Creating engineers insights from acoustic simulation
Post Processing - Visualization of data
The standard solution allows you to animate the structural modes as well as listing of modal characteristics that can greatly improve the understanding of the behavior of the modes. Acoustic modes too can be visualized and the presentation of acoustic responses as frequency functions means that problem frequencies are easily identified.
Panel Acoustic Contribution Analysis
The surface can be defined as a number of panels that represent different structural features of the radiating surface, and they can be easily defined using a number of identification criteria, such as feature angle, position, or material characteristics. Panel contribution analysis enables you to identify the contribution of each of these panels indicating which physical parts are radiating the most.
Grid Contribution
The LMS Virtual.Lab premium solution takes this analysis to a more detailed level allowing you to identify ‘hotspots’; particular locations where the acoustic radiation is high, and which require special attention or where modifications will be most effective.
Modal Acoustic Contribution Analysis
When assessing the effect of different structural modes, to the overall sound pressure level, the modal participation factor is not sufficient. The contribution to the global noise level is a combination of the modal participation factor, the vibration levels and the radiation factors. For a specific field point, the premium solution provides a dedicated view on the modes in a frequency range where high responses are obtained meaning that the relative contribution of different structural modes can be assessed.
Understanding the critical modes is crucial; the insight gained into the structural behavior of the vehicle leads to the most effective solutions. The use of such a distinct tool is essential due to large number of modes and the higher modal bandwidth.
Path Contribution Analysis
High levels of acoustic radiation are due not only to high loads (vibrations) but to easy transfer paths. So in addition to the panel contribution one has the ability to identify critical paths whereby the source of the noise is transferred to the acoustic radiation provides crucially important information. Only by understanding both the levels and the paths can a truly effective optimization of the design be accomplished.
Refinement and optimization
Whether there are distinct problems with target levels, or whether the objective is to improve the design, the goal is to use the analysis data to close the design loop. This can be done through scalable solutions, which involve increasing degrees of modification to the analysis data, the mesh or the structural model.
Fast Modification Prediction
Modification of the analysis data represents the tightest loop in the design process associated with trouble shooting and problem solving, which would be performed when the design of the vehicle is fixed and it is necessary to make small adjustments. Fast modification prediction allows you to adjust the behavior of the structural modes by the placement of physical characteristics at key locations on the structure. Such physical modifications can be the addition of a lumped mass, the effect of increasing stiffness or damping. Due to the integrated approach within LMS Virtual.Lab, these modifications can be made and the analysis results re-computed to determine the effect. Graphical representation of the modified locations and a graphical comparison of the ‘before and after’ response functions provide the information you need. This is a fast and efficient means to assess the effects that does not require any re-computation of modes by the original Finite Element solvers, this is available as part of the premium solution.
Mesh based modificationsMore significant modifications can be applied to the original FE mesh defining the structure. These modifications can include changing the properties of the existing mesh, by adjusting physical dimensions such as panel thicknesses, material properties or beam stiffness. The nature of the mesh can also be changed by the addition of elements, by changing the shape or by morphing the mesh. In both cases the FE solvers need to be re-run, butLMS Virtual.Lab provides drivers to run these external processes allowing you to remain within its integrated environment. The capture of the complete process means that mesh modifications are then carried through the entire analysis process.
Wave Based Substructuring
When a full acoustic analysis has been performed refinements of specific parts of the structure can be performed using a reduction technique termed Wave Based Substructuring. This provides an extremely rapid way of optimizing the design. A reduced and efficient assembly formulation is obtained by writing the interface displacements as a series of interface basis functions (waves). This drastically reduces the number of interface dofs involved and thus allowing faster assembly predictions.
The technique can be applied to optimization in areas such as rib stiffening, cowltops, glues, bead and seals by concentrating the optimization in a defined discrete area of interest.
As a comparison, after a full run up to 200Hz, taking around 19hours of computations, a WBS analysis can take just 20 minutes. {Automated Multilevel Substructuring 9 hours.}
This technique takes even greater importance as the frequency range of interest is increasing and the subsequent analysis costs associated with this augment in consequence.
