Research on the Performance of Sound Absorption Materials for Substations in Power Grid

As the environmental protection policies become increasingly strict, higher requirements will be proposed for low-frequency noise control problems, and research on control strategies for low-frequency noise in substations is imperative. In this work, principles of resistive acoustic absorption and resonance acoustic absorption materials were analyzed. Based on the measurement in acoustic impedance tubes, the sound absorption performances of two typical sound absorption materials, the glass wool and the micro-perforated panel, were tested. This study offers a new reference for power grid companies to select and combines suitable acoustic materials to realize the optimal application in substations based on noise reduction requirements of different frequencies.


Introduction
With the social and economic development and the continuous growth of electricity load, the smart grid construction with the ultra-high voltage grid as the backbone grid is steadily advancing, and the scale of substations is rapidly expanding.According to statistics, by the end of 2021, the number of substations above 35kV nationwide was more than 42000.However, with the increasing scale of the grid as well as the scarcity of land resources in the city, more and more substations are located deeper into the city center.Due to a series of reasons such as uneven manufacturing processes, aging equipment, long-term operation, and proximity to sensitive points, the noise pollution problem of urban substations has become increasingly prominent [1].In recent years, disputes and complaints related to noise problems in urban substations have been on the rise, and it has become an urgent problem that must be faced in the process of power grid construction [2].How to speed up the construction of power grids, ensure high quality and reliable power supply at the same time, and do a good job of urban substation noise control, to realize the organic combination of corporate benefits and social benefits, has become one of the current priorities.To improve the noise environment of the power transmission system and realize the mission of "dedicating clean energy and building a harmonious society", State Grid has included the control of substation noise into the "13th Five-Year Plan" environmental protection key work plan.
At present, with the acceleration of urbanization, noise nuisance and complaints and rights violations are occurring in North, East and Central China.Especially in the economically developed areas in the southeast, this kind of situation is more serious, even in the western region with relatively abundant land resources, there is a lack of power corridors and substation siting difficulties.From the current situation, whether it is an urban substation or suburban substation, the surrounding land resources are increasingly tight, substations around the gradual increase in building and population density, and substations adjacent to the area of the acoustic environment of the functional area category quickly changed from Class III to Class II or even Class I area, many substations in service has been, or is on the verge of becoming a super standard station, the increase in noise requirements for the harmonious operation of the substation is a huge test.The increase in noise requirement is a great test for the harmonious operation of substations.
China's urban substation noise is mainly generated by the transformer, reactor, capacitor, busbar, and fan cooling equipment, of which the main transformer and reactor are the main source of sound, and its main noise frequency is concentrated in the frequency and its high harmonic frequency, especially 100Hz, 150Hz, 200Hz, etc.The noise wavelength, slow attenuation, strong penetration ability for the building structure, and the general sound-absorbing materials for its poor absorption [3].This kind of noise wavelength is large, has slow attenuation, strong penetration ability for building structures, and general sound-absorbing materials for its poor acoustic effect [4].In addition, substation service characteristics, applied to noise reduction materials, components, and devices also need to meet fire, mildew, moisture, atmospheric corrosion, dirt, environmental protection, lightweight and easy to recycle and other requirements [5].Among them, rock wool is the most widely used noise reduction material (also including its preparation of mufflers, etc.) [6,7].Such materials have poor moisture resistance and are easy to moisture failure, and with the extension of service time, it is very easy to age and crack, producing a large number of dust [8].Its main drawback is that, in the 500Hz and the following lowfrequency band, the sound absorption coefficient is extremely low, fundamentally, and is not suitable for substations and other areas of noise reduction.
Acoustic material is a strong absorption of incident sound energy, in principle, the absorption coefficient should be greater than 0.2.Acoustic material can be categorized from the principle of acoustic absorption, can be divided into two categories of porous acoustic materials and resonance acoustic materials, both have a wide range of applications in the field of noise reduction.Porous soundabsorbing materials, the principle of sound absorption is mainly the sound vibration energy through the thermal or magnetic effect of consumption, to achieve the effect of sound absorption, increase the effect of consumption, often in the damping, energy needs of the service environment to use.
