Publications

It is well known today that Biot parameters are the intrinsic material properties of porous media such as foams and fibers. They are to porous media what Young modulus is to steel panels. Once these Biot parameters are accurately known, one can trust that a predictive simulation model will yield the corresponding level of accuracy. But how accurate must these Biot parameters be to warrant a safe level of accuracy of the resulting simulation models?

To build accurate NVH predictive simulation models of cars, airplanes, train cabins and the like, one has to properly represent the geometry of the structure, the acoustic fluid and the sound package. Furthermore, the model must contain accurate physical properties of the material used. Those physical properties are readily available for the structural elements but it is not the case for the sound package. When Biot parameters of various foams and fibers are not available, NVH engineers must “guestimate” the values and this can cause the predictiveness quality of the models to go down significantly. But how much of a risk are we taking by not knowing the physical properties of these NVH materials. This paper studies the effect of uncertainties of the Biot parameters on transmission loss and surface absorption. The method used in this paper rank the main sources of uncertainties of the response and can therefore identify the material properties that a NVH simulation engineer must know with precision to ensure his models are accurate and predictive in nature.

Integration of acoustic material concepts into vehicle design process is an important part of full vehicle design. The ability to assess the acoustic performance of a particular sound package component early in the design process allows designers to test various design concepts before selecting a final solution and long before a design freeze. This paper describes an innovative acoustic material concept which is easily integrated in a design process through the use of vibro-acoustic simulation and a database of intrinsic properties of acoustic materials: The Biot Parameters. Biot parameters are widely used in simulation in many industries (and used the most in the automotive industry) to describe the physical interactions between the acoustic waves travelling through foams, fibers or homogeneous metamaterials and the solid and fluid phase of these poro-elastic materials. Therefore, the surface absorption, the insertion loss and the added damping provided by the acoustic treatments on the base plate can all be predicted accurately. Simulation can be performed at component and full vehicle level using Biot parameters since these are the intrinsic properties of the porous material, the same way Young’s Modulus is an intrinsic property of steel. Furthermore, Biot parameters can be directly used in FEM (Finite Element Method), BEM(Boundary Element Method) and SEA (Statistical Energy Analysis) thanks to the existence of porous finite elements or the use of TMM (Transfer Matrix Method). This paper introduces a new acoustic material concept which provides a combination of absorption, transmission loss and added damping on the panel it is attached to. It has shown unique vibro-acoustics performance when tested on a German car manufacturer flagship vehicle and provides the ability to reduce the space needed for sound package component compared with classical solutions. It is manufactured by impregnating a fraction of total thickness of a PU foam. This results in two acoustic layers, one light foam and the other a heavy and high damping layer. A description of the Biot parameter measurements of each layer and test results foreach sample tested along with standard deviation are provided. Finally, a simulation analysis using TMM is performed to assess the airborne and structureborne acoustic performance of this new unique material.

Since the last decade, the automotive industry has expressed the need to better understand how the different trim parts interact together in a complete car up to 400 Hz for structureborne excitations. Classical FE methods in which the acoustic trim is represented as non-structural masses (NSM)and high damping or surface absorbers on the acoustic cavity can only be used at lower frequencies and do not provide insights into the interactions of the acoustic trims with the structure and the acoustic volume. It was demonstrated in several papers that modelling the acoustic components using the poroelastic finite element method (PEM) can yield accurate vibro-acoustic response such as transmission loss of a car component [1,2,3]. The increase of performance of today's computers and the further optimization of commercial simulation codes allow computations on full vehicle level [4,5,6]with adequate accuracy and computation times, which is essential for a car OEM. This paper presents a study of a fully trimmed vehicle excited by structureborne excitations with almost all acoustic trims such as seats, dash insulator, instrument panel, headliner…which are modelled as poroelastic finite element (PEM) parts. Simulation results are compared with extensive measurement results. The interactions between structure, acoustic trims and acoustic volume are illustrated and finally the analysis of several design changes such as trim material properties or geometry modifications is demonstrated.

In the automotive industry, it is common to have different departments designing different parts of the floor. The NVH department may define the overall structural stiffness and might request beading to be added to specific panels to reduce the vibration response locally. It may also request damping pads to be added on the remaining vibration hot spots of the floor. The acoustic group then needs to define the acoustic trim needed to meet the vehicle targets based on the constraints prescribed by the NVH department choices concerning structural stiffness, floor construction, beading, damping pads… In many cases, the NVH and acoustic groups are not communicating and some solutions proposed by one group are detrimental to the other group. For example, it has been shown that contrary to popular belief, adding beads to a structure can actually reduce noise at the first few modes of a plate while significantly increase noise radiation at higher frequency. This paper presents an investigation of how the structure, the beading, the damping pads and acoustic trim can be integrated into a holistic design process to evaluate the effect of all these components on the floor vibration and sound pressure level (SPL) inside a vehicle. In this study only a floor panel is studied to isolate phenomena. A vehicle cavity with seats and appropriate damping is used to represent the interior of the vehicle. The floor structure, beading, damping pads, acoustic trim and acoustic cavity are modeled using finite elements (FEM). Biot parameters are used to represent the poro-elastic layer physical properties. Different configurations including classical steel and innovative composite laminated panels are compared and associated with different types of beading, damping treatments and acoustic trim to evaluate the effect of the full floor component on the vibration response of the floor and most importantly the SPL at the driver’s ear.

