Research On Sound And Vibration Characteristics Of Multi-layer Acoustic Structures

With the development of automotive lightweight technology,the car body panel becomes thinner,which leads to the deterioration of NVH performance.In this situation,both radiation and propagation path should be considered to achieve the desired noise control.Therefore,the multi-layer acoustic structures composed of PCLD(Passive Constrained Layer Damping)and sound absorption structure are proposed to solve the issues,which can suppress the vibration and noise from the suppression of the structure vibration,sound insulation and sound absorption.The vibration and noise control of PCLD,sound insulation characteristic of PCLD and design of sound absorption structure are studied respectively,which summarizes the design rules applied to automobile NVH control and provides the theoretical basis for multi-layer acoustic structure.A new reduced passive constrained layer damping(PCLD)finite element model is proposed.The PCLD structure is a sort of sandwich plate made up of a viscoelastic core sandwiched between two elastic faces.The model is built by combining the first shear deformation theory with the Golla-Hughes-Mc Tavish(GHM)model that takes the frequency dependence of the viscoelastic material property into consideration.Due to the GHM,the stiffness,damping and mass matrices is doubled at least which requires a large amount of calculation.Then,a modified improved reduced system(M-IRS)method is proposed to reduce the order of the model.Finally,the proposed reduced model is compared to the Guyan reduction,the mode truncation,and the improved reduced system(IRS)models by two numerical examples.It demonstrates that the proposed M-IRS method is obviously superior to the other three classical methods and the presented PCLD model with the GHM is an effective and accurate sandwich model,which can be applied to the finite element software.Based on the model,the effects of the constrained layer thickness,density,elastic modulus and viscoelastic layer thickness,density,shear modulus steady state value are systematically investigated.A theoretical investigation on the sound transmission loss characteristics of four-side simply supported sandwich panels considering the flexural rigidity of the face sheet is presented.With the flexural rigidity of the face sheet taken into account,the sound transmission problem of the sandwich panels is derived from the governing equation of bending vibration.The sound transmission loss(STL)expression is also derived.The validation of the theoretical prediction model is validated by comparing with the high-accuracy finite element and boundary element simulation.Numerical analysis shows that the flexural rigidity of face sheet influences the natural frequencies obviously,and the theoretical prediction model proposed has high accuracy on predicting the natural frequencies and STL of four-side simply supported sandwich panels.The effects of the face sheet flexural rigidity,the thickness of face sheets and core layer,as well as the damping coefficient of the core on the STL are systematically investigated.A tunable dual-band perfect sound absorber is developed to improve the lowfrequency absorption based on the polyurethane foam.First,we propose a hybrid sound absorber composed of the polyurethane foam and Helmholtz resonator.The impedances of the Helmholtz resonator and the polyurethane foam are derived by the Helmholtz resonator theory and Delany-Bazley empirical formulations.Then the equivalent network is applied to present the impedance of the hybrid sound absorber.Next,numerical applications are implemented using MATLAB to predict the sound absorption coefficients of the hybrid sound absorber.It shows that the hybrid sound absorber can provide more sound absorption than the polyurethane foam in low frequency,while in high frequency,it provides more than the Helmholtz resonator.It also implies that the hybrid sound absorber is a tunable sound absorber in low frequency.Finally,experiments have been done by the two-microphone impedance tube method to validate the theoretical results.A three-aperture MPPA parallel with different divide cavities has been proposed in this work.The theoretical prediction model of sound absorption is established by the Maa's theory and electro-acoustical equivalent circuit principle.Numerical applications are implemented using MATLAB software to predict the sound absorption coefficients of the proposed absorber.Then,experiments are made to validate the accuracy of the prediction model.Based on the prediction model,the optimization of the perforation diameters,the cavity depths and the perforation ratios is realized by the way of Pointer optimization method.Optimal result shows that the optimal adjustment of the structural parameters can improve the sound absorption performance and broaden the sound absorption bandwidth.Finally,a plate-cavity system is used as the experimental object,and the multi-layer acoustic structure is used to suppress the vibration and reduce the noise.The frequency response function of different situations under a force excitation and an acoustic excitation is tested respectively.By contrast,PCLD structure has the strong ability to suppress the radiated noise under the force excitation and its noise control ability under the acoustic excitation is weaker.The multi-layer acoustic structure with foams has the strong noise control ability in the middle and high frequency range.Optimized hybrid absorber instead of foams provides more sound absorption than the polyurethane foam in low frequency,and it also retains its high frequency sound absorption effect.This provides a new way to solve NVH issues.