A ventilation silencer refers to noise reduction devices for air ducts with noise propagation, which can use管道 with an acoustic lining and elbows, or pipes with sudden changes in cross-sectional area and discontinuities in acoustic impedance to attenuate or reflect back the noise within the pipe. The former is called a resistive silencer, and the latter is called a reactive silencer; there are also impedance-combined silencers. There are many types of silencers, but in terms of their noise reduction mechanisms, they can be divided into six main types: resistive silencers, reactive silencers, impedance-combined silencers, micro-perforated plate silencers, small-hole silencers, and active silencers.