This paper discusses the effect of adding beads to a floor panel in respect to radiated sound inside a vehicle. It also describes the combined effect of using beads and a carpet to meet a predefined SPL (Sound Pressure Level) target at the driver’s ear location. It has been widely demonstrated in the literature that adding beads to a floor panel reduces vibration levels at low frequency by shifting the first few modes to a higher frequency.The gain can be significant especially to reduce the peaks at the first few modes of the unbeaded panel. In many cases, design decisions are taken based on vibration levels in conjunction with the so-called ERP (Equivalent Radiated Power) assumption that assumes a direct relation between radiated power and vibration levels. In fact, using ERP assumes that radiation efficiency is equal to 1 over the whole frequency domain. This is not the case in reality and care must be taken when designing beads to ensure that the radiated sound inside the car is not greater due to increased panel stiffness, increased radiating area and increased radiating edges provided by the beads. This papers presents a case study where beaded and unbeaded floor panel vibration, radiation efficiency and radiated sound power into an automobile interior cavity are compared.It also describes the impact of such beadings on the carpet design in order to meet SPL targets

Traditionally in the automotive industry, the acoustic trim has been modelled using a series of spring/mass/dampers added to an FE description of the structure. This simple method has deficiencies in accuracy even in the low frequency domain and could therefore not be used with confidence as a predictive method over 150 Hz. Other approaches literally ignored the effect of trim on the structure and focused at representing the acoustic trim absorption in the interior fluid. In recent years, new developments have shown that the physics of acoustic trim can be well represented when modelling the porous layers using Biot parameters. In fact, since the Biot theory describes the interaction between the acoustic trim, the structure and the fluid using intrinsic properties of the foams and fibers, this modelling approach can be considered predictive over the full audible frequency range. It has been shown for many years now that for high frequency analysis using Statistical Energy Analysis (SEA), Biot parameters play a critical role in the predictive character of a model and therefore its accuracy. Similarly,combining Biot theory with the use of Finite Element Method (FEM) to model acoustic trim in low frequency has shown a significant improvements in the accuracy of predictions and has pushed the upper limit of FE trim simulation over 400 Hz. This paper introduces the Biot theory approach and the interaction between trim, structure and fluid. It also presents a comparison between traditional and Biot theory approach on an academic and industrial case. Sensitivity of the response to Biot parameter values is also presented.

In the automotive industry, the influence of poro-elastic components on acoustic comfort has been mostly investigated for airborne noise at mid- and high frequency ranges; however, due to the lack of adequate theoretical formulations, the influence of poro-elastic in numerical vibro-acoustic simulation at lower frequency range has often been ignored or simplified by the use of distributed spring/mass on the BIW structure and impedance on the acoustic medium. In the last few years, new theoretical developments contributed to overcome this limitation by providing an efficient FEM formulation for poro-elastic material modelling. This FEM approach, implemented in VTM (Vehicle Trim Modeller) software developed by ESI Group, enables the computation of the coupled response of a fully trimmed vehicle by taking into account the BIW structure, the acoustic cavity and the poro-elastic components (carpet, dash insulator, headliner, seats…). This paper presents theoretical background and industrial application examples using these new developments.

In the automotive industry, the influence of poro-elastic components on acoustic comfort has been mostly investigated for air borne noise at mid and high frequency ranges. However, due to the lack of adequate theoretical formulations, the influence of poro-elastic material in numerical vibro-acoustic simulation at lower frequency range has often been ignored or roughly approximated by the addition of distributed spring/mass on the BIW structure. This paper presents the theoretical formulation for the calculation of the acoustic Transmission Loss (TL) of simple or double wall trimmed panels. The poro-elastic materials are described by a FEM model describing geometry and the intrinsic properties of the trim: the BIOT parameters. This method provides an efficient way of building predictive models of a trimmed structure. The resulting Transmission Loss module is implemented in ESI-GROUP software solutions. Numerical results are compared with other software tools available on the market and with experimental data.

In the automotive industry, the influence of poro-elastic components on acoustic comfort has been mostly investigated for airborne noise at mid- and high frequency ranges. However, due to the lack of adequate theoretical formulations, the influence ofporo-elastic in numerical vibro-acoustic simulation at lower frequency range has often been ignored or roughly approximated bythe addition of distributed spring/mass on the BIW structure. Recent theoretical developments removed this limitation byproviding a FEM formulation for poro-elastic material modelling. Using a FEM only approach, this new theory wassuccessfully used to compute the coupled vibro-acoustic response of a fully trimmed vehicle, which includes the BIW structure,the acoustic fluid and the poro-elastic materials (seats, carpet, dash insulator…). The latest development of powerful algorithmsallows the automatic model setup and high performance parallel calculation to be performed fast enough to be implemented intothe sound package design process of large OEMs. Most recent developments allow the automatic Transmission Loss (TL) modelbuilding process of complex components such as trimmed dash or floor. This paper presents a comparison between differenttypes of cabin side dash insulators using the development described above and compares these predictions with test.

This paper provides an overview of the building and validation process for an airborne SEA model of a typical automotive vehicle using the AutoSEA2® software. The emphasis is placed on identifying the transmission paths as well as sound package characteristics that are most critical to ensure accurate predictions using SEA. It also compares predictions with experimental results of well-controlled load cases. Correlation between predictions and tests is presented and briefly discussed.

There are many software tools in use today that are implementing the Biot, or complementary, method for the evaluation of foam and fiber materials. The justification of this process is to understand which mechanisms of the noise control material are contributing to the noise reduction and to optimize the material based on its acoustic properties. The disadvantage of this method is that it is quite complex and time consuming to test a material in order to extract the underlying properties that govern the acoustic performance. An alternative inverse method for material characterization based on simple impedance tube measurements has been developed lately. This paper recalls the physics and mathematics behind the method and concentrates on its validation.