds that characterize, or arise from, an ocean environment. The study of a soundscape is sometimes referred to as acoustic ecology. Disruption of the natural acoustic environment results in noise pollution : The potential effects of anthropogenic sound sources on marine mammals and fish that could include auditory damage. Changes in the ocean soundscape have also been driven by natural factors arising from climate change, including ocean acidification. The field of underwater acoustics enables us to observe and predict the behavior of this soundscape and the response of the natural acoustic environment to noise pollution. Underwater acoustics entails the development and employment of acoustical methods to image underwater features, to communicate information via the oceanic waveguide, or to measure oceanic properties. Modeling tools traditionally used in underwater acoustics have undergone a necessary transformation to respond to the rapidly changing requirements imposed by this dynamic soundscape. Additional advances have been achieved using energy-flux techniques that can simplify the interpretation of sound-channel models. Nonintrusive measurement approaches include new acoustic-transmission options to minimize impacts on aquatic life. Applied underwater acoustic modeling technologies have further evolved over the past several years in response to new regulatory initiatives that have placed restrictions on the uses of sound in the ocean. The mitigation of marine-mammal endangerment is now an integral consideration in acoustic-system design, installation, and operation. Marine-mammal protection research has focused on simulating anthropogenic sound sources including seismic-exploration activity, merchant shipping traffic , and a new generation of multistatic naval sonar systems. Additional sources derive from ocean-renewable energy resources, including the deployment of wind farms, tidal turbines , and wave-energy devices . Many underwater acoustic models presently used in environmental impact assessments consider only the sound-pressure component of sound, which is the means by which marine mammals hear; however, the primary mechanism by which fish and invertebrate species detect sound is through the particle-motion component of sound. To assist practitioners in the proper usage of acoustic models for assessing the impacts of MHK device noise on marine systems, selection guidance is provided for the current inventory of underwater acoustic propagation and noise models.
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