Drastic exterior noise reduction through optimization of acoustic shielding package

Drastic exterior noise reduction through optimization of acoustic shielding package Michaël THIVANT 1 ; Olivier MINCK 2 ; Benjamin BETGEN 3 Romain LENEVEU 4 1 VIBRATEC, France 2 MICRODB, France 3 VIBRATEC, France 4 VIBRATEC, France ABSTRACT In the context of the upcoming reduction of Pass-By-Noise limits in the EU regulations, automotive manufacturers need to implement new concepts of shielding package. ECOBEX is a French funded research project aiming at reducing the powertrain noise contribution of the vehicle, whilst restricting additional mass and cost. Bringing together OEM, raw materials suppliers, shielding manufacturers, universities and specialized consultants in this research program enabled innovations in materials, design, tests and computational methods. This paper will focus on a new procedure for the optimization of the shielding package, based on a precise 3D localization and quantification of the acoustic sources of the powertrain and on their implementation in an Energy Boundary Element model, computing the acoustic propagation. Intensity maps emphasized the dominant acoustic paths and highlighted mitigation opportunities in terms of absorption and insulation. The shielding efficiency could then be optimized with an improved accuracy, taking into account the spatial and frequency characteristics of both source and shielding. Materials were chosen to match local absorption and insulation optimums regarding their noise exposure spectra, whilst coping with all industrial constraints. Measured and computed results are encouraging, and the optimized shielding package is shown. Keywords: Sound, Insulation, Transmission traffic noise I-INCE Classification of Subjects Number(s): 76.1.1 Road 1. Introduction The new European standard (UN ECE R51-03) introduces a new procedure for pass by noise measurement, in addition to a decreasing value for the noise level over years. This will lead to reduce the pass by noise by 4dB in the coming 8 years. Considering this 4dB decrease, 97% of the actual cars became non-conform. In this context, Ecobex project has been launched in 2014. This project, funded by the French Government and some regions, includes 9 companies besides the leader Vibratec: the OEM Renault, suppliers RJP, MECAPLAST, and Isover, and research laboratories CrittM2A, Utc, Matelys, Esi and MicrodB. The goal is to develop an optimized engine acoustic package to reduce the engine contribution at a level compatible with the future regulation limits, taking into account reduction of other sources contribution, and following constrains like cost and mass for realistic mass production. The methodology consists in characterizing the powertrain as a 3D source to feed a simulation software for the prediction of the effectiveness of acoustic shields on the exterior noise. In a first part, the methodology for efficient measurement of the 3D sound field around an engine will be presented, and the post processing approach with validation on mockup will be detailed, and finally some results on the real engine will be presented. In the second part, the acoustic model of the vehicle is presented. Pass-by Noise simulation results are presented and validated with measurements. Then optimization of the shield package are proposed. 1 Michael.thivant@vibratec.fr 2 Olivier.minck@microdb.fr 3 Benjamin.betgen@vibratec.fr 4 Romain.leneveu@vibratec.fr 5246

2. Powertrain noise source characterization This first part summarizes the 3D source characterization methodology and results published in [1]. 2.1 Measurement method 2.1.1 Patch measurement 3D sound radiation around the powertrain of a Renault car is measured using an array of microphones and the intensity is computed directly on the mesh of the powertrain, which can be reused for simulation. For array measurement, as a synchronous signal is required between microphones, the ideal case would be to set a huge number of microphones around the engine, and to measure the field during a single run. But such a system, able to give results up to 6 khz, would require hundreds of microphones with an adequate frontend, and would then induce a high cost. A patch measurement approach has been preferred, as it only needs a single microphone array (HDCam 54 channels version). The experimental procedure involves moving the array around the object of interest. A pre-processing step is required to obtain a fully synchronized cross spectrum matrix (CSM). Using fixed reference sensors makes possible, like described in [2] to compute the complete CSM by using phase with references. The global CSM (SMM) is computed using: With: : Hermitian matrix of cross spectrum between reference sensors and microphones array, : pseudo-inverse of reference cross spectrum matrix. The quality of synchronization is strongly dependent on the quality of the inversion of the SRR squared-matrix. The chosen strategy is thus to decompose the matrix into principal components. For each component, a virtual S_MM matrix is estimated following the above-mentioned equation. The number of principal components to be kept is evaluated by minimizing the error between measured cross spectrums (on each single array position), and computed cross spectrums. In a final step, the computed global CSM is modified with the measured cross spectrum of each position. 2.1.2 Array positioning system The patch measurement requires the displacement of the array around the engine, and an accurate localization method to get the microphones positions relatively to the object of interest. MicrodB uses a positioning probe based on the time of flight, ensuring a 1mm accuracy. It allows a fast derivation of the change of basis from the coordinate system (C.S.) of the array microphones to the C.S. of the source mesh. 2.1.3 Estimation of transfer functions Once the geometry is known, acoustic transfer functions between nodes on the mesh and the microphones must be estimated. Free field is assumed in most acoustic imaging technics (green formulation). But such approximation doesn t take into account scattering and reflection due to the object. In terms of localization of sources on an engine for example, the resulting hologram would present sources with very loud ghost sources, and wrong source amplitudes. The free field assumption is less a problem when considering single faces of an object, in an anechoic chamber, if the main source is indeed on the measured face. A second approach would be to measure the transfer functions. But to cover a bandwidth up to 6 khz, a 5000 nodes mesh would be required, so the measurement would take a very long time, and would require managing big data. A third approach would be to use FEM or BEM simulation software, which has been successfully tested at MicrodB [3], and increases the stability of the results. However, it is often tricky to transfer the mesh and microphones positions, to run the simulation up to high frequency, and then import the results back into the testing software, to continue the post-process. A fourth approach is to integrate, into the measurement software, an algorithm able to estimate these transfer functions. This has been developed during T. Le Magueresse s PhD at MicrodB [4]. This method is based on equivalent source modeling (ESM) [5], adapted for transfer functions calculation, and with the following assumptions: Radiating object is rigid (normal velocity on the mesh is zero); Sommerfeld radiation condition. This approach has been validated in [1], both for a rigid sphere with theoretical comparison, and on 5247

