Medium Properties
Use the Medium Properties node to specify the speed of sound and density of the medium. This node is created by default and includes all of the selected domains for the physics interface. When the model is solved with unmeshed domains, the material data is specified in the Material Properties of Exterior and Unmeshed Domains.
Model Inputs
Select, User defined (the default), Common model input, or an input from another physics interface, if applicable.
The values of the quiescent (or background) Temperature T (SI unit: K) and Absolute pressure pA (SI unit: Pa) are entered in the Model Inputs section if required by the material properties (other inputs like relative humidity or salinity can also appear). The values of the model inputs can influence the material properties like, for example, the density and speed of sound, depending on their definition in the Materials node. In general, if the model includes a material property that depends on a model input, the corresponding text field will be enabled.
Details about the Model Input and the Default Model Inputs are found in the Global and Local Definitions chapter of the COMSOL Multiphysics Reference Manual.
Pressure Acoustics Model
If the ray intensity or power is computed in the model, select an option from the Fluid model list: Linear elastic (the default), User-defined attenuation, Atmosphere attenuation, Ocean attenuation, or Thermally conducting and viscous. This section determines which additional fluid properties can be specified in this section.
The Speed of sound c (Si unit: m/s) is always required. By default it uses values From material. For User defined enter a value or expression. The default is 343 m/s.
If the ray intensity or power is solved for, both the density and the speed of sound are required to compute the transmittance and reflectance at boundaries between different media. The default Density ρ (SI unit: kg/m3) uses values From material. For User defined, enter a value or expression. The default is 1.2 kg/m3.
Additional inputs are shown in this section depending on the option selected from the Fluid model list:
•
If Linear elastic is selected, absorption by the medium is neglected, so no other inputs are required.
•
If User-defined attenuation is selected, select an option from the Attenuation type list. The default is Attenuation coefficient, Np per unit length.
-
For Attenuation coefficient, Np per unit length enter a value or expression for the Attenuation coefficient α (SI unit: Np/m). The default is 0. Note that this is the attenuation coefficient of the pressure amplitude rather than the intensity. The ray intensity (neglecting wavefront curvature terms) actually scales as I(x) = I0exp(−2αx), where x is the propagation distance.
-
For Intensity attenuation coefficient, Np per unit length enter a value or expression for the Intensity attenuation coefficient m (SI unit: Np/m). The default is 0. Neglecting curvature terms, the ray intensity decreases as I(x) = I0exp(−mx).
In most room acoustics references, the attenuation coefficient given is the intensity attenuation coefficient m. When setting up models, it is important to know which definition is used and choose between the preceding two options accordingly.
-
For Attenuation coefficient, dB per unit length, enter a value or expression for the Attenuation coefficient α' (SI unit: dB/m). The default is 0.
-
For Attenuation coefficient, dB per wavelength, enter a value or expression for the Attenuation coefficient α(λ) (SI unit: dB). The default is 0.
•
For Atmosphere attenuation enter the following in the Model Input section:
-
Temperature T (SI unit: K, default 293.15 K)
-
Absolute pressure pA (SI unit: Pa, default 1 atm)
-
Relative humidity ϕw (dimensionless, default 0)
By default, these model inputs are all User defined, but they can also be taken from a Common model input.
•
For Ocean attenuation enter the following in the Model Input section:
-
Temperature T (SI unit: K, default 293.15 K)
-
Practical salinity Sp (dimensionless, default 35)
-
Depth D (SI unit: m, default 0)
By default, these model inputs are all User defined, but they can also be taken from a Common model input. Also, in the Pressure Acoustics Model section, enter a value or expression for the pH value pH (dimensionless). The default is 8.
The Practical salinity is defined on the Practical Salinity Scale. It represents a specific way of measuring salinity through electric conductivity. Practical salinity is a unitless quantity, although it can be thought of as given in units of g/kg. The default value for the Practical salinity Sp is 35. For details, see Ref. 22.
•
If Thermally conducting and viscous is selected, specify the following material properties. By default, all fluid properties take default values From material. Choose User defined to enter a value or expression for the:
-
Heat capacity at constant pressure Cp (SI unit: J/(kg·K)). The default value is 1005.4 J/(kg·K).
-
Ratio of specific heats γ (dimensionless). The default value is 1.4.
-
Thermal conductivity k (SI unit: W/(m·K)). The default value is 0.0257 W/(m·K). For ray acoustics, the thermal conductivity is assumed to be isotropic.
-
Dynamic viscosity μ (SI unit: Pa·s). The default value is 0.0181 mPa·s.
-
Bulk viscosity μB (SI unit: Pa·s). The default is 0.
To define the attenuation coefficient or another medium property as a function of ray properties such as the ray frequency or intensity, the ray variable must be enclosed in the noenv() operator. For example, to use the ray frequency rac.f in an expression for the speed of sound, it must be included as part of the expression noenv(rac.f).
Attenuation Within Domains