ansys 14 structural mechanics acoustics analysis speaker

© 2011 ANSYS, Inc. November 22, 2011

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Acoustics Analysis of Speaker


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ANSYS 14.0 offers many enhancements in the area of acoustics.

In this presentation, an example speaker analysis will be shown to highlight some of the acoustics enhancements in 14.0:

• Structural-acoustic coupling using the symmetric fluid-structure interaction (FSI) algorithm

• Postprocessing velocities

• Far-field postprocessing of acoustic field (output of pressure and SPL outside of meshed region)

Introduction


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Acoustics in ANSYS Mechanical involves solving the acoustic wave equation to determine the propagation of acoustic waves in a fluid medium:

• The above includes non-uniform medium and mass source terms, new in 14.0.

This is converted in matrix form to solve with finite elements:

Background on Acoustics

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Vibroacoustic problems can be solved by coupling the acoustic and structural equations together:

• The symmetric form of the harmonic FSI equations shown above is introduced in 14.0 for faster solution times. The fluid-structure coupling term is Cfs. An unsymmetric form from prior releases is still available.

• The sloshing term Sq exists for free surfaces.

• Since the equations are tightly coupled, the structural motions generate sound, and the acoustic waves can vibrate the structure.


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Perfectly Matched Layers or PML is a special formulation to absorb outgoing acoustic waves in harmonic response analyses to prevent waves from reflecting back into the system.

Sound Pressure Level or SPL is defined as follows:

• Prms is the root-mean-square of the pressure, or the amplitude divided by sqrt(2)

• SPL is measured in decibels

• The reference pressure in air is typically taken as 20 mPa.


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The geometry of the speaker in an enclosure is shown below. Note that ¼ symmetry is used:

Geometry & Mesh of Structure

For the speaker, forces are exerted on the voicecoil, causing it to move. The voicecoil moves the cone which is what displaces the air to produce sound. The surround and spider connect and stabilize the cone to the rigid frame.


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The air surrounding the speaker enclosure is shown:

Geometry and Mesh of Air

The air around the speaker is meshed with acoustic fluid elements. To absorb outgoing acoustic waves, perfectly-matched layers (PML) is used. This PML region is shown on the right.


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A “Commands (APDL)” object is inserted under the acoustic bodies

Activating Acoustic Elements

In the example shown on the right, the “et” command changes the element type to be an acoustic element using the new symmetric FSI algorithm. Density and speed of sound are also defined.

New in 14.0!


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In vibroacoustic problems solved in ANSYS Mechanical, the term FSI refers to coupling of the acoustic and structural equations

• ANSYS Mechanical can solve modal, transient, or harmonic response analyses with FSI

The acoustic linear wave equations are solved with the structural equations of motion in a coupled manner (in one matrix).

Fluid-Structure Interaction (FSI)


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A Named Selection of the truncated boundary is created for PML

Created Named Selection for PML

The outermost, truncated boundary should be specified through a Named Selection. This will be referenced with a “Commands” object, shown later


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A Named Selection of the FSI interface is also created

Create Named Selection for FSI

The surfaces between the acoustic bodies and structural bodies should be selected and placed in a Named Selection. This will also be referenced later in a “Commands” object.


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Another “Commands (APDL)” object is inserted under the “Harmonic Response” branch

Define PML and FSI Regions

The APDL commands on the right define the boundary condition on the PML region as well as apply the FSI flag to the Named Selections indicated previously.


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User-Defined Results allow for postprocessing acoustic pressure or calculating SPL

User-Defined Results for Pressure

Isosurfaces of sound pressure level are shown on the right. Identifiers and expressions in User-Defined Results provide flexibility to manipulate results


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Velocities can be plotted with a User-Defined Result using PGVECTORS

User-Defined Results for Velocity

Standard vector plot controls such as solid vectors, uniform vector distribution, uniform vector size are available. Here, “line” vectors at each node designating the velocity is shown.

New in 14.0!


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A “Commands (APDL)” object under the “Solution” branch allows for far-field postprocessing

Perform Far-Field Postprocessing

The lines shown in the highlighted section are used for far-field postprocessing. Namely, HFSYM defines symmetry planes, and PLFAR is used to plot results.

New in 14.0!


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The directivity plot at 1 meter (beyond mesh domain) is shown below

Perform Far-Field Postprocessing

One can determine how focused the acoustic signal is from this plot, which can help evaluate speaker performance.

New in 14.0!


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While a frequency sweep can be specified within a Harmonic Response analysis, one can also use Workbench Parameters to specify the sweep

Perform Frequency Sweep

Note that “Frequency” is a Workbench Parameter. The frequency for the analysis is made as a parameter equal to this value. The benefit to this approach is that users can add frequencies to the solution after solving without having the re-solve the entire frequency range


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By using this approach, users can also take advantage of Remote Solve Manager (RSM) to submit jobs on a cluster

• Instead of solving each frequency sequentially, if a user has more than one ANSYS Mechanical license, the jobs can be submitted through RSM

• Whether solving locally, on two machines, or on a cluster, multiple frequencies can then be solved simultaneously, thus decreasing overall solution time!

Perform Frequency Sweep with RSM


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After the solution is complete, one can plot results within the Workbench Parameters page

Review Frequency Sweep Results

An output of SPL in front of the speaker, designated earlier, is tracked in this example. In speaker design, a constant response is sought within the frequency range of interest. This example shows that structural resonance around 800 Hz is causing undesirable behavior.


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In the past, ANSYS Mechanical solved these two physics simultaneously with unsymmetric matrices, which required double the memory and more CPU time. In ANSYS 14.0, symmetric option is introduced to cut memory requirements in half and significantly decreasing CPU time.

New Symmetric Option in 14.0

Cores Solver Option Speed-up

1 Sparse Unsym 1.00

1 Sparse Sym 1.64

2 Sparse Unsym 1.00

2 Sparse Sym 1.56

4 Sparse Unsym 1.00

4 Sparse Sym 1.50

The table on the right compares the overall solution time speed-up for 275k DOF solved on dual quad-core Intel Xeon E5530. Note that the symmetric option is about 1.5 times faster for this model on this model on this particular hardware.

New in 14.0!


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The GPU Accelerator can also help decrease solution time for vibroacoustic problems. GPU Accelerator performs the solver computation on the graphics card cores.

Using GPU Accelerator

Cores Solver GPU Speed-up

1 Sparse off 1.00

2 Sparse off 1.52

4 Sparse off 2.12

1 Sparse on 2.24

2 Sparse on 2.68

4 Sparse on 3.00

The table on the right compares the overall solution time speed-up for 275k DOF solved on dual quad-core Intel Xeon E5530. Note that the GPU Accelerator provides noticeable speed-up for this model on this model on this particular hardware.


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There are a myriad of other new acoustics features not covered in this presentation:

• Non-uniform acoustic medium, which can be a function of temperature or static pressure

• Acoustic scattering capability and ability to output total or scattered pressure

• Ability to input bulk viscosity to model viscous losses

• Mass sources, impedance sheet, normal velocity b.c.

• Near-field postprocessing

• Ability to define external planar wave, monopole, dipole sources

Other New 14.0 Features in Acoustics

ansys 14 structural mechanics acoustics analysis speaker

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