Abstract: The impact of central air conditioning system noise on the environment and the use of rooms cannot be ignored. How to carry out noise reduction, sound insulation, sound absorption, and shock absorption of air conditioning systems, in the actual engineering design and construction process, to make the noise in the use of rooms and surrounding buildings meet the standard requirements and meet people's requirements for environmental comfort.

The main sources of noise are as follows:

① The noise and vibration of refrigeration units, cooling towers, as well as their auxiliary equipment such as water pumps and water treatment;

② Air is blown out from the air supply outlet to form wind sounds;

③ The noise generated by the frictional vibration of air flowing in the air duct;

④ The flow of frozen water in the frozen water pipe produces water flow sound and noise generated by pipe vibration;

⑤ Mechanical noise generated by the operation and vibration of equipment such as air conditioners and fan coil units;

⑥ Other external noise sources may produce resonance sounds with the aforementioned noise sources, etc.

The noise of air conditioning systems mainly comes from ventilation and air conditioning systems.

2 Central air conditioning noise control methods

The noise control of air conditioning systems involves noise reduction, sound insulation, sound absorption, and vibration isolation. The propagation modes of air conditioning noise include airborne sound and solid borne sound. Solid state sound propagation mainly includes the propagation of vibration of refrigeration units, cooling towers, air conditioners, fan coil units, pipelines and other equipment, while airborne sound propagation includes the propagation of noise from air ducts and direct radiation of end noise.

2.1 Choose appropriate low-noise equipment to control noise from the sound source. ① Adopt reasonable air conditioning forms to reduce noise. ② To reduce wind and water flow noise, the chilled water flow rate should be controlled at around 1.5m/s, the branch pipe wind speed should be ≤ 3.5m/s, and the main air duct wind speed should be ≤ 4m/s. Suitable wind speed and chilled water flow rate should be used Advanced low-noise equipment should be selected. For refrigeration units, compressors with relatively low vibration should be chosen. For water pumps, low-speed pumps with a speed of ≤ 450rpm should be selected as much as possible. Fresh air fan equipment and fan coil units should be located in public areas, office areas, and rest areas. Therefore, products with good quality and low noise must be selected, as their noise can be directly transmitted to the crowd.

2.2 Air conditioning system muffler is an airflow duct with sound-absorbing lining or special structural form that can effectively reduce noise. It can not only effectively reduce noise, but also make the airflow pass smoothly. Usually, a muffler needs to be installed in the ventilation duct to reduce the noise pressure level. The main purpose is to control the noise of air conditioning equipment such as air conditioning units, which is transmitted through ventilation ducts to the air flow noise in the air conditioning service area and air ducts. In noise control technology, mufflers are the most widely used noise reduction equipment. They are used for noise reduction in the air supply and return ducts of air conditioning systems, as well as the inlet and outlet of cooling towers. They are applied for noise reduction in the inlet and outlet of equipment rooms such as air conditioning rooms, boiler rooms, and refrigeration rooms.

2.3 Vibration isolation measures for air conditioning equipment The method of attenuating vibration is to eliminate the rigid connection between the vibration source and the receiver. To control the noise of air conditioning system equipment, it is necessary to control the solid sound propagated by the vibration of air conditioning units and refrigeration equipment, while reducing the fan noise propagated by ventilation ducts and equipment noise transmitted through the enclosure structure. Only in this way can the air-conditioned room meet the predetermined allowable noise control standards. There are two ways to control equipment vibration isolation: one is to reduce the efficiency of vibration transmission, and the other is to reduce the vibration of the vibration source. Controlling vibration at the vibration source is the most effective method, but this method is often not feasible in practical engineering because it requires redesigning or modifying the vibration source equipment to control vibration in the vibration propagation path. Common methods include increasing the damping of the vibration propagation path, such as adding isolation hoses. Its main purpose is to absorb the energy of vibration propagation (converted into heat); Add spring isolators or rubber pads. The commonly used vibration isolation hoses currently include various rubber flexible connections and stainless steel corrugated hoses. Rubber hoses have excellent vibration isolation and noise reduction effects, but their use is limited by the temperature and pressure of the medium, and their corrosion resistance is poor. Stainless steel corrugated pipes are widely used due to their ability to withstand high temperatures, high pressures, and corrosive media, as well as their durability and excellent vibration isolation effect. But it has a high cost. In the vibration isolation control of air conditioning pipelines, various rubber hoses can be used for low-temperature and low-pressure water pipes, while stainless steel corrugated pipes need to be used for refrigerators, air compressors, and high-pressure water pumps.

