Muffler is a device that uses the principles of sound absorption, reflection and interference to reduce airflow noise in ventilation and air conditioning systems. According to the different principles of noise elimination, it can be divided into resistance, resistance, resonance type and composite type.
1、 Resistive muffler
The resistive muffler uses the sound absorption function of the sound absorption material to eliminate noise. Its structure is to fix the sound-absorbing material on the inner wall of the air flow pipe, or arrange it in the pipe or shell in a certain way to form a resistive muffler. The sound-absorbing material can partially absorb the sound energy incident on it. The reason why sound energy can be absorbed is due to the porosity and looseness of sound absorbing materials. When the sound wave enters the pore, the air and material in the pore will produce tiny vibration due to friction and viscous resistance. So that a considerable part of the sound energy is converted into heat energy and absorbed.
It has a good effect on high-frequency and medium-frequency noise, but has a poor noise reduction performance on low-frequency noise.
1. Pipe muffler
The tubular muffler is the simplest muffler. It only sticks a layer of sound-absorbing material around the pipe wall, so it is also called "pipe lining". It is characterized by convenient fabrication and small resistance, but it is only suitable for small air ducts, and the diameter is generally not greater than 400mm. The tubular muffler can only eliminate noise at medium and high frequencies. The low frequency performance is very poor.
2. Chip and format muffler
The tubular muffler has poor low frequency performance, and is easy to pass through medium and high rate noise. When the pipe section area is large, it will affect the noise elimination effect of high frequency noise. This is because the high frequency sound wave (short wavelength) propagates in a narrow beam in the pipe. When the pipe area is large, the contact between the sound wave and the sound-absorbing material on the pipe wall is reduced, thus reducing the noise elimination amount of high frequency sound. Therefore, the section of the air duct with large section can be divided into several grids, This is the chip and format muffler. The chip muffler is widely used and has simple structure. The effective sectional area of the format muffler should be no less than the section of the air duct, so its volume is large. The size of each cell should be controlled at 200mm × About 200 mm. The disc spacing of the disc muffler is generally within the range of 100~200mm. When the disc spacing increases, the noise reduction will decrease accordingly.
2、 Resonance muffler
Sound absorption materials usually have low absorption capacity for low-frequency noise. It is not economical to increase the thickness of sound absorption materials to improve the sound absorption effect. In order to improve the sound absorption effect of low-frequency noise, resonant mufflers are usually used. The form of resonant muffler is to use the pipe opening to connect with the resonant cavity, and use the air column at the hole and the air in the cavity to form an elastic resonance system. When the external noise frequency is the same as the natural frequency of the resonance system, the air column in the hole resonates and rubs violently with the hole wall, and the friction can consume sound energy, thus achieving the purpose of noise elimination.
This kind of muffler has strong frequency selectivity, that is, the effective frequency range is very narrow, and generally can produce large attenuation for low-frequency noise elimination. The air flow resistance is small, but the structure is too large due to the resonance cavity.
3、 Reactive muffler
The resistance muffler is formed by connecting the pipe and the chamber. The muffler makes use of the sudden change of the section of the air duct to make the sound wave reflect back in the direction of the sound source and play the role of noise elimination.
The muffler has a good noise elimination effect for medium and low noise, and has a simple structure. Since no sound absorption material is used, it is not affected by high temperature and corrosive gases. Eliminating low-frequency noise has certain effect.
In order to ensure a certain silencing effect, the expansion ratio (ratio of large to small cross-sectional area) of the muffler should be greater than 4. Therefore, the application is limited in situations where the building space of the machine room is small.
4、 Composite muffler
The composite muffler combines the advantages of the resistive and the expansion muffler, and has a good noise elimination effect in the low frequency to high frequency areas. Its structure is shown in Figure 14.5. For example, the low-frequency noise reduction of the 1.2m-long composite muffler can reach 10~20dB. In addition, for the air conditioning system (such as the purification air conditioning project) that cannot use fiber sound-absorbing materials, the micro-perforated plate muffler with metal (aluminum, etc.) structure can achieve good results.
5、 Other types of mufflers
In addition to the types of mufflers discussed above, in practical engineering, proper treatment of some air duct components can also play a role in eliminating noise. In addition, they also have the advantage of saving building space. Commonly used muffler components include:
1. Silencing elbow
When the area of the machine room is small or it is necessary to improve the noise elimination effect of the original building, the noise elimination elbow can be used. There are two types of silencing elbow. The common muffler elbow uses the sound absorption material attached to the inside to eliminate noise. Generally, the inner edge of the elbow is made into an arc, and the outer edge is pasted with sound-absorbing material. The length of the sound-absorbing material should not be less than 4 times the width of the elbow. The other kind of muffler elbow is called resonant muffler elbow. Its outer edge uses perforated plate, sound-absorbing material and cavity. The resonance sound-absorbing structure is used to improve the poor effect of ordinary muffler elbow on low-frequency noise elimination.
2. Silencing static pressure box
A static pressure box with sound-absorbing material inside is set at the fan outlet or in front of the air distributor, which can both stabilize the air flow and eliminate noise. The silencing capacity of the anechoic static pressure box is related to the performance of the sound-absorbing material, the area of the sound-absorbing material inside the box, and the area of the air duct at the outlet side. The anechoic static pressure box can also be used as the air distribution static pressure box.
