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Advanced solver options

The Solver Advanced tab in the preferences form allows for more detailed control of solver behavior. Extensive documentation on all types of solvers is available in Reference [3].

General Options

  • Autosave: Save the active project file after each solve, if the project has a save file.
  • Remove levels below seafloor: dBSeaModes and dBSeaPE calculate levels in the seafloor as part of a solve. By default, dBSea ignores these levels.
  • Levels must decrease with distance from the source: The solver output is often more detailed than required. This option ensures that sound levels decrease monotonically with distance from each source, which is useful when a conservative design is required. This can be applied or removed without recalculating levels in the scenario.
  • Stop marching solution upon reaching land: Whenever land is encountered, the solver will stop the propagation calculation. This is useful for limiting computational load next to a large landmass but is not suitable when many small islands are present.
  • Frequency oversampling: dBSea can solve for multiple evenly spaced frequencies in each frequency band. For example, if the band center is 1 kHz, the oversampling factor is 5, and the bandwidth is 1 octave, dBSea will solve at 757.9, 870.6, 1000, 1148.7, and 1319.5 Hz, and then average the results to give the level for that band. This option is useful for modeling broadband noise sources and/or fast-changing environments. It increases computation time.
  • Radial smoothing factor and exclusion zone radial smoothing factor: The solver output is often more detailed than required. dBSea smooths results or exclusion zones radially around each source, with a triangular kernel of length 2×factor + 1. Set the factor to 0 for no smoothing.

Maximum Attenuation Layer Thickness (in wavelengths)

For dBSeaPE and dBSeaModes, the sediment is set to attenuate sound so that no sound is reflected from deep in the sediment. The user can set the depth at which maximum attenuation occurs, relative to the wavelengths in the scenario.

dBSeaPE

  • Depth and range oversampling: The output from dBSeaPE is calculated at the range points and depth points grid. These oversampling factors control the size of the oversampled calculation grid that dBSeaPE uses internally. Increasing these factors increases the accuracy of the calculated levels at the cost of increased solve time and memory requirements.
  • Starting field: dBSea currently uses dBSeaModes to generate a starting field. This approach can fail, in which case dBSeaPE uses Greene's starter [3], a simpler (but less accurate) method of generating a starting field.

Number of Padé Terms

Specify the number of Padé terms to use. A higher number will increase the accuracy of the prediction but decrease solving speed. If 0, dBSea will use "Greene's approximation" [2]. Setting the number of Padé terms at 5 is thought to be sufficient for most scenarios [1]. Note that using just one Padé term is inferior to using Greene's approximation.

If the modal starter fails to find any modes, dBSea will show a message: "PE solver used analytical starter". This indicates that dBSea is using an analytical starter (i.e., Greene's) for the specified frequencies and slice numbers.

dBSeaRay

  • Coherent rays: The contributions from each ray are added coherently (dBSeaRay internally holds the real and imaginary pressure components) or incoherently (dBSeaRay ignores ray phase information).
  • Calculate volume attenuation at each step: dBSeaRay can calculate the accumulated attenuation in water for each ray at every step. With this option turned off, the average of the source and receiver depths is used. The volume attenuation is typically negligible except for at high frequencies.
  • Number of rays, start, and end angles: dBSea splits the sound leaving the source into N evenly spaced rays between the start and end angles. 0 degrees is horizontal from the source, -90 degrees is up, and +90 degrees is down.
  • Max number of reflections from seafloor: After this number of reflections, the ray is terminated. In shallow water scenarios, this number should be high (>1000).
  • Initial step size (m): Size of the first step in meters. Following step sizes depend on solver grid resolution and ray path.

dBSeaModes

  • Max number of modes: Enter a value other than zero to limit the number of modes to search for. Setting zero lets dBSeaModes use all modes found by the eigenvalue search routine.

References

[1] M.D. Collins, Higher-order parabolic approximations for accurate and stable elastic parabolic equations with application to interface wave propagation. J. Acoust. Soc. Am. 89, 1050–1057 (1991).

[2] R.R. Greene, The rational approximation to the acoustic wave equation with bottom interaction. J. Acoust. Soc. Am. 76, 1764–1773 (1984).

[3] F.B. Jensen, W.A. Kuperman, M.B. Porter, & H. Schmidt, Computational Ocean Acoustics, 2nd edition, Springer (2011).