Abstract: This paper briefly analyzes the noise sources of wind turbine generators, focuses on the principles and methods of damping vibration reduction and noise reduction control and noise propagation control, and proposes noise control measures and methods for wind turbine generators. Keywords : wind turbine generator damping vibration reduction and noise control 0 Introduction Energy is the foundation of modern society and economic development. Under the dual pressure of conventional energy shortage and global ecological environment deterioration, wind energy, as one of the new energy and renewable energy sources with the most promising development prospects and the most mature technology, has become a hot topic of global energy industry. Since the 1970s, wind energy development and utilization have developed very rapidly in developed countries in Europe and America, and the technology of wind power generation has become increasingly mature. In March 1996, the State Planning Commission of China formulated the "Wind Power Development Plan" to promote the industrialization of wind farm construction by localizing the production of wind turbine generators. The plan aims to adopt the form of technology and trade combination, introduce advanced foreign technologies, and achieve the ability to independently develop, design and manufacture large wind turbine generators through digestion and absorption [1]. Wind energy development can reduce air pollution and water pollution, but if not handled properly, it will increase noise pollution. In recent years, with the continuous expansion of the localization of wind turbines, and the fact that China's manufacturing industry still lags behind that of developed countries in Europe and America, the vibration and noise problems of domestically produced wind turbines have gradually emerged. Residents near wind farms have increasingly complained about the noise pollution caused by wind turbine generators, even threatening the normal domestic industrialization of wind turbines. Therefore, vibration reduction and noise control of wind turbines is very important and necessary. This article will focus on the application of damping vibration reduction and noise reduction technology and noise propagation reduction technology in the noise control of wind turbine generators. 1 Noise Source Analysis During the operation of wind turbine generators, under the excitation of wind and moving parts, the blades and generator components generate significant noise. The main noise sources are: (1) Mechanical noise and structural noise ① Gear noise. Meshing gear pairs or gear sets, due to mutual collision and friction, excite the vibration of the gear body, and radiate gear noise through the solid structure. ② Bearing noise. Noise is generated by the vibration radiation caused by friction and vibration between relatively moving elements in the bearing and the imbalance of rotating parts or the impact between relatively moving elements. ③ Noise excited by periodic forces. Noise generated by the periodic force of rotating mechanical parts such as rotating shafts. ④ Motor noise. Unbalanced electromagnetic force causes the motor to generate electromagnetic vibration and radiates electromagnetic noise through solid structure. Mechanical noise and structural noise are the main noise sources of wind turbine generator sets and are the most annoying to people. This part of the noise can be controlled. The main way is to avoid or reduce impact force, periodic force and friction force, such as improving the processing technology and installation accuracy, and keeping gears and bearings in good lubrication conditions. In order to reduce the vibration of mechanical parts, the vibration transmission path can be cut off near the force source, such as replacing rigid connection with elastic connection; or high damping material can be used to absorb the vibration energy of mechanical parts to reduce vibration noise. (2) Aerodynamic noise Aerodynamic noise is generated by the interaction between blades and air. Its magnitude is related to wind speed and increases with increasing wind speed. The difficulty in dealing with aerodynamic noise is that its sound source is in the propagation medium, so it is not easy to separate the sound source area. (3) Ventilation equipment noise Noise generated by auxiliary equipment such as radiators and fans. 2 Noise Control Noise control can be approached from three aspects: noise source, noise propagation path and noise receiver[2]. Noise control technology mainly focuses on acoustic control methods of noise. Specific technical approaches generally include sound insulation treatment, sound absorption treatment, vibration isolation, and damping vibration reduction. Sound insulation treatment and sound absorption treatment belong to noise propagation and noise reduction control; vibration isolation and damping vibration reduction belong to damping vibration reduction and noise reduction control. The mechanism of these noise control methods is that energy is consumed through the interaction between noise sound waves and acoustic materials or acoustic structures, and vibration waves and damping materials or damping structures, thereby achieving the purpose of noise reduction. 2.1 Damping Vibration Reduction and Noise Reduction Control Damping vibration reduction and noise reduction technology utilizes the characteristics of damping materials and the reasonable design of damping structures to dissipate the vibration energy of structural components in order to achieve the purpose of vibration reduction and noise reduction. Damping vibration reduction technology has developed rapidly in recent years, especially in aerospace, automotive industry, instrumentation, weaponry, construction industry and home appliance industry. Whether in terms of basic theory or in the research and development and application technology of new materials, it has grown into an independent scientific branch. 2.1.1 Damping materials and their characteristics Material damping refers to the ability of a material to dissipate vibration energy during vibration deformation [3]. Damping materials are also called viscoelastic damping materials or viscoelastic high-damping materials. It is a material that combines the characteristics of some viscous liquids and elastic solids. Viscous liquids have the ability to dissipate energy but cannot store energy; on the contrary, elastic materials have the ability to store energy but cannot dissipate energy. Viscoelastic materials are between the two. When they generate dynamic stress and strain, part of the energy is converted into heat energy and dissipated, while another part of the energy is stored in the form of potential energy. The phenomenon of energy conversion and dissipation is manifested as damping characteristics. It can be used to suppress the vibration peak at the resonance frequency, reduce the transmission of vibration along the structure, and reduce structural noise. All kinds of damping materials are affected by the ambient temperature and working frequency. Different temperatures and different working frequencies result in different damping characteristics. As a good damping material, it should have a high loss factor over a wide temperature range and a wide frequency range, as shown in Figure 1. 