At present, the research and application of sound-absorbing materials at home and abroad are developing steadily, in which foreign research on sound-absorbing materials is more focused on the functionalization of the application environment.For example, for environmental protection, weathering and other needs, in recent years, foreign countries have developed new acoustic materials such as: ceramic fiber, metal fiber, spray fiber, foam metal, etc. have been widely used.Among them, ceramic fiber including ordinary alumino-silicate fiber, high alumina alumino-silicate fiber, containing Cr 2O3 or ZrO2 or B2O3 alumino-silicate fiber, etc., with good acoustic performance, good insulation, long-lasting weathering and other characteristics.Metal fiber including fiber aluminum, fiber copper, fiber stainless steel, etc., which is characterized by high strength, not affected by the climate, high heat transfer efficiency, good acoustic performance for low and medium frequency, recycling is convenient and pollution-free.There are many kinds of sprayed fibers, the most famous is the United States K-13 series of sprayed fiber acoustic materials, using direct spraying method of construction, do not need keel, no seams on the surface, and its 25mm thick acoustic layer of sound reduction coefficient of NRC up to 0.75, the adhesive force is greater than 704kg/m 2 ; the United States Pai Locke (Pyrok) production of particles of acoustic coatings are vermiculite and cement as the main raw material, does not contain asbestos and mineral fibers, general acoustic coatings, and the main material is asbestos and mineral fibers.Containing asbestos and mineral fiber, the general acoustic coating thickness of 20-50mm, noise reduction coefficient of 0.60-0.75, with fire prevention, thermal insulation, water resistance, moisture resistance, anti-condensation, anti-corrosion and excellent acoustic performance.Foam metal materials including foam metal aluminum, foam metal copper, foam metal iron zinc aluminum, etc., due to the foam material compared with the traditional material has a large modulus of elasticity, high temperature resistance, good fire resistance, no aging and other advantages.
For the field of power transmission, compared with foreign technology, domestic research is more inclined to specific acoustic performance improvement, such as higher low-frequency sound absorption coefficient, wider absorption band, etc.However, in large-scale engineering applications, due to price factors, the most widely used sound-absorbing materials or glass wool, rock wool and other mineral wool acoustic materials, such materials have no strength, easy to moisture, moisture basically does not absorb sound, easy to aging failure, in some service conditions, 2-3 years that is the beginning of the volatilization of rock wool microdust, an average of less than ten years will be very serious pollution.In addition, such materials for low-frequency noise absorption coefficient is low, such as the need to achieve noise reduction requirements, often need to increase the thickness of the material on the structure, so that the installation, layout are more disadvantages.Within the power grid, due to the demand for service reliability, the service durability of the material extends the more stringent requirements.
For sound-absorbing materials, foreign countries in the direction of porous sound-absorbing materials to invest in the development of more.Such as metal fiber acoustic materials, including fiber aluminum, fiber copper, fiber stainless steel, etc., which is characterized by high strength, unaffected by climate, high heat transfer efficiency, for low and medium frequency acoustic performance, recycling is convenient and non-polluting; inorganic fiber acoustic materials are many kinds of acoustic materials, of which the most famous is the U.S. K-13 sprayed fiber acoustic materials, which is for the medium and low-frequency noise absorption coefficient is high, non-polluting, but K-13 fiber heat transfer, poor, high cost, easy to aging, not easy to recycle.However, K-13 fiber heat transfer is poor, high cost, easy to aging, not easy to recycle.There are many kinds of foam metal materials, including foam metal aluminum, foam metal copper, foam metal iron zinc aluminum, etc.Compared with the traditional materials, the foam materials show the high modulus of elasticity, high temperature resistance, good fire resistance, no aging, etc.However, the foam metal suffer from the high price of raw materials, and the processing cost is also high, due to the strict control process of the porosity, the processing is difficult, so it is currently only applied to high-value service conditions; while the resonance acoustic materials are mainly used for low-frequency noise absorption coefficient of high pollution.Resonance soundabsorbing materials mainly use the Helmholtz resonator principle to achieve sound absorption, which is the most mature and widely used micro-perforated sound-absorbing structure researched by academician Ma Dayou of the Chinese Academy of Sciences.On this basis, foreign countries also carry out secondary development, design and research in the form of a large number of combined acoustic structure.Such as impedance matching composite acoustic structure, variable cross-section acoustic structure.On the basis of the above material research, foreign countries also through structural simulation and optimization, the design of a large number of excellent performance muffler, acoustic enclosure and other devices, and is widely used in machinery, factories, mines, energy and other fields.

