The Pressure Acoustics, Frequency Domain Interface and The Ray Acoustics Interface include two semi analytical fluid models that are calibrated with extensive measurement data. This is the Atmospheric attenuation and the Ocean attenuation models. Both include effects due to viscosity and thermal conduction and relaxation processes of molecules.

The Atmospheric attenuation model defines attenuation in atmospheric air that follows the ANSI standard S1.26-2014, see Ref. 3 (Appendix B), 4, 5, and 6 for details. The model describes attenuation due to thermal and viscous effects (primarily pure air), the relaxation processes for nitrogen and oxygen, and the dependency on atmospheric pressure (absolute pressure), temperature, and relative humidity. The attenuation in air is important for propagation over large distances and for high frequency processes. This also means that the attenuation effect is more important in ray tracing simulations where propagation can be simulated over much larger distances and especially at higher frequencies. In the next section, we will see when the full details of this model are necessary when compared to boundary layer losses.

The Ocean attenuation model defines attenuation in seawater of the ocean. The model is based on a semianalytical model with parameters that are based on extensive measurement data. No standard exists for the attenuation in sea water. It includes effects due to viscosity in pure water, the relaxation processes of boric acid and magnesium sulfate, as well as depth, temperature, practical salinity, and pH value. For further details see Ref. 7, 8, 9, 10, and 11. Like the atmosphere model, the ocean attenuation model is important in ray tracing simulations where propagation can be simulated over much larger distances.