an engine mockup, with experimental comparisons. ESM algorithm shows a really good behavior, and enables a correct quantification. One advantage of this approach is that the same mesh is used for transfer function simulation and for post processing of the array measurements. It means that no complex import/export operation is required for source quantification. 2.2 Post processing 2.2.1 Generalized Acoustic Holography (GAh) Knowing both the propagation model given by the transfer function and the synchronized array measurements, it is possible to deduce the amplitude level and the position of sources on the mesh of the powertrain. Many methods exist in the literature to reconstruct sources field from array measurements. Nearfield Acoustical Holography reconstructs the sound field using the plane wave decomposition, Helmholtz Equation Least Squares (HELS) using the spherical harmonics decomposition, to mention just a few of existing methods. It has been shown that all of these methods can be generalized using a probabilistic approach in a simple 2D set-up [6] or in a complex 3D shape structure [5]. This probabilistic approach, combined with the procedure explained above concerning estimation of the transfer function, defines the Generalized Acoustic Holography (GAH). Further information about probabilistic models of the noise and the unknown sources can be found in references [5,6]. The method offers a solution describing the source distribution on the mesh which minimize both the least square error (residue) and the stability of the solution in terms of energy, which leads a to minimization the following cost function: (1) Where : - H is the transfer function matrix, describing the acoustic propagation from a source placed on each node of the mesh toward each microphone of the array. This matrix is computed using the ESM method presented above; - q is the unknown vector of the complex amplitudes of point sources on nodes of the mesh; - p is the complex pressure vector calculated from the whole re-synchronized CSM; - λ is a regularization parameter computed using a Bayesian approach [6]. The minimization of the cost function (1) leads to the well-known Tikhonov solution: (2) Once the estimation of volume velocities of the point sources is available, the projection on the 3D mesh is done, in order to quantify sound power for each element of the mesh. 2.3 Application on the powertrain 2.3.1 Engine test setup A Renault car powertrain has been installed in a full anechoic chamber at CRITT M2A, and some specific running conditions have been selected. It consist in stationary configurations for different engine speed, and gearbox ratio engaged, representative of maximal noise during pass-by tests with the new standard. All the engines faces are measured for each selected engine speed / gearbox ratio, and also for idle state. Figure 1 shows the installation, with 5 different positions of the array measured (virtual array of 270 microphones). 8 reference microphones are measured for synchronization. Figure 1. Engine in the test cell (left), mesh with the measured array positions (right) 5248