OptimizationIn today’s highly competitive environments, the optimum design is one that is close to the design limits. The LMS Virtual.Lab Optimization package offers an integrated set of powerful capabilities, tools, techniques that give engineers rapid insight into all possible design options that meet their requirements.
All the capabilities are integrated into a module that allows users to specify the design objective, set design parameters and their distribution, automates, controls and monitors the optimization routines.
The Design of Experiments (DOE), technique can be used to carry out virtual experiments, the results of which can be viewed using various Response Surface Modeling (RSM) techniques giving critical insight into design parameters and the tradeoffs involved.
Once an optimum is achieved it is important to investigate the robustness of the optimum, due to tolerances on the design parameters, input design parameters must be considered as distributions rather than single deterministic values. Variation of the design around its optimum values can be evaluated to meet robustness reliability and quality criteria.
ConclusionLMS Virtual.Lab Interior Acoustics meets the needs of all engineers involved in achieving vibro-acoustic targets. The standard solution operates within the familiar FE environment and offers a simple and easy management of the analysis and yields direct information about whether targets are met.
For the engineer who needs to improve and optimize the design based in the vibro-acoustic criteria, the premium and complementary solution offers all the analysis data required, plus the effective, efficient techniques and methodologies to adjust the design parameters and accurately and efficiently assess design changes. Our trim paneling modeling technique using wave reduction offers a fast alternative to the traditional Biot approach (using 3D SOLID elements), reducing computation time even further without affecting the accuracy of your model.
The integrated LMS Virtual.Lab environment provides an effective and efficient means of managing the data and the process. Data from a variety of sources; meshes, loading functions, design parameters are incorporated into the one analysis. The process itself is captured in a tree structure which defines and controls the entire process including executing external procedures. Input data, analysis conditions and results are clearly defined, and easy to reference thus enabling a convenient and well managed process.
The second and more premium solution offers extra detailed analysis capabilities. It will provide the additional information needed to better pin point the problem areas and to more efficiently fine tune the design. This is done using acoustic boundary elements or acoustic finite elements based on acoustic transfer vectors (ATV’s).
In order to analyze interior noise, the acoustic cavity of the vehicle needs to be both defined and meshed. As for any FE analysis it is important to create an accurate, realistic model. In order to analyze the interior noise, the interior should be meshed such that the vibration from the structure can be transferred to the cavity across the outer envelope of the cavity mesh. LMS Virtual.Lab provides tools, which enable the cavity mesh to be generated directly from the structural model, thus ensuring the close proximity between the two. A mechanism for detecting and repairing holes and thus defining the cavity is employed first, before a high quality mesh is created automatically. The level of automation required can be determined by the user; a flexibility that allows either the entire vehicle cavity or just volumes of specific 
interest to be meshed. The meshing algorithm can competently handle sharp and smooth features, seats and footprints. The envelope mesh is detected automatically. These dedicated tools mean that anaccurate and effective mesh is generated in hours.
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conditions for the acoustic radiation problem. The draw back of this approach is that these boundary conditions vary with loading conditions and so a different solution must be found for each loading condition. The LMS Virtual.Lab implementation uses Acoustic Transfer Vector (ATV) technology. ATVs are transfer functions that link the input of the structural velocity of the radiating surfaces and the Sound Pressure Levels at the desired output field points. They are dependent only on the geometry and media characteristics of the acoustic domain, the acoustic surface characteristics (impedances and admittances), the frequency and the location of the field points. They are independent of the loading, which means that they are especially well suited for multi-case forced responses and optimization of the structure. This represents an enormous advantage in that loading and design parameters can be varied without having to run original solvers again. Depending on your need, you can apply the ATV technology to either a boundary element mesh or a finite element mesh. For the FEM one can model 3D volume absorbers more in detail while in the BEM approach one only takes surface impedances. 
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