Principle of sound absorption material
Porous acoustic materials from low-frequency to high-frequency acoustic absorption coefficient shows an increasing trend, in which the type of fiber, nature, diameter, porosity directly determines the acoustic performance of porous fiber acoustic materials, through the selection of fiber materials can flexibly change the physical and chemical properties of the fiber materials, fire retardant properties and weathering properties.Also through different types (metal, non-metal), the combination of different density fiber layer and the combination of air cavity to achieve good low-frequency acoustic performance and wide absorption band.Therefore, artificial fiber materials or modified fiber is a development direction to replace rock wool/glass wool, such materials have been more research in foreign countries.Although the porous fiber acoustic materials have many advantages, but its weathering and fire retardant properties depend on the nature of the fiber itself, and to achieve good fire performance and aging performance depends mainly on the composition of the fiber molecules and the role of added components.At present, foreign research has been carried out more fully, while the domestic civil field for harsh service conditions of the fiber research is still in its infancy.Among them, it is a simple and effective way to improve fire resistance through flame retardants.Hydroxide flame retardant is the earliest class of flame retardant applied to polymer flame retardant, mainly including aluminum hydroxide (ATH), magnesium hydroxide (MH), etc., and its surface modification and filler amount have a great influence on the flame retardant and other properties of composite materials.Porous sound-absorbing materials have good adsorption ability for medium and high frequency noise, but it is difficult to achieve effective control in a certain target frequency band, such as for the low-frequency band <500Hz, its absorption coefficient is generally not high.The resonance sound-absorbing structure makes up for the shortcomings of the porous sound-absorbing material, therefore, through the optimization of the structure of the two, grouping, to achieve good low-frequency sound-absorbing performance and a wider absorption band is one of the future development trends.

Principle of resistive sound absorption
The principle of resistive acoustic absorption lies in the fact that when the sound wave enters the interior of the material through the fine pores on the surface of the material, most of the air in the pores will vibrate [9], but the part of the air near the wall of the pores is not easy to vibrate due to friction, thus forming a difference in vibration speed, which produces a friction and viscous force, so that a considerable portion of the acoustic energy is converted to heat dissipation, thus making the acoustic wave attenuation [10].In addition, the air in the pores of the material also plays the role of a low-pass filter, which can attenuate high-frequency sound waves and distort the noise spectrum; the multiple interfaces between the pores and the fibers can make the sound waves reflect back and forth and vibrate, resulting in internal dissipation, which reduces the intensity of the noise and changes the propagation path, to achieve the effect of acoustic noise reduction.The resistive sound absorption principle diagram is shown in Figure .1.

Principle of resonance sound absorption
Resonance absorption principle is the use of Helmholtz resonance principle, when the sound wave into the sound-absorbing body, relying on the resonance of the sound wave and sound absorbing body to the loss of acoustic energy, when the sound-absorbing body of the intrinsic frequency and the frequency of acoustic wave is consistent with the resonance effect, the amplitude of acoustic wave excitation soundabsorbing structure to reach the maximum, due to the friction of the loss of acoustic energy Wa and the speed of vibration of the U is proportional to the square, the formula can be expressed as: Wa = RaU 2 , for the sound resistance of a certain sound absorption body, the larger the speed of vibration is also larger.Acoustic resistance of a certain sound-absorbing body, the greater the vibration speed, the greater the consumption of acoustic energy, the greater the absorption coefficient.Resonance sound absorption principle diagram is shown in Figure .2. In addition, by filling resistive sound-absorbing materials behind the resonant sound-absorbing structure plate, the air resistance into the resonant sound-absorbing structure is increased, which effectively improves the sound-absorbing coefficient of a specific frequency band and expands the width of the absorption band.

