Research On Acoustic Design Of Muffler Test Bench For Comprehensive Performance

Muffler actually work in the state of high-speed flow,the static transmission loss test does not fully represent the actual performance of the muffler.The use of engine bench to making transmission loss test not only need to spend a lot of manpower and material resources,and difficult to meet the high precision test requirements.Therefore,the design of the muffler transmission loss test bench which can be conducted in the flow conditions and suitable for different specifications is particularly important.It's aimed at conducting the muffler transmission loss under high speed flow condition.Through the analysis of the flow field and sound field requirements of the test environment,the four major forming systems was determined which are fan system that provides high-speed air flow for the test bench,sound source system that provides a highlevel white-noise signal,test system that can perform high-precision measurements and muffler system that eliminates airflow noise.The overall layout of the test bench was also designed.Based on the basic acoustics principle and numerical calculation method,the design of a new sound source structure has been designed.It includes that(1)By the analysis of the characteristics of the sound source speaker,selected the combination form of low frequency and middle frequency speaker,because of the different working frequency;(2)The sound field of the speaker and the bypass structure is compared,the bypass sound source structure with better frequency characteristics is selected;(3)Through the simulation of the sound field of the bypass structure,the sound source structure was determined that using the form of single low frequency bypass pipe and double middle frequency bypass pipe combination.(4)The design of the pipe connection uses a perforated structure.Compared with the flow field simulation with no perforation structure,it is shown that the perforated structure is beneficial to reduce the instability of the flow shear layer at the pipe joint,and lead to a lower noise level.At the same time,through the simulation of the sound field of the perforated structure,it shows that the perforation structure has little effect on the acoustic signal propagation.(5)Based on the research of high-order digital equalizer algorithm,a parametric equalizer can be designed to compensate the sound source electrical signal.The experimental results show that good results can be obtained.In order to improve the accuracy of the test under the condition of flow,the microphone was selected and a special sampling tube structure was designed.Through the analysis of the flow field and sound field of the sampling tube,it is shown that the sampling tube is suitable for acoustical measurement at high flow rate.The error introduced when using a tapered transition tube for different specifications of the muffler was evaluated.The results show that choosing the small inlet/outlet ratio and long length will lead to a little error.The error in passing frequency is also little.A modified algorithm for inversion of transfer matrix is developed by using the principle of two-load method.The experimental results show that the error can be effectively reduced.By developing the appropriate transmission loss test and correction algorithm,the development of the test software is carried out.Finally,it is shown that the new sound source system with the combination of low frequency and intermediate frequency loudspeaker designed in this paper can provide flat high sound pressure level sound signal in the frequency range of the test.The test system designed in this paper can carry out high-precision transmission loss measurement under the condition of flow,and greatly simplifies the test flow and improves the test efficiency.