Study On Acoustic Performance Of Complex Exhaust Muffler

Equipping the muffler is the simplest and most effective means to control exhaust noise and complex muffler is widely used due to its excellent acoustic performance.This paper aims to study the acoustic performance of the complex exhaust muffler.The acoustic properties of the sound-absorbing materials aluminum silicate and glassfiber are studied.Based on the transfer function theory,the characteristic impedance ratio and complex wave ratio of the two materials are obtained with the four-microphone method and MATLAB programming.The experimental results are then curve-fitted and empirical formulae are obtained respectively.The sound absorption coefficients are measured by the double microphone method and the experimental results are compared.It is found that the sound absorption performance of aluminum silicate at low frequencies is much better than that of the glassfiber.Meanwhile the influence of the thickness of sound-absorbing material on the sound absorption coefficient is studied with glassfiber as an example.It is found that increasing the thickness of the sound absorbing material can effectively improve the sound absorbing ability at low frequencies.The effects of different complex structures on the acoustic performance of the muffler are explored.Based on FEM,the transmission loss of the muffler is calculated using COMSOL software.With expansion chamber muffler as an example,cases like perforated plate only,all rigid walls in the chamber,sound-absorbing material only,sound-absorbing material in contact with perforated plate,and glasscloth sandwiched between sound-absorbing material and perforated plate are studied.The transmission loss results in different cases are compared and analysed.It is found that the perforated plate can effectively increase the transmission loss compared with all rigid walls in the simple expansion chamber;when the perforated plate and the sound absorbing material are attached,the effect of the perforated plate is almost negligible;the difference in TL between material attached to plate and sound-absorbing material only is small,indicating that the sound-absorbing material plays a major role;compared with the complex structure of the sound-absorbing material attached to perforated plate,the transmission loss with the glasscloth sandwiched between is smaller,especially at higher frequencies,which shows that the glasscloth serves as sound insulation to some extent,being a disadvantage to improve the acoustic performance of the muffler.The effects of extended inlet and expansion chamber on the acoustic performance of the muffler are explored.Based on the simulation,the transmission loss of muffler is measured by the two-load method.The experimental and calculation results are in good agreement.When the length of the expansion chamber is fixed,increasing the length of the extended inlet can move the transmission loss curve to lower frequencies,indicating that increasing the length of extended inlet is beneficial to control low frequency noise;when the length of extended inlet is fixed,changing the length of the expansion chamber barely affects the transmission loss,indicating that adjusting the length of the expansion chamber is not an effective way compared with the adjustment of extended inlet.