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Tutorial
Category: LMS Virtual Lab Acoustics
Topic:BEM coupled harmonic

In this tutorial, the coupling of a loudspeaker will be simulated. The vibro-acoustic coupling between
the membranes with its structural behavior and the loudspeaker will be calculated in the Vibro-Acoustic
Response Analysis Case of LMS Virtual.Lab Acoustics.

Figure 1:loudspeaker

Overview

1 Creating a new CATAnalysis ............................................................................................................ 2

2 Definition of the meshes.................................................................................................................... 2
3 Properties definition .......................................................................................................................... 4
4 Checking ID conflicts......................................................................................................................... 4
5 Modes handling ................................................................................................................................. 6
6 Loads handling in the Load Function ................................................................................................ 6
7 The Vibro-Acoustic case ................................................................................................................... 8
8 Sound Directivity ............................................................................................................................... 9

Prerequisites

 loudspeaker.bdf: acoustic mesh of a loudspeaker

 membranes.bdf: structural mesh of the membranes
 modes-memb.pch: structural modes of the membranes

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 1 Creating a new CATAnalysis
We will start with a new file:

 Start  Acoustics Acoustic Harmonic BEM

 Save As: vibro_acoustic.CATAnalysis

2 Definition of the meshes

The Acoustic mesh was build with IDEAS and pre acoustics. The structural mesh is “locally
compatible” with the acoustic mesh.

 Import the structural mesh ‘membranes.bdf’ (select m - kg - s). Check that both, the
analysis case and dynamic loads options, are toggled off;
 Import the acoustic mesh ‘loudspeaker.bdf’ (select m - kg – s), toggle of Create
Analysis Case. You can speed up the model creation by choosing File Import
Acoustic MeshModel mesh. This way the Fluid Material and Properties will be
created.

The position of the microphones will be defined on planes

 Create one plane field point mesh: Insert Field Point Meshes  Plane
FieldPoint Mesh, see Figure 2, left.
 Createa secondPlane Field Point Mesh, see Figure 2, right.

Figure 2:Definition of Field Point Mesh 1 (left) and 2 (right)

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 Figure 3:specification tree with the imported meshes and the Field Point meshes

 Check / set the mesh types for all the meshes with Tools Set Mesh Parts Type
(membranes = structural, loudspeaker = acoustical, planes = field point).

Figure 4: assignment of the Mesh Parts Type

 Update Nodes and Elements;

 Check normal direction (right click on Nodes and Elements Acoustic Element
Normal Vector Symbol Image). You should observe that the orientation is not
consistent for an Indirect BEM analysis. Orientation must be corrected;
 To correct normal orientation an Acoustic Mesh Preprocessing Set must be inserted:
Insert Acoustic Mesh Preprocessing Setand if not automatically done, select the
previously imported acoustic mesh (loudspeaker);
 Update after right clicking on Acoustic Mesh Preprocessing Set.

We can now check the normal mesh orientation, the correction will be observed. The mesh has
not actually been modified, only set to meet the Indirect BEM‟s requirements.

 Check also the Material Maximum Frequency Quality Criterion by generating an image
on Properties.

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 3 Properties definition
The fluid properties will be defined (if the import was done by „Import Acoustic Mesh‟ you should
rename them only)
 Main menu  Insert  Materials  New Materials New Fluid Material;
 Leave the values for velocity and density as default constant (air). However they can
be frequency dependent if one creates or imports a table; name it Air
 Properties  Insert  Properties  New AcousticProperties New Acoustic Fluid
Properties;
 Select the acoustic mesh for the location list and give a feature name for the
properties: AIRSTD. Select the material and press OK;
 The Properties and Materials item are now out of date. Right click on them and
choose Update.

4 Checking ID conflicts
Because both meshes, the Structural and Acoustical, contain identical numbers for nodes and
elements, this can lead to conflicts as long both meshes are contained in the same CATAnalysis
document. There is a tool in Virtual.Lab to check and correct them:

 Tools ID and Model ChecksCheck Id Conflicts;

 Press the button Check;

Figure 5:ID Conflict check

 Select the Nodes line in the list and press the button Details.