As the transfer functions simulation only contains the powertrain mesh, and not the supporting structure, all the beams of the bench have been covered with acoustic material, to avoid reflections. 2.3.2 Results Figure 2 is an example of the hologram obtained for at 2 khz for maximum torque, third gear, and a selected engine speed. The localized area correspond to the oil pump of the engine. Figure 2. Intensity hologram at 2 khz for different faces view, full torque, third gear. In order to validate the acoustic power levels computed with array measurement, an additional test with intensity probe measurement has been done at low idle speed, and with a partial torque with fourth gear engaged. Power determination using intensity probe follows the ISO 9614-2 standard, scanning each face of the powertrain. According to indicators obtained during the intensity scan (engineering grade), the uncertainty range reaches 2dB. Figure 8 shows the comparison with the array based method. Both curves stay within the 2dB uncertainty range given by the ISO standard. This 3D intensity hologram presents lots of advantages: Analysis is much easier because it is display on the mesh, and doesn t need the comparison of 2D faces hologram, A function enables the selection of sub-area, directly on the mesh, to compute a reliable ranking of their emitted power. Figure 3. Power level comparison, partial torque-4th gear, between intensity probe (blue), and Generalized Acoustic holography (orange) The following step consists in integrating the powertrain characterization results in a simulation software able to compute the radiation of the powertrain installed in the car. The objective is to optimize the acoustic shields package. 5249

3. Vehicle acoustic model A boundary element model of the vehicle is built in order to compute the noise propagation from the powertrain to the pass-by noise certification microphones. The model proposed here uses surface boundary elements to describe sources, absorbing and reflecting surfaces. The resolution is based on a boundary element energy method described in [11]. The objective is to compute the noise contribution of the powertrain during Pass-By tests, and then to minimize it by optimizing the acoustic shield package. 3.1 Vehicle mesh The vehicle parts and its environment are meshed with boundary elements. As the solution is based on power balance, without taking into account waves interferences, there is no wavelength based criteria. Only the geometry and the distribution of sources and materials must be respected. The powertrain mesh is the same that was used for its acoustic characterization. A farfield cube is meshed with 1m² square element. It is not mandatory, but it gives the opportunity to compute the directivity of the noise radiation and the global radiated power. Point receivers are used to compute pressure levels at 7.5m microphone positions corresponding to certification microphones for discrete positions of the vehicle on the track. The ground is meshed under the vehicle to account for noise reflection back to the vehicle body. Outside the vehicle area, the ground is not meshed: reflected acoustic paths are accounted for by considering pressure levels on image point receivers at 7.5m. Only the vehicle parts that may impact the noise propagation toward the certification microphones are meshed. Figure 4 Vehicle, ground and farfield mesh 3.2 Material configurations For a given mesh, several material configurations can be defined, by assigning chosen materials to selected groups of elements. The location of the acoustic shields package in standard configuration is shown on figure 5, and on figure 6 for the vehicle body, the tires and the ground. Figure 5 Right : Standard configuration of acoustic shields Left : absorption and transmission coefficients 5250

Figure 6 Absorption and transmission coefficients of the ground and the vehicle body 3.3 Source configurations Power spectrum is exported for each element as function of the frequency, and imported in Sonor simulation software as a source configuration. The noisiest test conditions (full-load, third gear ratio at 1900 rpm) are chosen to quantify the powertrain noise contribution, and to optimize the shields package. The powertrain noise map, consisting in one third-octave band power spectrum per element, is loaded into the model. 3.4 Solve Theoretical details can be found in references [7] to [10]. However, the basic theory is recalled in the following. 3.4.1 Model assumptions Uncorrelated waves are assumed. Interferences between waves are neglected. This assumption is valid for acoustic problems at high frequencies, with broadband sources and complex environments. Sources are described by their emitted power. Sound power is uniformly distributed on the source surface and radiates diffusely in all directions. The incident field on each element is diffuse. Absorption, reflection and transmission on the boundaries are locally diffuse (random incidence) and described by energy coefficients i, r i and ij in accordance with the equations below. If an element i receives a power w incident(i) from the other elements it will absorb part of the incident power depending on its diffuse absorption coefficient α i following equation (3): w absorbed ( i) w ( i) (3) i incident Diffuse reflection is governed by the reflection coefficient r i w reflected (i) r w ( i) (4) i incident The power transmitted from face k to face i of an element is given by equation (5), where τ ki is the transmission coefficient in diffuse field. w transmitte d(i) ikwincident( k) (5) Coefficients α i, r i and τ ki are related by equation (6), deriving from the power balance on element i. 1 r (6) i ik i 3.4.2 View factors computation The energy exchanged by a couple of boundary elements depends on their mutual view factors. These factors are only related to geometry (elements area, orientation, presence of obstacles between the 2 boundary elements). In the absence of obstacles, the view factors can be obtained analytically. To account for the presence of obstacles, the computation of view factors is performed by an algorithm based on Hemicube method. 5251

