Loss Mechanisms
Loss mechanisms in acoustics due to viscosity, thermal conduction, relaxation processes, and other processes cause absorption and dissipation of the acoustic energy. These result in a reduction in the pressure wave amplitude. This is not due to geometrical spreading where no energy is lost, but because heating actually takes place. When an acoustic wave undergoes absorption, this process is in general also accompanied by dispersion, that is, the dependence of the speed of sound on frequency.
Loss mechanism in acoustics can roughly be divided into four categories, but can of course also happen simultaneously. The division is mostly conceptual:
1
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.
2
Viscous and thermal boundary-layer losses occur at hard walls because of the effective no-slip and isothermal conditions. These are most important in geometries of small dimensions comparable to the boundary layer thickness. The losses can be included in a very general manner using one of the Thermoviscous Acoustics Interfaces. The Narrow Region Acoustics feature can be used in narrow waveguides of constant cross section, while the Thermoviscous Boundary Layer Impedance condition can be used an any boundary as long as no boundary layers are overlapping. The latter two features are great engineering approximations (often yielding exact results) to include the losses at a lower computational cost using the The Pressure Acoustics, Frequency Domain Interface.
3
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).