Analysis of Noise Control Methods for Central Air Conditioning in High-rise Buildings

1. Noise sources of central air conditioning systems

The noise sources of the air conditioning system include the noise of the fan in the air conditioning unit, the airflow noise of the supply and return air ducts, the noise of the terminal air outlet, the noise and vibration of the refrigeration unit and its auxiliary equipment (including water pumps, water treatment equipment, etc.), the noise and vibration of the cooling tower, etc. The transmission methods of air conditioning noise include airborne sound transmission and solid-borne sound transmission. Airborne sound transmission includes noise transmission through air ducts and direct radiation from terminal noise, etc. Solid-borne sound transmission mainly includes the vibration transmission of equipment such as refrigeration units, cooling towers, and pipelines. Air conditioning noise control involves noise reduction, sound insulation, sound absorption, and vibration isolation, among other aspects.

2.Noise control methods for central air conditioning

2.1 Noise reduction of the air conditioning system

To control the noise from air conditioning equipment such as fans from entering the air conditioning service area and the airflow noise within the air ducts through ventilation ducts, it is usually necessary to install silencers in the ventilation ducts to reduce the noise sound pressure level. A silencer is a device that not only allows the airflow to pass smoothly but also effectively reduces noise, or in other words, A muffler is an airflow duct with sound-absorbing inner linings or special structural forms that can effectively reduce noise. In noise control technology, mufflers are the most widely used noise reduction devices. In air conditioning systems, mufflers are applied to the noise reduction of air inlets and outlets in air conditioning rooms, boiler rooms, refrigeration rooms and other equipment rooms, as well as the noise reduction of supply and return air ducts in air conditioning systems. And the noise reduction of the air inlet and outlet of the cooling tower, etc. In actual engineering, the noise reduction performance of the muffler significantly decreases with the increase of wind speed, and sometimes even the noise reduction amount is negative. The main reason is the regenerative noise of the airflow. In the pipeline, the main mechanisms of airflow noise are: The first type is the vibration generated by the airflow stimulating the pipe wall and other components. This solid noise is mainly of medium and low frequencies and generally follows the law of the fourth power of flow velocity. Another type is the direct sound produced when the airflow vortex detels from the boundary layer. This noise generated by airflow turbulence is essentially a dipole radiation, characterized by medium and high frequencies, and roughly varies according to the sixth power of the flow velocity. These two types of noise coexist. When the flow velocity is low, the former is dominant, and as the flow velocity increases, the latter gradually becomes dominant.

2.2 Vibration isolation of Air Conditioning System

To control the noise of air conditioning and refrigeration equipment, in addition to reducing the fan noise transmitted through ventilation ducts and the equipment noise passing through the enclosure structure, it is also necessary to simultaneously control the solid sound transmitted by the vibration of air conditioning and refrigeration equipment. Only in this way can the air conditioning room meet the predetermined allowable noise control standards. The vibration of air conditioning and refrigeration equipment is transmitted along the building structure in the form of elastic waves to all rooms adjacent to the machine room, and is felt by people in the form of airborne sound. The method to attenuate the vibration is to eliminate the rigid connection between the vibration source and the receiver. Vibration isolation for air conditioning and refrigeration equipment involves vibration isolation of equipment foundations and pipelines. It can be controlled through two channels;

(1) Reduce the vibration of the vibration source

(2) Reduce the efficiency of vibration transmission.

Controlling vibration at the vibration source is the most effective approach, but this may require a redesign or modification of the vibration source equipment, and thus cannot be implemented in many projects. Common methods for controlling vibration along the vibration transmission path include:

(1) Introduce elastic vibration damping elements to reduce the vibration transmission rate. For example, introducing spring vibration isolators or rubber pads

(2) Increase the damping of the vibration propagation path to absorb the energy of vibration propagation (converted into heat).

