Study On Computational Method And Experimental Measurement Of Acoustic Performance Of Exhaust Muffler

The traditional calculation and measurement of muffler transmission loss are based on the assumption of plane wave sound field emerging in inlet and outlet ducts previously. The traditional method can not accurately predict the acoustic performance of muffler when the three-dimensional sound field excitation by higher-order modes emerge in inlet and outlet ducts. According to the acoustical theory of duct and nodal lines for transverse pressure distribution in circular duct, the computational method and experimental measurement for acoustic attenuation performance of muffler in middle frequency field are studied.Adding radial bulkheads to the inlet and outlet ducts of muffler can divide the duct into several sectors. This method can suppress the excitation of asymmetric modes in the circular duct. By calculating the excitation frequency of higher modes in the sectors, adding four bulkheads forming four 90° sectors will suppress excitation of (1,0) order mode, adding six bulkheads forming six 60° sectors will suppress excitation of (2,0) order mode. Thus the cut-off frequency of plane waves in the inlet and outlet ducts can broaden to excitation frequency of (0,1) mode. Adding asymmetric tubule in the inlet and outlet ducts of muffler will separate circular duct to one small circular duct and one the ring duct. This method can suppress excitation of (0,1) radial mode, thereby increase the cut-off frequency of the plane wave in the duct. When the sound fields of the inlet and outlet ducts are plane wave, radial dividing tube method and asymmetric dividing tube method may be used to calculate transmission loss of mufflers. The calculated results consistent with the experimental results and conventional finite element calculated results very well.The test bench of exhaust system is designed and built. The isolators, the intake muffler and exhaust muffler are installed to control the noise and vibration problems of the roots blower. Experiment measuring segments are designed for the radial dividing tube method and applying acoustical decomposition method to test the transmission loss of the resistive muffler.Excepting the low frequency and individual frequency, the measurement result and finite element numerical result are in good agreement. At last, according to existing experimental conditions, analysis errors of the experiment results systematically.Applying radial dividing tube method and asymmetric dividing tube method to calculate acoustical performance of large diameter mufflers, and compared with the finite element calculation result of traditional method, the applicability of these methods for calculating transmission loss of large diameter mufflers is validated.