Resistive silencers, also known as absorption silencers, utilize sound-absorbing materials to dampen noise. These materials have a loose, porous structure, where sound waves entering the pores cause slight vibrations. Due to friction and viscous resistance, a significant portion of the sound energy is converted into heat and absorbed. Sound-absorbing materials are generally loose or porous, such as glass wool, foam plastics, slag wool, felt, asbestos wool, sound-absorbing bricks, aerated concrete, and wood fiber boards. Their main characteristic is the numerous tiny holes贯穿 the material, known as an open-cell structure, which differs from the closed-cell structure of most insulating materials.
The resistive silencer exhibits good absorption effectiveness for high and mid-frequency noises but has poor noise attenuation performance for low frequencies. It is primarily used for eliminating noises dominated by high and mid-high frequencies.
The tubular silencer is designed for ducts with diameters not exceeding 400mm, featuring a simple construction with minimal resistance, achieved by applying an acoustic material only to the inner circumference of the tube wall.
For larger diameter ducts with sheet or format silencers, increasing the contact area between the sound-absorbing material and sound waves reduces the direct transmission of mid-to-high-frequency noise to ensure effective noise reduction.
The resistance silencer, also known as an expansible silencer, consists of a tube connected to a chamber, which utilizes the abrupt change in the wind tube cross-section to reflect noise. Its structure is simple, does not use absorbent materials, and therefore is unaffected by high temperatures and corrosive gases. The resistance silencer offers excellent noise reduction for low and mid-frequency noise, primarily used for eliminating noise dominated by low and low-mid frequencies.
To ensure adequate soundproofing effects, the expansion ratio of the silencer (the ratio of the larger to smaller cross-sectional area) should be greater than 4, which limits its application in smaller spaces such as server rooms.
Resonant mufflers utilize pipeline openings connected to resonance chambers, forming an elastic resonance system with the air inside the cavity. When the external noise is at the same frequency, resonance occurs, and sound energy is dissipated through friction at the perforation point and the wall. Resonant mufflers fall under the resistive category, with strong frequency selectivity, meaning their effective frequency range is narrow, primarily used for eliminating low-frequency noise, narrowband mid-frequency noise, or noise peaks. They have low resistance, do not require absorbent materials, but are larger in structure due to the resonance chamber. When the perforation plate of the resonant muffler is reduced to ≤1mm in diameter, it becomes a micro-perforated plate muffler. The structure is simple, does not require absorbent materials, and is particularly suitable for muffling in pipeline systems with high temperatures, humidity, and cleanliness requirements.
The impedance composite silencer combines the advantages of both resistive and reactive silencers, offering excellent noise reduction for frequencies ranging from low to high. The low-frequency noise reduction of a composite silencer measuring 1.2 meters in length can reach 10 to 20 dB.
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