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Bulk or volume losses are associated with the propagation of waves over long distances or at very high frequencies (also known as internal damping). The (plane wave) attenuation coefficient α (SI unit: 1/m) is often associated with this mechanism. Several loss models are included in the Pressure Acoustics model (see also Theory for the Equivalent Fluid Models) including: User-Defined Attenuation Fluid Model, Atmosphere Attenuation Fluid Model, Ocean Attenuation Fluid Model, or Thermally Conducting and/or Viscous Fluid Model. Note that the Thermally Conducting and/or Viscous Fluid Model should not be confused with boundary layer losses (see next point). Bulk losses are due to several different mechanisms including viscous and thermal dissipation, relaxation processes, and other loss mechanism.
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In porous materials losses are again due to viscous and thermal boundary layer losses, here inside the channels of the porous matrix. The losses are also caused by damping because of the coupling to the porous matrix structure. Detailed modeling is done using The Poroelastic Waves Interface that solves the full Biot model for the coupled pressure and structural waves. A simplified so-called equivalent fluid description can be done using the Poroacoustics feature (see also Theory for the Equivalent Fluid Models).
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Losses can also occur due to interaction with the surroundings like solids and membranes. This is best modeled using multiphysics, see Acoustic-Structure Boundary in the Multiphysics Couplings chapter. Here, simplified models exist using the many options available with the Impedance boundary conditions (see Theory for the Boundary Impedance Models).
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