Investigation On NVH Performance Of Automotive Magnesium Allov And Acoustic Optimization Design For Magnesium Dash

Magnesium alloy is the lightest application structural material at present, and has large specific stiffness, great damping and excellent shock absorption. It had been made into automotive dash successfully. If original automotive metal materials are replaced by magnesium alloy, the vehicle NVH performance must be affected. Hence, researching on acoustic performance of automotive components is imperative. Applying magnesium alloy to automotive dash might meet acoustic problems. In this dissertation, these acoustic problems were investigated.Damping performance of magnesium alloy AZ31was researched with the impulse response decay method (IRDM). The sound pressure near the magnesium alloy sheet substituted for vibration signals of sheet surface. It could avoid additional mass and boundary constrained damping caused by contact vibration sensors, and got more accurate test results. The near-field sound pressure signals were filtered by band-pass filter. Based on these filtered signals, damping loss factor(DLF) of one modal in low frequency range and average damping loss factor of one frequency band in high frequency range were calculated. DLF of magnesium alloy AZ31is2-25×10-4in0-3000Hz. DLF of steel was measured meanwhile. The results showed that damping performance of magnesium alloy is better than steel.In the respect of research on sound insulation of magnesium alloy plate, based on acoustic boundary condition on interface of acoustic medium and Newton’s laws, sound impedance radio and sound transmission loss theoretical formulas of single-layer plate in the plane-wave and diffuse incident acoustic field were derived respectively. Noise reduction of circular magnesium alloy plate in normal sound incident field was measured with self-designed standing wave tube. Compare the results of single-layer plate and double-layer plate to analyze the influence factors of sound insulation performance, such as thickness of plate, thickness of air layer between two plates and order of two plates.This dissertation advanced a simulation method to be used for predicting sound insulation performance of complex components——structural-acoustic coupling method. Validated by experiments, the method is accurate and reliable. The sound insulation performance of Chrysler Viper dash and Cadillac CTS dash in low-mid frequency range were predicted with this method. The research results showed that replacing steel by magnesium alloy, dash reduced weight by 30%-40%, while acoustic performance didn’t deteriorate. Because of damping, sound insulation performance even improved in low frequency range. Improving seal of flanking paths and enhancing connect stiffness can increase overall sound insulation performance of dash.Based on panel acoustic contribution analysis,"key regions" in "key frequency" range of magnesium alloy dash for sound insulation were identified successfully. Based on Biot-Allard model, acoustic admittance of sound package (transmission-like condition) was calculated. Applying this admittance to laminated magnesium alloy dash, sound insulation performance of the dash was obtained accurately. Research indicates that sound transmission loss of laminated dash doesn’t increase monotonically with the increase of thickness of porous-absorbed material. So when designing laminated dash, cost and "sound insulation performance deterioration" in low frequency range should be taken into account simultaneously and chose appropriate thickness of absorbing material.Established structural-acoustic coupling finite element model of a vehicle, and divided the laminated dash into five panels, the structural-acoustic sensitivities of sound pressure at driver’s ear position with respect to each layer’s thickness of each panel of laminated magnesium alloy dash were calculated. According to the sensitivities, the target point’s acoustic performance was optimized with "feasible direction method" to minimize its sound pressure in whole frequency range. The weight of optimized laminated magnesium alloy dash is reduced3.6kg, and overall sound pressure level at target point drops from78.1dB to76.1dB. The light-weight, low-noise design for magnesium alloy dash is achieved.