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 In the Detail window there is the possibility to renumber the meshes. For this case, it is very
important to keep the numbering of the structural mesh otherwise we will lose the link between
the structural mesh and the structural modes. The structural modes have in fact been calculated
on the imported *.bdf file. We therefore decide to fix the conflict by renumbering all the meshes
except for the structural one.
 Under Mesh Parts, select the membranes mesh;
 Under Conflicts, select the loudspeaker mesh;
 Press the Fix button;
 Close the window.

keep numbering change numbering

Figure 6: Details of ID conflicts

Now the conflicts between the membranes and the loudspeaker are fixed, by renumbering the
loudspeaker.

Figure 7:ID conflicts window after renumbering the node numbers

Now the node ID conflicts are solved. The element ID conflicts can be solved in the same way,
by avoiding the renumbering of „membranes nodes and elements‟.
 Select the Elements in the list above;
 Press the button Details;
 Now Fix the element ID conflicts in a similar way.

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 5 Modes handling
Now that the ID conflicts are solved, the rest of the model data can be imported.

 Insert a Mode Set: Insert Vector & Functions Sets Mode Set;
 Add a Data File to the Data source and select the modes_memb.pch as data file;
 Click OK;
 Double click on the Modal Editing;
 Select All Modes;
 Edit the values for Viscous Damping in clicking on the Edit Value Button, enter 1% as
viscous damping and click OK;
 ClickOK.

We can create images (Option “Translational Displacement magnitude”) to look at the structural
modes shapes.

 Right click on Mode Set.1 and Generate Image;

 Choose Translational displacement magnitude OK.

6 Loads handling in the Load Function

 Create a Load Function Set: InsertVector & Functions Set Load Function Set;
 Use the default options and select Force as Physical Data Type  OK;
 Right click on Load Function Set.1 Add Edited LoadFunction;
 In the Attributes tab, select DOF +Z;
 In the Values tab, enter one value: 0.1N at 440 Hz.

Figure 8: Load Function Editor with entered values

 Click OK.

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 Off course the right excitation points need to be created. These points are part of the structural
mesh. The easiest way to select them is to hide the other meshes.

 Right click on the Acoustic Mesh Preprocessing Set and choose Hide/Show;
 Right click on the Field Point Meshes and choose Hide/Show;

Now only the structural mesh will be visible.

 Double click on Input locations [Force] under Load Function Set.1;

 Insert Single Point and choose a node on the center of the woofer, select only Z for
the degree of freedom and Close the window;
 Do the same with a node on the center of the tweeter;
 Click on Update before clicking Close.

It is very important to select the right DOF for the excitation points. During the attachment of the
loads, every selected DOF will need to be attached to a corresponding load function. If they are
not, the load function set will not be valid.

 Double click on Load Conditions, select Automatic for the definition mode;
 Double click on Load Attachments, choose Select Load Id as the Load Attachment
Type;
 Select then the created Edited Load Function with the Id LoadResponseID.1 for both
Locations.

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 7 The Vibro-Acoustic case
 Insert a Vibro-Acoustic Case: Insert Vibro-Acoustic Forced RespionseModal
SuperpositionVibro-Acoustic Response Case;
 Select No Boundary ConditionSet, reference the existing Load Function Set, the
existing Structural Mode Setandcreate a new Vibro-Acoustic Mesh Mapping;
 Define the Mesh Mapping (membranes as Structural Mesh,loudspeaker as Acoustical
Mesh, Mapping Data with MaxDistance, 1 node, 0mm);
 Double click on the Solution set and define the user frequency: 440Hz  Addand
chooseSave As Vectorsunder Results at Field Points;
 Save the CATAnalysis;
 Compute the solution set. Do not forget to save before and after the computation!
 Generate Image for the Vibro-Acoustic Response Solution Set, choose Pressure
Amplitude dB(RMS) OK.

Figure 9:Pressure on Field Point Mesh at 440 Hz

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 8 Sound Directivity
A sound directivity analysis allows the visualization of the acoustic field quantities (pressure,
velocity, intensity) on a sphere or a circle with a given center and radius. With this tool the
directivity of these acoustic quantities can be evaluated: in which direction is the acoustic
response large (or small)?

 Save the open document.

