Mufflers are the cause of noise emitted by cars. The car muffler is divided into multi-stage separation and partial pressure combination, and the principle of noise reduction is to use the reasonable combination of cavity and tube to achieve two functions. One method is to use sudden changes in the cross-section of the pipeline to reflect sound waves along the pipeline towards the direction of the sound source, causing the sound energy to be reflected back to its original location; Secondly, by utilizing the reflections of several interfaces, the initially scattered sound waves are returned to their original points, then folded back and propagated forward. This point merges with the second forward propagating sound wave without reflection, with equal amplitude and an odd multiple of 180 degrees in phase difference, thus interfering and canceling each other out.