According to the interference principle, there are two basic noise elimination methods of exhaust muffler: absorption method and reflection method. The absorption muffler reduces the energy of waste gas by rubbing sound-absorbing materials such as glass fiber, steel fiber and asbestos. Reflective muffler has several harmonic cavities in series and porous reflection tubes with different lengths, in which the exhaust gas is reflected, collided, expanded and cooled for many times to reduce its pressure, vibration and energy. In fact, most cars use different combinations of silencing principles to design exhaust silencers, and at the same time, small passenger cars with high comfort requirements will use multiple silencing units for multi-stage silencing and noise reduction control.
According to the principle and structure of noise elimination, car muffler can be divided into three types: resistance muffler, resistance muffler and impedance compound muffler. 1 The resistance muffler is a kind of muffler device, such as expansion chamber and vibration chamber. Among them, sound waves reflect and interfere in the process of propagation, thus reducing acoustic energy and eliminating noise. The frequency band of reactive muffler is limited, and it usually works well in middle and low frequency bands, but it works poorly in high frequency. Most freight cars use reactive silencers. 2. The resistive muffler E is a muffler that fills sound-absorbing materials around the pipeline through which the internal exhaust gas passes to absorb sound energy and achieve the purpose of noise elimination. It has a good noise elimination effect on middle and high frequencies, and is rarely used as an automobile exhaust muffler. It is usually used in conjunction with a reactive muffler. Three-impedance compound muffler is composed of impedance muffler unit and sound-absorbing material, which has the same characteristics as impedance muffler. It has a good noise elimination effect on low, medium and high frequency noises.