Elastic vibration damping elements can be added at any point along the vibration transmission path, but it is most effective to introduce them at or near the vibration source. At present, the commonly used vibration isolation hoses include various rubber flexible connections and stainless steel corrugated hoses. Rubber hoses have a very good vibration isolation and noise reduction effect, but their disadvantage is that their use is limited by the temperature and pressure of the medium. At the same time, its corrosion resistance is relatively 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. However, it is relatively expensive. 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 should be selected for refrigerators, air compressors and high-pressure water pumps. The vibration isolation effect of the hose is related to its own material and structure. The reasonable length of the hose, the pressure of the medium inside the pipe, and the way the pipe is fixed are all related. After hoses are installed between the equipment and the pipeline, the vibration of the equipment can be attenuated from being transmitted through the pipeline. However, the vibration caused by the medium inside the pipeline can still be transmitted to the building structure through the components that fix the pipeline. Therefore, isolation measures must be taken. The common methods are to use elastic hangers with springs or lay elastic vibration isolation materials on the hangers.

2.3 Sound Absorption and noise reduction

The noise from the air conditioning system spreads into the rooms where the air conditioning is used through the air conditioning terminals or the building structure. Part of the sound energy directly reaches the ears and is called direct sound, while the majority of the sound energy is reflected multiple times through various interfaces in the room and then spreads to the human ears. It is called reverberation sound. The sound heard by the human ear is the superposition of direct sound and reverberation sound. If sound-absorbing materials or sound-absorbing structures are placed at the interfaces such as the ceiling, walls or floor in the room, part of the reflected sound energy can be absorbed, which can weaken the reverberation sound. This is the principle of sound absorption and noise reduction. At present, the method of "sound absorption and noise reduction" for noise control has become very common both at home and abroad. Generally, the noise reduction can reach 6-10dB. It should be noted that sound absorption and noise reduction can only reduce reverberation sound and cannot reduce direct sound, nor can it absorb all the noise in the room. If the original room had very little sound absorption, the effect of noise reduction by using sound absorption is obvious. If the original room already has a certain amount of sound absorption, then increasing the same amount of sound absorption will result in a smaller noise reduction. Therefore, it is usually impossible to attempt to reduce the noise level by more than 10dB solely relying on sound absorption.

2.4 Sound insulation through sound insulation walls

(1) Sound insulation performance of single-layer homogeneous solid walls.

The sound insulation performance of a single-layer homogeneous solid wall is related to the frequency of the incident sound wave. Its frequency characteristics depend on factors such as the unit area mass, stiffness, internal damping of the material, and boundary conditions of the wall itself. Within the main sound frequency range, the sound insulation performance of a single-layer homogeneous solid wall is mainly controlled by mass, conforming to the "mass law", that is, the larger the unit area mass of the wall, The better the sound insulation effect, the more the sound insulation quantity increases by 6dB for every doubling of the unit area mass. From this, it can be seen that to enhance the sound insulation of walls, thick walls should be used as much as possible.

(2) Composite wall.

By increasing the thickness of the wall, its sound insulation can be enhanced. However, merely increasing the thickness of the wall to enhance the sound insulation is clearly uneconomical. Increasing the thickness of the wall adds to the weight of the structure and also limits its application range. Multi-layer composite partition walls increase sound insulation by utilizing the reflection and attenuation absorption of sound waves when they pass through different media. This method can effectively enhance sound insulation and the walls can be made very light. The sound insulation of composite walls can be enhanced by leaving an air layer in the middle. The air interlayer can be regarded as the "spring" connecting the wall panels. When sound waves strike the first layer of the wall panels, they cause the panels to vibrate, and this vibration is transmitted to the second layer of the wall panels through the air layer. Due to the vibration damping effect of the elastic deformation of the air interlayer, the vibration transmitted to the second wall is greatly reduced, thereby increasing the total sound insulation of the wall. The sound insulation of a double-layer wall can be estimated by adding the additional sound insulation of an air interlayer to the sound insulation of a single-layer wall whose unit mass is equal to the sum of the unit masses of the double-layer walls. The additional sound insulation of the air interlayer is related to its thickness. If sound-absorbing material One is placed within the air interlayer without filling it, the sound insulation can be further enhanced. Through these measures, it is very easy to make the sound insulation of lightweight walls reach the level of heavy walls, and they have a very good sound insulation effect. The sound insulation of lightweight walls with double-sided and double-layer 12mm gypsum board, light steel keel and glass filling inside can be comparable to that of 24cm brick walls, while their weight is only 1/100 of that of brick walls. Although the comprehensive sound insulation of lightweight composite walls can reach the level of heavy solid walls, their low-frequency sound insulation is generally lower than that of heavy walls. Therefore, in Spaces where low-frequency noise isolation is the main task, if the load-bearing structure allows, heavy walls should be used as much as possible.

