The Pressure Acoustics, Kirchhoff-Helmholtz Interface
The Pressure Acoustics, Kirchhoff-Helmholtz (pakh) interface (), found under the Acoustics>Pressure Acoustics branch () when adding a physics interface, is used to model radiation problems at high frequencies using the Kirchhoff-Helmholtz integral formulation. The acoustic field is assumed to be locally plane. This technique is also often referred to as high frequency BEM, HFBEM, or simply HFB. The method is often used for computing the radiated acoustic field from vibrating structures at high frequencies, without the need to model the surrounding fluid. The method is valid as long as the acoustic wavelength in the fluid is smaller then the structure and the structural modes. The method is only applicable for open radiation problems. For a flat vibrating surface the method reduces to solving the Rayleigh integral. A user-defined option gives access to the full Kirchhoff-Helmholtz integral formulation for defining both pressure and its normal gradient. The harmonic variation of all fields and sources is given by using the +iω convention.
When this physics interface is added, these default nodes are also added to the Model Builder — Pressure Acoustics, Radiating Object (with Wall sub-feature), and Exterior Field Calculation. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and source. You can also right-click Pressure Acoustics, Asymptotic Scattering to select physics features from the context menu.
Domain and Boundary Nodes for the Pressure Acoustics, Asymptotic Scattering Interface
Settings
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is pakh.
Sound Pressure Level Settings
The settings selected here are only used if the transient solution solved is transformed into the frequency domain using the Time to Frequency FFT study. The zero level on the dB scale varies with the type of fluid. That value is a reference pressure that corresponds to 0 dB. This variable occurs in calculations of the sound pressure level Lp based on the root mean square (rms) pressure prms, such that
where pref is the reference pressure and the star (*) represents the complex conjugate. This is an expression valid for the case of harmonically time-varying acoustic pressure p.
Select a Reference pressure for the sound pressure level based on the fluid type:
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Use reference pressure for air to use a reference pressure of 20 μPa (20·10−6 Pa).
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Use reference pressure for water to use a reference pressure of 1 μPa (1·10−6 Pa).
User-defined reference pressure to enter a reference pressure pref, SPL (SI unit: Pa). The default value is the same as for air, 20 μPa.
Discretization
In this section you can select the discretization or element order used for resolving the pressure and its normal derivative at the scattering surfaces. No dependent variable is solved by the interface, but the representation of curved surfaces is influenced by this setting as well as the number of elements needed to resolve the problem.