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 Figure 5. Sound modeling south of Long Island, NY, using conditions from an ocean model. Top left: surface current and temperature snapshot showing eddies. Top right: derived parameter (metric) thought to govern the horizontal refraction of sound, which could create areas of strong sound and shadow zones. Bottom: simulation results for sound propagating outward from a 1,500 Hz source at 10 m depth at 2 times, 3 hours apart. Energy reduction in the 10-m depth plane is shown in dB re source level. The up/down heaving of the sound speed layering from tide- driven internal waves changes the refraction and gives a strong reduction of sound energy near the surface over the time interval.
 underway. Because the true ocean will differ, the background estimates could be used to plan a survey that would establish the full impact of the local dynamics on acoustic propagation.
This same area is known to support internal tides (tide- forced internal waves) with scales as short as a few kilometers. Because these features are small and changing rapidly, they are challenging to study with in situ sampling methods. How- ever, because they affect sound speed, they have an acoustic impact. Figure 5 shows how a modeled sound level changes over a three-hour period. After 12 hours the simulated sound returns close to the initial conditions, confirming the strong modeled changes to be tidally related. The effects of the full spectrum of eddies and internal waves on sound remain to
be established, with some aspects treated deterministically and some stochastically (Colosi, 2016; Duda, 2017). For example, a system using stochastic ocean simulations and acoustic simulations can infer the most likely environments (Coelho et al., 2015).
Noise Modeling
Computational modeling of underwater noise can be broken down into the component parts of determining the sources of noise and their parameters, tracking the sources and updating their parameters, propagating each sound, and then adding fields or field energies. Example sources would be ships, and parameters would be sound signatures as a function of speed, location, and speed.
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