Research On Prediction And Control Methods Of Automobile Middle And High Frequency Noise

Automotive interior noise can be directly perceived by passengers and can also influence the preferred orientation of buying a car for customers. Automotive exterior noise which affects ambient environment directly is one of pollution sources of environmental noise. Some rules are presented to restrict automotive interior and exterior noise. In order to improve competitiveness of the automobile, automotive interior and exterior noise are predicted and controlled applying different methods by vehicle manufacturers. Also, the development period can be shortened, and the development and research costs can be reduced. Statistical energy analysis (SEA) is an effective method for high frequency prediction, while, hybrid finite element and statistical energy analysis (FE-SEA) is a powerful tool for middle frequency prediction. Thus, middle and high frequency prediction can be realized combining SEA and hybrid FE-SEA methods. Sound packages can be optimized using grey relational analysis (GRA) and design of experiment (DOE). Furthermore, automotive interior and exterior noise can be controlled.Taking a domestic passenger vehicle as research object, a SEA model of the vehicle was created. According to the basic principles and hypothesis of SEA, the SEA model was divided into 45 subsystems. The modal densities, damping loss factors (DLF) and coupling loss factors (CLF) of the subsystems which could be simplified into flat plat and beam were calculated using analytical formulas. The modal densities and DLFs of the complicated subsystems such as door, fender, etc. were measured using admittance and steady-state energy flow methods, respectively. The DLFs of the acoustic cavities of the passenger and luggage compartments were tested using 60 dB decay method. The CLFs of the point junctions in the SEA model were calculated using analytical formula. The CLFs of the polyline, arc, and arbitrary curve junctions were derived based on the theory of straight line junction. Numeric calculation was performed for these four kinds of junctions. Also, the CLFs of the line juctions in the SEA model were calculated using the derived formulas. Lastly, the CLFs of the face junctions including structure-cavity and cavity-cavity were calculated.Four kinds of different excitations of the SEA model were obtained through experiments and numeric simulation. Sound radiation excitations were acquired on a roller bench at three different conditions in a semi-anechoic room. The excitations of engine mounts and road roughness were also measured on an asphalt road at three different conditions. A computational fluid dynamics (CFD) model of the vehicle was created, and some monitoring points were set to obtain wind excitations of the outer surfaces of the automobile.In order to improve the analysis precision of SEA method, the modal densities of the SEA subsystems were calculated using finite element method (FEM). The automotive interior noise was predicted using the SEA model. The prediction results of the interior noise were compared by applying different modal densities calculated between simplified method and finite element method. Both of the prediction results of the interior noise were verified, and the validation results show that the prediction precision can be effectively improved applying the modal densities calculated by using FEM. The effect degree of the modal density to automotive interior noise was presented and analyzed. The modal densities of the subsystems have little effect as the velocity changes. The excitations of the SEA model were acquired in the process of the forward design of automobile noise, vibration and harshness (NVH). Firstly, the rotation speeds of the engine were determined as 2500 r/min and 3200 r/min corresponding to the vehicle speed of 50 km/h and 120 km/h. The sound of the engine was tested in the semi-anechoic room. The radiation distances of the sound source were determined. Furthermore, the sound radiation excitations were calculated on six surfaces in the engine cabin using point sound source method. Secondly, an accelerometer was placed at the engine side of the engine mount, and the perpendicular accelerations of the engine mounts were measured at the engine side at the corresponding rotation speed of 50 km/h and 120 km/h. The engine mount excitations to body can be obtained accordint to the requirement of the transmissibility and the vibration isolation ratio formula. Thirdly, both of the multi-body system dynamics model of the vehicle and B-level road model were created. The road excitations were measured when the multi-body system dynamics model of the vehicle was running on the B-level road at even speed of 50 km/h and 120 km/h. Forthly, the wind excitations were calculated using CFD method. The excitations including sound radiation excitations of the engine, engine mount excitations, road excitations and wind excitations, and parameters including modal densities, damping loss factors and coupling loss factors obtained in in the process of the forward design of automobile NVH were added into the SEA model, and the automotive interior noise was predicted at speed of 50 km/h and 120 km/h.A SEA model of automotive exterior noise was created based on the SEA model of the automotive interior noise. The DLFs of the exterior acoustic cavities were determined. Also, the modal densities of the exterior acoustic cavity and the CLFs of the exterior cavity-cavity and structure-cavity were calculated using analytical method. The condition of the exterior noise prediction was determined as 55 km/h. The excitations including sound radiation excitations of the engine, engine mount excitations, road excitations and wind excitations were measured or calculated at speed of 55 km/h. Then, the automotive exterior noise was predicted by the A SEA model of automotive exterior noise with the calculated excitations. Another SEA model of the automotive exterior noise was also created based on semi-infinite fluid; meanwhile, the exterior noise was also predicted with this model. The prediction results of the automotive exterior noise were compared with the experiment result. The comparison result shows that both of the SEA models of the automotive exterior noise are credible. The excitations including sound radiation excitations of the engine, engine mount excitations, road excitations and wind excitations were obtained through experiments and numeric simulation in the process of the forward design of automobile NVH. All the excitations and parameters acquired in the process of the forward design of automobile NVH were added into the SEA modal of the automotive exterior noise. The prediction of the automotive exterior noise was 74.05 dB(A). The near field exhaust noise and tire noise were both measured. Eventually, the automotive exterior noise was determined as 74.0528 dB(A) through adding prediction result of the SEA model, exhaust noise, front and rear tire noise together. The adding result is 1.45 dB(A) larger than the experiment result. The relative error is 2.00%. The absolute error is less than 2.00 dB(A). The prediction result is satisfied by the engineering precision requirement. It is fully demonstrated that the prediction of the automotive exterior noise is feasible and credible in the process of the forward design of automobile NVH.In order to control the interior and exterior noise, the absorption and insulation performances of the commonly used materials were measured by using impedance tube of B&K 4206 type. The contributions of the body plates to the interior noise were analyzed. Optimization of the sound packages of the firewall and front floor was presented coupled with grey relational analysis (GRA) and design of experiment (DOE) methods. The weight of the sound packages of the firewall and front floor were reduced by 7.72 kg. The automotive interior noise was predicted at 50 km/h and 120 km/h after optimization. The prediction results were compared with the interior sound pressure level (SPL) before optimization. The comparison results show that the automotive interior noises were reduced by 1.41 and 1.21 dB(A) at 50 km/h and 120 km/h, respectively. It is fully demonstrates that GRA is an effective method for optimization of automotive sound packages. Based on the optimization of the automotive interior sound packages and the structure and thickness ratio of the sound package of the firewall, the sound packages of the fender and engine cover were newly designed. The weights of the newly designed sound packages of the fender and engine cover were reduced 0.8616 kg and 1.3444 kg, respectively. Meanwhile, the exterior noise of the vehicle was predicted by the SEA model with the newly designed sound packages. The prediction result of the exterior noise was 73.15 dB(A). The exterior noise is reduced by 1.44 dB(A). In order to improve the prediction precision of the middle frequency noise, a simplified hybrid automotive FE-SEA model was created. Some basic principles of the hybrid FE-SEA modeling were presented. An exact hybrid automotive FE-SEA model was created based on these principles. The interior noise of the vehicle was predicted by both of the simplified and exact automotive hybrid FE-SEA models. The prediction results were compared with the experimental results. The comparison results show that the absolute error is within 2.00 dB(A) which is satisfied by the engineering requirement; and the prediction precision of the exact automotive hybrid FE-SEA model is larger than the simplified one. Eventually, it is fully demonstrated that the exact FE-SEA model is credible.