Effects of baffle on acoustic attenuation performance of micro-perforated tube muffler

Abstract: The correctness of the numerical finite element method to calculate transmission loss (TL) of the micro-perforated tube muffler is validated through the measurement of the transmission loss of a micro-perforated tube muffler sample. The transmission loss is measured by a two-load technique that is usually applied to measure the acoustic impedance of micro-perforated panel (MPP) and the transmission loss of various mufflers. A theoretical calculation model of the transmission loss of the micro-perforated tube muffler with a baffle is then established and afterwards verified by the finite element method that has been previously validated. The theoretical model is based on the analysis of a simple pass-through micro-perforated muffler and the plane wave theory. The transfer matrix method is applied to connect the sound pressure and particle velocity of the inlet and the outlet of the muffler. Additionally, boundary conditions and continuity requirements are considered in the theoretical modeling process. Finally, several analyses of the effect of the baffle on the transmission loss of the micro-perforated tube muffler are conducted respectively, which are based on the theoretical model mentioned above. Specifically, two key factors: the axial position of a baffle and the number of the baffles arranged uniformly in the axial direction of the muffler, are under consideration. Five different positions of the baffle lead to different transmission loss curves. In addition, a detailed analysis is given concerned with the baffles number that increases from 0 to 7. Besides, a comparison among three different micro-perforated tube mufflers is given to illustrate the relationship between the baffle and the air cavity length from the aspect of the transmission loss. Consequently, the results of all analyses reveal that, on one hand, the position of the baffle located in the air cavity of the micro-perforated tube muffler alters the main sound attenuation frequency band and the corresponding transmission losses. Furthermore, when the baffle is installed closer to the central cross-section of the air cavity, the first sound attenuation frequency domain arch extends towards the higher frequencies and also the corresponding transmission losses become greater than before. On the other hand, the transmission losses of the micro-perforated muffler with baffles arranged uniformly in the axial direction of the muffler improve with the increasing number of baffles. However, they will not significantly improve once the number of baffles reaches a certain value. Ultimately, we concluded that baffles installed in the air cavity of the micro-perforated tube muffler are beneficial for muffler design to shorten the axial length of the air cavity, thereby shortening the axial length of the entire muffler, without the degradation of sound attenuation performance of the muffler, which is very favorable to reduce the axial dimension of the micro-perforated muffler and thus instructs its optimization and design.