Defining acoustic boundary conditions and simulation objects (Acoustics BEM)

You can define acoustic loads and boundary conditions for acoustic and vibro-acoustic analyses such as panel transmission loss and muffler transmission loss.

You can define the following acoustic loads and boundary conditions:

• Acoustic monopole

• Acoustic dipole

• Acoustic plane wave

• Acoustic panel normal velocity

• Transfer admittance

Acoustic monopole

• An acoustic monopole source is a pulsating sound source that radiates sound (acoustic energy) equally in all directions. The radiation pattern of a monopole is a spherical wave.You position a monopole by selecting an arbitrary point (coordinate) or a node. You define the acoustic magnitude by specifying amplitude values or by specifying acoustic power and a phase angle.Acoustic monopole sourceAmplitude — You can specify the following:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.Acoustic power — You can specify constant acoustic power and a phase angle of the equivalent monopole amplitude values.For more information see, Defining acoustic sources and loads (Acoustics BEM).

Acoustic dipole

An acoustic dipole is a pair of monopoles of equal strength and opposite phases. You can consider a dipole as a pair of sound sources, except that one is a source and the other is a sink. A dipole has acoustic directional properties. The directivity pattern has a maximum along two opposite poles and no radiation at the poles perpendicular to the maximum axis.

Acoustic dipole source

You can define an acoustic dipole by specifying magnitude and direction, or by specifying its three directional components.

• Magnitude and direction — You can specify the following:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.

• Components — You can specify the following:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.

You can use an acoustic dipole source to simulate simplified fans or speakers.

Acoustic plane wave

A plane wave is an acoustic wave that radiates in a single direction. The vibration direction is the same as the direction of travel. An acoustic plane wave generates a plane wave on only one side of the source, along the positive direction of the source vector.

(1) Incident field(2) Vector(3) Position(4) Non-incident field	Acoustic plane wave source

You define an acoustic plane wave by specifying a location in space and by specifying the magnitude and direction.

You can specify the following:

• A constant complex excitation with real and imaginary parts.

• A constant complex excitation with magnitude and phase angle.

• A frequency dependent complex field value.

The typical application for an acoustic plane wave is in underwater acoustics for the computation of a scattered field. You can also use an acoustic plane wave to simulate panel transmission loss of spacecraft.

Acoustic panel normal velocity

You can apply acoustic panel velocity to acoustic fluid elements using an Acoustic Panel Normal Velocity boundary condition.

You can specify the following:

• A constant complex excitation with real and imaginary parts.

• A constant complex excitation with magnitude and phase angle.

• A frequency dependent complex field value.

You use this boundary condition to represent rigid planes, perfectly reflective surfaces, hard surfaces, or a ground plane, such as in a muffler transmission loss solution.

Transfer admittance

Transfer admittance is a simulation object that lets you define a relationship between the acoustic velocities and pressures on opposite sides of a surface. For example, you can define the transfer admittance on two sides of an acoustic surface, for example to represent a perforated sheet, air filter, heat exchanger, loudspeaker cover, or so forth.

Transfer admittance is not supported by the direct BEM acoustic solver, or by the Fast-Multipole and H-matrix BEM methods.

You can define transfer admittance using any of the following input methods:

• Admittance and Source Coefficients — The transfer admittance defines a relationship between the acoustic velocities and pressures on one side of a surface, and the acoustic velocities and the pressures on the other side.

• Rigid Transfer Admittance — You can specify the following:A constant complex value with real and imaginary parts.A constant complex value with magnitude and phase angle.A frequency dependent complex field value.

• Geometric Parameters — Mechel’s formula is used to calculate the β admittance value, based on the geometric characteristics of perforations in the surface. The dynamic viscosity and mass density use the default parameters corresponding to air at sea level and at 250C.

• Transfer matrix — The transfer matrix method defines a relationship between the acoustic velocities and pressures on one side of a surface, and the acoustic velocities and the pressures on the other side. The transfer matrix formulation is given by:where:P1 = Inlet pressureP2 = Outlet pressureV1 = Inlet velocityV2 = Outlet velocityT = transfer matrix parameters

Where do I find it?

Creating an acoustic monopole, or an acoustic dipole, or an acoustic plane wave

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter 3D Acoustics BEM as the specified solverDirect Acoustic or Indirect Acoustic or Indirect Vibro-Acoustic as the specified analysis type
Simulation Navigator	Expand the Solution Step node of an active solution→Loads→right-click the Loads container→New Load→Acoustic MonopoleAcoustic DipoleAcoustic Plane Wave****Surface Dipole

Creating a transfer admittance

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter 3D Acoustics BEM as the specified solverIndirect Acoustic or Indirect Vibro-Acoustic as the specified analysis type
Simulation Navigator	Right-click the Simulation Objects container of an active solution→New Simulation Object→Transfer Admittance

How do I

Move a boundary condition to a new folder

Create a boundary condition folder

Edit a boundary condition

Delete or remove boundary conditions

Copy boundary conditions

Rename a boundary condition

Learn more

Organizing boundary conditions in folders

Look up more details

Acoustic Monopole dialog box (Acoustics BEM)

Acoustic Dipole dialog box

Acoustic Plane Wave dialog box

Transfer Admittance dialog box (Acoustics BEM)

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Defining acoustic boundary conditions and simulation objects (Acoustics BEM), Simcenter 3D 2021.1 Series

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