3.4.3 Solution of the boundary element problem To derive the system of equations to be solved, the power balance is written on each boundary element. For acoustic problems, the power w(i) emitted by the element i is the sum of reflected, transmitted and source powers: w(i) wreflected ( i) wtransmitte d ( i) wsource( i) (7) Reflected and transmitted powers are expressed in terms of incident powers using equations (4) and (5). The incident power w incident(i) is the sum of the powers w(j) emitted by the other elements j, weighted by the view factors F ji: w incident( i) F w( j) j i ji Introducing equations (4), (5) and (8) in equation (7) gives (8) w(i) (1 ) F w( j) F w( j) W i ik j i ji ki j k jk Source i (9) One obtains a system of n equations (with n the number of boundary elements), with n unknowns w(i) being the powers emitted by each element i. This linear, well-conditioned system is solved using sparse algorithm SuperLU. 3.5 Results analysis 3.5.1 Emitted intensity map The resolution of the linear system first provides the power emitted by each element face. This result can be plot as an emitted intensity map. Looking at this map from a receiver point of vue gives a straight insight to the dominant propagation path. On figure 7 it is clear that most of the radiation originates from the right region of the engine compartment and propagates to the right hand side receiver through apertures beside the shield under the powertrain and then by reflection on the ground. 40 db(a) 40 db(a) Figure 7 Emitted intensity Right view Bottom view 3.5.2 Incident Intensity map From Equation (8) the incident power can be computed and plotted as incident intensity map (figure 8). This gives useful information on best locations for adding absorption material 40 db(a) Figure 8 Incident intensity map 5252

3.5.3 Transmitted and reflected intensity maps From equations (4) and (5) the reflected and transmitted powers are computed and plotted in terms of intensity maps. Figure 9 shows such maps for the shield under the engine bay. It appears clearly that most of the emitted power (figure 7) is due to reflected power and very little is transmitted through the shield. It becomes clear that the transmission coefficient of the shield is high enough, and that a gain could be obtained by increasing absorption on the bottom side of the shield, in order to calm down reverberation between the shield and the ground. 40 db(a) Figure 9 Reflected (left) and transmitted (right) intensity maps plotted on the shield under the engine bay 3.5.4 Power balance Power balance can be computed from the different intensity fields, in order to quantify the powers incident/reflected/transmitted by a given shield, radiated to the farfield, incident on potential locations for absorbing material or on panels likely to transmit noise to the cabin interior. 3.5.5 Pressure maps and spectra From equation (10), the pressure levels can be computed on point receivers and plotted as pressure maps (figure 10) or pressure spectra (figure 11). W( i) 2 cvfij i Si (10) p²( j) 0 Where p²(j) is the square pressure on point receiver j, W(i) is the power emitted by element face i, and VF ij is the solid angle between point j and element face i. 10 db(a) Figure 10 Powertrain global noise contribution level on receivers at 7.5m. Noise levels are higher on the right hand side of the vehicle and on image receivers (reflected path) 5253

5 db(a) Figure 11 - Pressure contribution levels of engine sides, gearbox and global powertrain, on the certification microphone at 7.5m corresponding to maximal noise during pass-by Engine faces contribution on figure 11 are obtained by switching on only the corresponding source in a specific loadcase. This result is useful to focus optimization on the predominant source itself by engine shielding or vibration troubleshooting, or on its transfer path by improving the noise shield nearby. 3.6 Validation The computation results are validated by acoustic measurement with operating vehicle. 3.6.1 Global radiated power Figure 12 shows the global power radiated by the vehicle at low idle speed. The grey curve corresponds to computed results using the power map on the powertrain mesh measured on an engine bench and the partial mesh of the vehicle as described before, with the standard shield package. The orange curve corresponds to measurements of the power radiated by the whole vehicle in a reverberant room according to ISO standard relative to power measurement from pressure measurements. Computation seems to underestimate the global power level by 1.4 dba. This is a rather good result, considering - the measurement uncertainties for source characterization, - the measurement uncertainties for vehicle power estimation, - the reproducibility of engine running condition after engine removal, installation on the engine test bench, reinstallation on the vehicle - the possible influence of the air intake and of the exhaust (partially masked but not deduced from the global power) - the simulation model uncertainty 5 db(a) Figure 12 Global power radiated by the vehicle at low idle speed 5254

