Acoustic chamber meshes for panel transmission loss analyses

In a panel transition loss analysis, you analyze the transmission loss of acoustic power through a structural panel between a chamber that contains the diffuse acoustic field (the reverberant chamber) and an adjacent chamber (the anechoic chamber). The acoustic transmission loss is the ratio between the acoustic power transmitted through the structural panel and the input acoustic power. You can use the Acoustic Chamber Mesh command to automatically generate two separate 3D acoustic fluid meshes to model the acoustic and reverberant chambers on either side of an existing 2D structural mesh on the panel.

The Acoustic Chamber Mesh command:

• Eliminates the need to create any temporary CAD geometry to define the outer boundaries of the acoustic fluid meshes.

• Automatically fills any holes in the 2D structural mesh on the panel.

• Generates the acoustic fluid meshes to represent the reverberant and anechoic chambers.

• Creates the groups of free element faces or nodes that you need to later define boundary conditions, such as to define an Automatically Matched Layer simulation object to represent the non-reflective acoustic boundary conditions on the reverberant and anechoic sides or to define the fluid-structure coupling surfaces.

The acoustic meshes created by the Acoustic Chamber Mesh command:

• Are not connected to the elements in the 2D structural mesh.

• Are not connected to each other. The acoustic meshes on either side of the structural mesh are completely independent of each other.

The underside of the top acoustic chamber mesh. Notice how the mesh is indented slightly to accommodate the structural mesh on the acoustic panel.	The underside of the bottom acoustic chamber mesh. Notice the concave profile of the mesh to accommodate the structural mesh on the acoustic panel.	The structural mesh on the acoustic panel is shown in green together with the top acoustic chamber mesh in blue.

Specifying the 2D structural mesh

In the Acoustic Chamber Mesh dialog box, you must specify the 2D mesh that represents the structural panel around which to create the acoustic chamber meshes. You can select:

• One or more 2D meshes

• 2D elements

• 3D element faces

All meshes, elements, or element faces that you select must be connected to each other (they must share one or more element edges). Additionally, all elements that you select must be of the same order.

Note:

If the input mesh contains holes, the software fills the holes with elements of the same type and order as the surrounding mesh. Importantly, the software does not modify the original 2D mesh during this process. The software creates a temporary copy of the input mesh and makes all modifications to the copy.

Defining the plane to use to define the outer boundary of the acoustic mesh

The software uses the input mesh together with a plane that you specify to determine the outer boundary of the acoustic chamber meshes. The software projects the nodes from the input mesh to the plane and then offsets those nodes based on an Offset Method and Offset Distance that you specify to define the outer boundary.

The plane:

• Must not intersect the input mesh.

• Should be aligned so that the projection of the input mesh covers the maximum area of the plane. This ensures the best results.

Note:

Although you can manually specify any plane that meets these criteria, you can click the Infer button to have the software compute the most appropriate plane for the input mesh that meets these criteria.

The direction of the plane's normal defines the direction in which the software defines the location of the top chamber mesh.

Specifying the acoustic mesh element size

You can use the Element Size option to specify the size of the elements in the acoustic meshes. You can specify this value as an element's edge length or define the value as a function of frequency. For more information, see Element size for acoustic analysis.

Note:

The Number of Element Layers in Acoustic Chambers customer default controls how many layers of elements that the software creates through each acoustic chamber mesh.

Controlling how the software offsets the outer boundary of the acoustic mesh

You can use the Offset Method and Offset Distance options to control how the software offsets the nodes from the plane to define the outer boundary of the acoustic mesh.

• Select Along Plane Normal to offset the nodes in along the normal of the specified plane.

• Select Along Node Normal to offset each node along its own normal.

To define the Offset Distance you can specify an absolute distance value. You can also click Infer button to have the software compute a distance that is equal to the Number of Element Layers in Acoustic Chambers customer default * the specified Element Size for the acoustic mesh.

Using the Remesh Input option

The Remesh Input check box controls whether the software can modify the 2D structural mesh to facilitate the creation of the acoustic chamber mesh. These modifications can include:

• Coarsening the 2D structural mesh to a size that is more appropriate for an acoustics analysis. If you select the Remesh Input check box, you can specify a new element size for the 2D structural mesh, or you can click Infer button to have the software compute an appropriate size.The software uses an interpolation of the original 2D structural mesh to determine the outer boundary of the acoustic chamber mesh. If the 2D structural mesh is too fine, the size of the outer boundary of the acoustic mesh may be overly large for the analysis.

If you clear the Remesh Input check box, you must ensure that the 2D structural mesh:

• Has a boundary that consists of a single outer loop.

• Is meshed with an appropriate element size (not overly small).

Working with the automatically created groups

When the software creates the acoustic chamber meshes, it automatically creates several groups that help you define boundary conditions for the analysis. For each acoustic chamber mesh, it creates:

• A group that contains the free element faces that you need to define an Automatically Matched Layer simulation object to represent the non-reflective acoustic boundary conditions on the reverberant and anechoic chambers.For more information, see Modeling exterior acoustic problems using automatically matched layers.

• A group that contains the free element faces that are in contact with the 2D structural mesh, and a group that contains the nodes on the 2D structural mesh. You can use these two groups together to define a Fluid-Structure Interface Modeling Parameters modeling object that models the interaction effects between the structural panel and the acoustic chamber meshes.For more information, see One-way fluid-structure interface coupling.

Working with an acoustic chamber mesh

The software stores the acoustic meshes in separate nodes in the Simulation Navigator.

When you create an acoustic chamber mesh, the software stores the options and settings you used to create the mesh. If you edit the mesh and change a setting, such as the element type or offset distance, the software recreates the mesh with the new settings.

Where do I find it?

Acoustic Chamber Mesh command

Application	Pre/Post
Prerequisite	A FEM file as the work part and displayed part
Command Finder	Acoustic Chamber Mesh

Number of Element Layers in Acoustic Chambers default

Menu	File→Utilities→Customer Defaults
Location in dialog box	Simulation→ Pre/Post→ Meshing→ General tab→Acoustics Parameters group

How do I

Reset node and element IDs

Learn more

Manually creating elements

Extruding elements

Projecting elements

Reflecting elements

Revolving elements

Rotating elements

Translating elements

Splitting shell elements

Swapping the diagonals between triangular elements

Splitting 1D elements

Detaching elements from a mesh

Attaching elements into a mesh

Creating a mesh from a cloud of points

Rib detection and removal within a mesh

Thickening a 2D mesh

Convex meshes for FEM acoustics analyses

Open duct end meshing for acoustics

Combining triangular elements

Modifying element order

Modifying the type of elements

Moving nodes in 2D elements

Modifying element labels

Modifying element connectivity

Deleting elements

Extracting elements from a mesh

Locking and unlocking a mesh

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Acoustic chamber meshes for panel transmission loss analyses, Simcenter 3D 2021.1 Series

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