 Open a new document with Start  Acoustics  Acoustic Harmonic BEM;
 File  Import and choose vibro_acoustic.CATAnalysis.

Two field point meshes will be inserted: one sphere and one circle.

 Insert  Field Point Meshes Spherical Field Point Mesh with a center of X=0 mm,
Y=150 mm, Z=-200 mm, a radius of 3000 mm and a refinement of 10.

Figure 10:Definition of the Spherical Field Point Mesh

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  Insert  Field Point Meshes Directivity Field Point Mesh with the following
specifications:
o Center: Coordinates
Values: 0, 0, -200mm;
o Normal Direction: Vector by Axis System
Value: Global, V-Axis;
o Initial Angle Direction: Vector by Axis System
Value: Global, W-Axis;

Figure 11: Definition of the Directivity Field Point Mesh

 Insert New IO Set;

 Double click on IO Set.1 and give it the name Sphere;
 Click on Insert Multiple Node;
 Click on the Spherical Field Point MeshApply  Close;
 Close the IO Set window;
 Insert New IO Set;
 Double click on IO Set.2 and give it the name Circle;
 Click on Insert Multiple Node;
 Click on the field point mesh Directivity FPM Apply  Close;
 Close the IO set window;
 Insert Pre- and Post-ProcessingAcoustic Field Response Case;
 Choose the Modal Based Vibro-Acoustic Response Solution Set in the imported
CATAnalysis and take Sphere as output points.

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 Figure 12:Inserting an Acoustic Field Response Analysis Case

 Save the document and give it a meaningful name like

Speaker_Directivity.CATAnalysis;
 Compute/Update the Acoustic Field Response Analysis Case.

The acoustic response on the sphere has been calculated. Several visualizations are now
possible (pressure, velocity, intensity). The instructions below are for the intensity.

 Right click on Acoustic Field Response Solution Set  Generate Image;

 Choose Acoustic Intensity Symbol Vector;
 Double click on the image in the specification tree. In ‘Occurences’ tab, click on the
More >> button;
 In the Complex part field, choose Real part. This way we will only see the Active
Intensity;
 In the Selections tab, activate only the Spherical Field Point Mesh.1, see Figure 13,
left.

Figure 13: Image Edition in the Selections tab (left) and in the Visu tab (right)

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 You have the image with vector arrows. You can easily visualize directions of acoustic energy
flow (Z-Axis).

 Go to the tab Visu, select Average iso in Types, select Normal Component in Criteria,
see Figure 13, right.

The image displays the active acoustic intensity projected onto the normal vector to the FPM
surface (which points outwards). This is the density of acoustic energy flowing outwards the
spherical field point meshes.

 Flag on Deform according to Normal of nodes in the Visu tab;

 Use to fit image on screen.

Note that this is not a visualization of vectors. It is simply a scalar (the real part of the acoustic
intensity) that causes the deformation according to the normal of the nodes.

 Close the image edition dialog with OK.

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  In the main menu bar, select Tools  Options
 In the Acoustics options, go to the Display tab and set dB shift for images on
deformed mesh to 40, seeFigure 14

Figure 14:Set the dB shift to 40 in Tools  Options  Acoustics

 Edit the image and select Normal component dB with shift in criteria list

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 Figure 15: Acoustic intensity Normal Component dB with shift, on Sphere

Two-dimensional analysis of the sound directivity is also possible. To accomplish this, a field
point response analysis on the circle has to be done.

 Insert Pre- and Post-ProcessingAcoustic Field Response Analysis Case;

 Choose the Modal Based Vibro-Acoustic Response Solution Set in the imported
CATAnalysis and take Circle as output points;
 Compute the Acoustic Field Response Analysis Case;
 Save your document;
 Right click on the Acoustic Field Response Analysis Case Generate Image;
 Choose Pressure (nodal values) OK;
 Double click on the image in the specification tree, go to the tab Selections and
activate only Directivity FPM;
 In the Visu tab, choose Deform according to Normal of nodes and Scalar dB(RMS)
with Shift;

 Use to fit the image on screen. If nothing is displayed, make sure that the render

style is correct ( ).

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 Figure 16: Field Response pressure on Circle

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