2.1.2 Surface Damping Treatment Surface damping treatment is mainly applied to thin components or thin plate parts that are mainly subjected to bending vibration. Wind turbine nacelles and partitions are thin plate vibrating parts, so surface damping treatment can be applied to wind turbines. Surface damping treatment is usually divided into two categories: free damping treatment and constrained damping treatment. (1) Free Damping Treatment A layer of viscoelastic damping material of a certain thickness is pasted on the surface of the substrate. When the substrate undergoes bending vibration, the damping layer vibrates with the base layer, and tensile-compressive deformation is generated inside the damping layer. According to the energy dissipation mechanism of damping material, when alternating stress is generated inside the damping material, the damping material will convert ordered mechanical energy into disordered thermal energy, thereby playing the role of energy dissipation. The free damping structure is shown in Figure 2. The thicker the damping layer, the greater the damping loss factor and the better the vibration damping performance. a) Free state b) Vibration tension-compression deformation state 1—Basic layer 2—Damping layer Figure 2 Free damping treatment structure (2) Constrained damping treatment An elastic layer is pasted on the outer surface of the damping layer in the free damping treatment. This elastic layer should have an elastic modulus much larger than that of the damping layer. When the damping layer undergoes bending vibration along with the basic structure layer, causing tension-compression deformation, since the elastic modulus of the outer elastic layer is much larger than that of the damping layer, this elastic layer will play the role of constraining the tension-compression deformation of the damping layer. Therefore, this elastic layer is called the constrained layer, and the damping layer constrained by the elastic layer is called the constrained damping layer. Since the tension-compression deformation generated on the surface of the damping layer in contact with the basic layer is different from the tension-compression deformation generated on the surface in contact with the constrained layer, shear deformation is generated inside the damping material. Therefore, in the constrained damping treatment structure, the damping layer not only bears tension-compression deformation, but also shear deformation. Both of them can play the role of energy dissipation, as shown in Figure 3. The constrained damping structure dissipates more energy than the free damping structure, so it has a better vibration reduction and noise reduction effect. [align=center] a) Free state b) Vibration tension-compression and shear deformation state[/align] 1—Basic layer 2—Damping layer 3—Constraint layer Figure 3 Constrained damping treatment structure In practical applications, the basic layer and the constraint layer are often made of the same material and have the same thickness, which is called a symmetrical composite damping material structure. 2.1.3 Types of damping materials and their applications Damping materials are divided into two main categories: damping plates and damping coatings. Damping plates are further divided into asphalt damping plates and rubber damping plates according to the matrix composition. Damping plates have good vibration reduction and sound insulation performance and stable performance, but have high requirements for the surface shape and installation process of the structure. Damping plates can be used on the surface of structural components near vibration sources, or they can be attached to the surface of thin shell structures to make free damping treatment structures. Damping coating is a special coating that can be applied to the surface of various materials and complex shapes of structures. It has the functions of vibration reduction, noise reduction, vibration isolation and sealing. Damping coating can be sprayed or scraped onto the surface of thin shell structures to make free damping treatment structures. Damping coatings are easy to apply and are particularly suitable for coating complex-shaped shells, achieving an overall aesthetically pleasing result. Since the main components of a wind turbine are installed inside the nacelle, the vibrations generated by these components are directly transmitted to the nacelle, causing nacelle vibration and radiating noise. Therefore, damping materials can be applied to the inner surface of the nacelle to perform surface free damping treatment, attenuating vibrations, reducing structural radiated noise, and simultaneously isolating internal nacelle noise from propagating outwards. 2.2 Noise Propagation Control Noise propagation control, also known as passive noise control, uses acoustic materials or structures along the noise propagation path to isolate or absorb a portion of the sound energy, causing sound waves to attenuate as they pass through, thus achieving noise reduction. Noise propagation control and damping vibration reduction control are complementary and inseparable; damping plays a significant role in improving the sound insulation performance of materials. Damping and sound insulation materials are applied to the inner surface of the wind turbine nacelle for damping and sound insulation treatment. When the noise generated by the gearbox, motor and other components inside the nacelle is incident on the surface of the nacelle shell, it is converted into the following main parts: 1) Part of it is reflected back into the nacelle; 2) Part of it is converted into other forms of energy or waveforms and absorbed when passing through the nacelle surface. For example, part of it is converted into heat energy and lost by the high damping material attached to the nacelle shell, and another part is converted into structural radiation noise or other forms of waveforms; 3) The remaining part is transmitted to the external environment through the nacelle. The energy distribution during the noise propagation process is shown in Figure 4. Equation (2) shows that the smaller the reflected sound energy, the larger the sound absorption coefficient of the material, and the better its sound absorption performance. The sound insulation performance and sound absorption performance of the material are often inversely proportional. The better the sound insulation performance, the worse its sound absorption performance. The primary purpose of noise control for wind turbines is to reduce ambient noise, specifically to control the propagation of noise from inside the turbine nacelle that could disturb nearby residents. Therefore, damping and sound-insulating materials, such as damping coatings, asphalt damping plates, or rubber damping plates, should be primarily considered for the turbine nacelle to achieve this goal. Additionally, attention should be paid to sealing and heat dissipation during the damping and sound-insulating treatment of the nacelle. 3. Conclusion Damping vibration reduction and noise control, along with sound insulation and noise reduction control in noise propagation, can serve as important means of noise control for wind turbine generators. They offer advantages such as simple and convenient construction, no maintenance required, no need to modify existing structures, and relatively low investment. They can be used not only as an independent method but also in conjunction with other noise reduction measures to enhance the noise reduction effect. Zhuzhou Times New Material Technology Co., Ltd. is dedicated to research in this area, developing high-performance damping coatings and damping plates for high-power wind turbine generators to dampen and insulate the turbine nacelle. Experiments have demonstrated their effectiveness in reducing wind turbine noise.