2.4 Sound insulation for air conditioning equipment. Refrigeration units, chilled water pumps, cooling water pumps, and other units with high noise levels should be located in the basement as much as possible to reduce the impact on the use of rooms on the ground. Sound waves should be isolated by the walls and underground floors of the computer room; If it can only be set on the ground, equipment rooms and soundproof walls should be set up Composite wall sound insulation. Composite walls can increase sound insulation by leaving an air layer in the middle. When sound waves are incident on the first layer of wall panels, the wall panels vibrate, and the air layer can be seen as a "spring" connecting the wall panels. This vibration is transmitted to the second layer of wall panels through the air layer Single layer homogeneous solid wall sound insulation. The larger the unit area mass of a wall, the better the sound insulation effect. In the main audio frequency range, the sound insulation performance of a single-layer homogeneous solid wall is mainly controlled by quality. For every doubling of the unit area mass, the sound insulation amount increases by 6dB. Therefore, the choice of wall should be as thick as possible to improve the sound insulation of the wall.

2.5 Air conditioning noise control and building noise prevention design planning. The collaboration between air conditioning design and noise control mainly involves noise prevention planning within the building, allocation of building space, and building construction. From the perspective of noise control, the air conditioning equipment room should be far away from air conditioning rooms and rooms with high noise control requirements. This can increase the natural attenuation of noise and reduce the impact of air conditioning noise on air-conditioned rooms. To reduce the airflow noise of the air duct, the architectural designer should reserve as much space as possible for the air conditioning system, including vertical shafts and suspended ceiling space. In the layout of the air conditioning room, rooms with high noise control requirements should be concentrated in the interior area of the building, and auxiliary or office rooms with low noise control requirements should be used as sound barriers to isolate external noise interference. In terms of building construction, for rooms that generate noise and rooms that require quietness, their enclosure structures need to have sufficient sound insulation and generally need to be made into thick and dense structures. If not handled properly during the architectural design phase, it may require a high cost to compensate for noise control.

2.6 Reasonable construction methods and main measures to reduce noise

① To effectively control the air supply noise of fan coil units, it is necessary to increase the air outlet area and reduce the wind speed.

② Installation of main pipe support for chilled water: In order to effectively eliminate noise and vibration, spring dampers should be added to the rigid support. This method is commonly used for chilled water main pipes with large diameters and slight vibrations, in order to effectively control the noise between the floor and the rigid support through the spring dampers.

③ Installation of air ducts: The production and installation of air ducts must strictly follow national regulations for construction. Air conditioning and fresh air silencers, like static pressure boxes, are coated with high-quality sound-absorbing materials inside and insulated with high-quality insulation materials outside. Sound reducing elbows are installed at the bend of the air duct, and sound absorbers are installed at appropriate positions of the air duct. The production and installation of air ducts must strictly follow national regulations for construction. Sound reducing louvers are used at the fresh air inlet, and impedance silencers are installed at the inlet and outlet of the fan. For ducts with a large cross-sectional area, rubber damping pads should be used as much as possible for duct hangers to ensure that the ducts do not produce vibration noise. If the installation strength and overall stiffness of the ducts are not sufficient, friction and vibration noise may occur.

④ Installation of water pipes: The installation of water pipes must strictly comply with national regulations. Hangers cannot be fixed on the floor slab, but should be fixed on the beam as much as possible. Spring vibration reduction hangers should be used for the main chilled water pipes and cooling water pipe hangers. When water pipes pass through the floor slab or wall, sleeves must be used and non combustible materials must be used for sealing.

⑤ Equipment installation: The fresh air and air conditioning units are installed with damping spring shock absorbers, the fan coil unit is installed with spring hooks, the fan coil unit is connected to the water pipe with flexible hoses, the fan is connected to the air pipe with flexible connections, and the fresh air unit is connected to the water pipe with flexible joints. Sound absorption treatment should be carried out in the air conditioning room to prevent the external transmission of equipment noise. For example, soundproof materials can be used to make the enclosure structure in the air conditioning room. In order to enhance the sound absorption effect, concave convex three-dimensional sound-absorbing panels can also be used to make the walls or ceiling panels of the room. Sound absorbing materials should be pasted in the room, and doors and windows should be minimized to minimize the external transmission of equipment noise. The doors and windows used should also use sound-absorbing windows or louvers.