Sound absorption coefficient enhancement principle
The sound absorption coefficient is an important index reflecting the good or bad acoustic performance of porous fiber acoustic material, which is determined by the acoustic characteristic parameters of the material.Characteristic impedance Ze and propagation constant γ are the most basic acoustic parameters of porous acoustic materials, and other characteristic parameters can be derived from these two parameters.The characteristic impedance rate and propagation constant are complex numbers, which can be expressed as: Where R is the material characteristic acoustic resistivity, X is the material characteristic acoustic impedance, α is the attenuation coefficient and β is the phase coefficient.
Characteristic impedance rate and propagation constant are power function relationships between flow resistance rate and frequency, modeled as: ) ) (3) where ρ0 is the air static density, C0 is the air sound velocity, f is the frequency, c1-c8 are eight fixed coefficients, and r is the fiber material flow resistivity, which can be expressed as: Where η=1.85×10 -5 Pa•s, is the air viscosity coefficient, d is the average diameter of the fiber, ρf is the density of the fiber, and ρm is the bulk weight of the fiber material.
Assuming that the fiber acoustic material with thickness a is placed on the rigid wall surface, and another piece of acoustic material with the same thickness is placed immediately in front of a piece, using the impedance transfer method, the acoustic impedance rate Zs on the surface of the fiber acoustic material with thickness a can be expressed as: According to the formula (2-1), Zs can be expressed as: Where A is the surface acoustic resistance of the material and B is the surface acoustic resistance of the material.
When the sound wave is vertically incident on the surface of fiber acoustic material from the air, the sound reflection coefficient H can be expressed as: According to Eqs. ( 1)-( 8), the main factors affecting the sound absorption coefficient of fiber acoustic material are frequency f, flow resistance rate r, and thickness a, while, where η = 1.85 × 10 -5 Pa•s is a fixed value, so the fiber diameter d, fiber density ρf, and the fiber material bulk weight ρm all affect the size of the flow resistance rate r.Therefore, by optimally designing the material structure parameters such as fiber diameter d, fiber density ρf, fiber material bulk weight ρm, and fiber material thickness a of the porous fiber acoustic absorbing material, a good match between acoustic resistivity and the acoustic impedance of the porous fiber acoustic absorbing material can be achieved, and the acoustic absorption coefficient in a specific frequency band can be improved.

Theoretical analysis model
For single-layer acoustic materials, a commonly used theoretical analytical model is an experimental model for sound wave propagation in porous materials proposed by Allard and Champoux, known as the Johnson-Champoux-Allard (JCA) model.To solve for the normal absorption coefficient, the model first derives the equivalent dynamic density and the equivalent dynamic bulk modulus: The σ,Φ,α ∞ , Λ, Λ ′ denotes static flow resistivity, porosity, curvature factor, viscous characteristic length, and thermal characteristic length, respectively; ambient atmospheric pressure; angular frequency; and, are the air density, kinetic viscosity, specific heat rate, and Platt's constant for saturated air, respectively.The characteristic acoustic impedance of the material is: For a single-layer sound-absorbing material, the transfer matrix is constructed using the interfacial sound pressure P and the normal vibration velocity of the plasma point u as the transfer parameters, then the sound pressures P1, P2 and the normal vibration velocities of the points u1, u2 on both sides of the material satisfy the following relationship: The sound absorption coefficient of the material is: 1st International Conference on Applied Physics and Mathematics Journal of Physics: Conference Series 2729 (2024) 012021