2.5 Air Conditioning Noise Control and Building Noise Prevention Planning

How architectural design, air conditioning system design and noise control can complement and collaborate with each other is a very worthy issue to study and pay attention to. If air conditioning design and noise control are well considered in the architectural design stage, it often leads to twice the result with half the effort and a mutually reinforcing effect. Conversely, if there is a lack of consideration during architectural design, making it difficult to design and arrange the air conditioning system, resulting in an unfavorable situation for noise control, then making up for it will often be twice the effort for half the result. Similarly, air conditioning design should also be carried out in combination with the actual situation of the building and noise control requirements, and low-noise schemes or schemes that are more convenient for noise control should be selected as much as possible. The collaboration among architectural design, air conditioning design and noise control mainly involves noise prevention planning within buildings, allocation of building space and building structure, etc. From the perspective of noise control, the machine room of air conditioning equipment 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 conditioning rooms. To reduce the airflow noise of air ducts, the architectural design party should reserve as much space as possible for the air conditioning system, including shafts and ceiling Spaces. In the layout of air conditioning rooms, rooms with high noise control requirements should be concentrated in the inner area of the building, and auxiliary rooms or office rooms with low noise control requirements should be used as sound insulation barriers to isolate the interference of external noise. In terms of building structure, For rooms that generate noise and those that require quietness, their enclosure structures need to have sufficient sound insulation and are generally made into thick and dense structures. If not handled properly during the architectural design stage, it may take a very high cost to make up for the noise control.

Design Program for noise Control of Central Air Conditioning system

When conducting noise control design for air conditioning systems, it is essential to have a clear understanding of the content of the work, professional relationships, and design procedures. This enables each specialty to take into account the requirements of noise reduction and vibration isolation during the scheme design stage. This is also conducive to division of labor and collaboration, smooth progress of the work, and improvement of work efficiency. Noise control in air conditioning systems mainly includes two aspects: noise reduction and vibration isolation. Noise reduction design is aimed at noise control of fans, while vibration isolation design covers noise control related to air conditioning and refrigeration equipment. However, whether it is noise reduction or vibration isolation, the aim is to make the air-conditioned room meet the determined allowable noise standards.

4 Conclusion

Noise control in central air conditioning systems involves various specialties such as HVAC, acoustics, architecture, and structure. It requires a comprehensive consideration and design by integrating knowledge from all these specialties. Only through close cooperation among all specialties can effective and reasonable control be achieved. However, in many current projects, Because architects and HVAC engineers often focus only on the research and design of temperature, humidity, air flow organization and air quality during the design process, they fail to realize the severity of noise hazards and neglect noise control design, resulting in air conditioning noise exceeding noise control standards, affecting people's lives and studies. At the same time, At present, most air conditioning professionals are not familiar with the field of acoustics either. It is difficult to do a good job in the design of air conditioning noise control, and professionals in architecture and acoustics often have little knowledge of the air conditioning field, making it hard for them to independently do a good job in the design of air conditioning noise control. Therefore, to effectively control the noise of central air conditioning systems, architects and HVAC engineers need to pay sufficient attention to noise control. They should consider how to carry out noise control from the very beginning of architectural design, and conduct an overall design by comprehensively considering factors such as the acoustic environment, indoor microclimate environment, and indoor air quality.