Study On Mode Coupling Mechanism And Design Method Of Car Body To Lower Interior Noise

Developments of modern analysis method and control technology, appearance of high performance materials, increase of cars’performance make interior noise a noticeable problem. To lower car interior structure-borne noise, the control methods of dynamic features of structure-acoustic coupled systems is one of the important aspects of researches on interior noise. Deep understanding of the structure-acoustic mode shape and frequency coupling mechanism and achieving the determinant parameter of sound pressure level (SPL) are key basements of accurate predicting and controlling of car interior structure borne noise. Because of the unclearness of the structure-acoustic mode shape and frequency coupling mechanism and the imperfect of the determinant parameter of sound pressure level (DPSPL), relative research work is carried out in this thesis. First the structure-acoustic mode shape and frequency coupling mechanism is deeply analyzed. Then DPSPL is discussed and a new way for car interior noise control is given out that offers strong surpport for design of car body during design stage.The virtual prototype to analyze car interior structure-borne noise at low frequency is established. That includes the finite element models of BIW, BIW with windshields, car doors, trimmed body only with car doors and windshields, trimmed body, acoustic cavity and the structure-acoustic coupled system. The accuracy of the models is proved by comparing the analytical and experimental modes. Moreover, though comparisons of the analytical and experimental results of the static stiffness and interior sound pressure, the correctness of the models is further verified.Deep analysis of the structure-acoustic mode shape coupling mechanism is done. The new function to calculate the mode shape coupling coefficient (MSCC) of any shaped coupled systems by FEM is derived and validated by comparing the FEM and analytical results which prove the new function. Then MSCCs of regular shaped, irregular shaped, and a car panel-cavity coupled system are calculated and analyzed in detail. Finally the conclusion is achieved that the mode shape similarity and complexity on the contact surface determine MSCC. That consequently determines the coupling degree between the structure and acoustic cavity. The lower the mode shape similarity and the higher the mode shape complexity on the contact surface, the weaker is the coupling degree; on the contrary, the stronger is the coupling degree. Along with the increase of the shape complexity of the coupled system, MSCC tends to general small values leading to general weak coupling between structure and acoustic cavity.Deep analysis of the structure-acoustic frequency coupling mechanism is done. When two uncoupled structure and fluid modes couple to each other, making the differences of their uncoupled natural frequencies changed in certain range, the coupled frequencies have certain distribution and domination features. The distribution feature is that two uncoupled modes will create two coupled modes; the higher coupled frequency is higher than the higher uncoupled frequency, the lower one is lower than the lower uncoupled one; with the increase of the frequency difference, the difference between the higher coupled and uncoupled frequency tends to small, and so as the lower one.The domination feature is that if the uncoupled natural frequency of structure equals to the acoustic one, the lower coupled frequency is dominated by both subsystems and the higher one is a little strongly dominated by fluid; if the uncoupled structure natural frequency is lower than the acoustic one, along with the increase of the frequency difference the lower coupled frequency is more strongly dominated by structure, and the higher coupled one is more strongly dominated by fluid; if the uncoupled structure natural frequency is higher than the acoustic one, with the increase of the frequency difference, the lower coupled frequency is more strongly dominated by fluid, and the higher coupled frequencies is more strongly dominated by structure.Base on the analytical function of car interior SPL, the parameter for rapidly predicting car interior noise is achieved, which is DPSPL of car interior structure borne noise. From DPSPL the primary modes of structure and fluid that lead to higher SPL are clearly focused. By decrease the amplitude of modes of structure and acoustic fluid on the contact surface, SPL would be lowered. That gives clearly guidance for structure improvements. And it offers a brand new way for car interior noise control.A brand new design method of car body to lower interior structure borne noise is given out. Base on DPSPL, the interior noise is analyzed and optimized with the result of the highest noise reduction of 10dB at the driver’s right ear. This proves the control of DPSPL could rapidly focus the modes of structure and acoustic cavity which lead to higher interior SPL and by lower DPSPL the coupling degree is weakened resulting in the decrease of the car interior SPL.