Figure 13 shows the Insertion Loss computed between two different vehicle shields configurations, obtain from computation model (grey curve) and vehicle power measurement in reverberant room (orange curve). Curves do fit much better (0.3 dba difference on the global I.L. levels). It is likely that both measurement and simulation errors compensate when computing I.L. from power ratios. The model can be considered as validated for Insertion Loss computation, which characterize the efficiency of vehicle shields packages. 0.5 db(a) Figure 13 - Insertion Loss between two different shield configurations 3.6.2 Pressure Levels To compare pressure levels obtained in the condition of Pass-By Noise tests, the vehicle has been installed on a roller bench in a semi-anechoic room, and the engine speed and gear ratio are chosen to fit the running conditions during acceleration phase of PBN test, at the position of maximal noise. Pressure levels are measured at 2m from the vehicle axle rather than 7.5m, due to technical feasibility in the room. Both front wheels are equipped with circulation tyres, however chosen amongst the quietest. As the tires are turning on the roller-bench during full load test, their noise contributions at exterior microphones, measured at low idle speed with tyres forced by rollers at 50km/h, have to be deduced from the operating pressure levels, in order to compare the only powertrain noise contributions. This introduces an important uncertainty, as tyres noise contribution levels happened to be of the same order of magnitude as powertrain noise contributions. The powertrain noise contribution, averaged over 4 microphones are compared with the simulated powertrain noise contribution on figure 14. Although the powertrain noise contribution is overestimated in low frequencies and slightly underestimated at third-octave band 2000Hz to 3150Hz, the global level is overestimated by only 0.9 dba, standing in the range of measurement uncertainty. The method seems therefore to be well suited for pass-by noise simulation of powertrain contribution. 5255

5dB(A) Figure 14 Measured and simulated noise contribution of the powertrain at noisiest running conditionsduring PBN test 3.7 Optimization of the vehicle shields package From the results obtained with the standard shield package and the analysis of transfer paths using the intensity maps, four optimized configurations have been computed to foresee their expectable acoustic attenuations. The maximal shield package, named opti4, is shown on figure 15. It leads to a computed IL of 7 db(a) regarding the noise contribution of the powertrain at 7.5m (Figure 16). The optimized shields package is now being studied in details by industrial partners in order to converge to a realistic yet acoustically optimized solution. Optimisation of innovative material is also under development. Figure 15 - Maximal shields package (configuration opti4) 5256

1 db(a) 300 500 1000 2000 4000 serie opti1 opti2 opti3 opti4 Figure 16 - Simulated Insertion Loss 4. CONCLUSIONS In the context of the upcoming reduction of Pass-By Noise limits in the EU regulations, Vibratec and MicrodB propose a global method to quantify a powertrain on the basis of a 3D boundary element mesh, and then introduce this source in an acoustic model of the vehicle, that can be used to predict PBN levels and to optimize acoustic shields. Experimental validations show fairly good agreement and tools are fully developed for industrial applications. An optimization of the shields concept of a Renault car has been performed, reaching a global 7dB(A) Insertion Loss. The design study of the shields is being performed, taking into account all industrial constraints, and the final vehicle prototype is expected by the end of 2016. ACKNOWLEDGEMENTS This work is part of ECOBEX project, funded by French state (BPI, DGCIS), regions Rhone-Alpes, Nord Pas de Calais and Grand Lyon. Authors also acknowledge all Ecobex industrial partners for their support and helpful collaboration. REFERENCES 1. Minck, Olivier & Al. Array based acoustic power measurement, Renault Pass-By Noise. In: Proceeding of NVHG Congresse, Japan; 2016. p. 887-909. 2. Yoon. S.H, Nelson. P.A. A method for the efficient construction of acoustic pressure cross-spectral matrices [J] Journal of sound and vibration, 2000, 233(5): 897-920. 3. Le Magueresse. T, Minck. O. Source localization inside cabin using calculated Green s function. [J]. Automotive NVH Confort, SIA 2012. 4. Le Magueresse T. Approche multidimensionnelle du problème d identification acoustique inverse», PhD Thesis, 2016. 5. Pavic. G. A technique for the computation of sound radiation by vibrating bodies using multipole substitute sources. [J]. Acta Acustica united with Acustica, 2006, 92(1):112-126. 6. Antoni. J. A Bayesian approach to sound source reconstruction: optimal basis, regularization, and focusing. JASA, 2012. 7. M. Thivant, P. Bouvet, A. Cloix and N. Blairon, Boundary Element Energy Method for the acoustic design of vehicles shileds, Proceedings of ISNVH, Graz (2008) 8. A. Le Bot, A vibroacoustic model for high frequency analysis. In Journal of sound and vibration, vol.211, no.4,, p. 537-554 (1998) 10. B. Betgen, M. Thivant, P. Bouvet, A. Cloix and A. Bocquillet, Boundary Element Energy Method for the acoustic prediction of vehicle external and interior noise Validation on a mock-up and industrial application In Proceedings of DAGA (2011) 11. B. Betgen, M. Thivant, P. Bouvet, A. Cloix and T. Lambert, Boundary Element Energy Method for the acoustic prediction of automotive external noise In Proceedings of DAGA (2013) 5257