Glass wool
In this study, the acoustic performance of glass wool and its composite material with cavity was tested.
With glass wool as the outer layer and the back-cavity, the composite acoustic structure was combined into a composite acoustic structure, which greatly broadened the acoustic band in the low-frequency region of the material, and improved the low-frequency acoustic absorption coefficient of the material.
According to the national standard GB/T 18696.1-2004"Measurement of sound absorption coefficient and acoustic impedance in acoustic impedance tubes", Part 1 standing wave ratio method, the Danish B&K4206 double microphone impedance measuring tube was used to measure the glass wool (with back-cavity of 70 mm), and the curve of the sound absorption coefficient is shown in Figure 3.The absorption coefficient measurement of glass wool was under different frequencies using the standing wave tube (Microphone Spacing: 0.05 m, Diameter: 0.1 m, Lower Frequency Limit: 50 Hz, Distance to Sample from Mic.: 0.35 m).The thickness is 2.5 cm, 5 cm, and 5 cm with 7 cm back-cavity.
It can be seen that with the increase in thickness (from 2.5 cm to 5 cm), the acoustic absorption coefficient of the glass wool material is greatly improved in the whole range of 50 Hz-1600 Hz.At 1600 Hz, the absorption coefficient increases from 0.651 to 0.963.Compared with the 0.5 cm thickness of pure glass wool, the composite material with the introduction of a 0.7 cm back cavity has a higher absorption coefficient in the low-frequency band below 200 Hz.This broadens the range of the absorption band and extends the absorption band to lower frequencies.Generally speaking, the absorption coefficient of glass wool material in the absorption band of 200~1,600 Hz can not be guaranteed to be stable at more than 0.8, and it is difficult to meet the noise reduction requirements of substation equipment.From Figure 4, it can be seen that with the increase of cavity thickness (from 10 mm to 30 mm), the acoustic absorption coefficient is greatly improved in the range of 50 Hz-1600 Hz.From 30 mm to 100 mm, the acoustic absorption peak shifts to the low-frequency region.The sample with 50 mm cavity thickness shows the broadest absorption band, which shows the potential of the noise reduction material of substation equipment.However, the sample with 100 mm shows a better absorption coefficient in the low-frequency region and weaker absorption in the region of 600 Hz-1600 Hz.In practical application, the size of the cavity between the micro-perforated plate and the shell, as well as between the microperforated plate, varies flexibly according to the frequency bands to be absorbed, which can absorb low, medium and high frequencies.

Conclusion
In conclusion, from both theoretical and experimental studies, this article systematically describes the performance of sound absorption materials for substations in power grids.Through the analysis of the principle of sound absorption coefficient enhancement, the optimized design of fiber diameter d and other structural parameters can achieve a good match between the acoustic resistance and acoustic impedance of porous acoustic materials, and improve the acoustic absorption coefficient of a specific frequency band.Compared with glass wool, micro-perforated panel material shows a broader absorption band and a potential for noise reduction material in substation practical application equipment.

Figure 3 .
Figure 3. Absorption coefficient measurement of glass wool 3.2 Micro-perforated panel Micro-perforated plate acoustic structure refers to the thickness of less than 1mm in the thin metal plate drilling many micro-holes, these micro-holes of the aperture are generally 0.5~1mm or so, and the perforation rate of 1% to 3% of the perforated plate in the perforated plate after a certain amount of cavity.The micro-perforated plate is an acoustic element with high acoustic resistance and low acoustic quality.Compared with glass wool, micro-perforated panel-based composite material with cavity shows boarded absorption frequency range.(back-cavity thickness of 10 mm, 20 mm, 30 mm, 50 mm and 100 mm) under different frequencies using the standing wave tube (Microphone Spacing: 0.05 m, Diameter: 0.1 m, Lower Frequency Limit: 50 Hz, Distance to Sample from Mic.: 0.35 m).From Figure4, it can be seen that with the increase of cavity thickness (from 10 mm to 30 mm), the acoustic absorption coefficient is greatly improved in the range of 50 Hz-1600 Hz.From 30 mm to 100 mm, the acoustic absorption peak shifts to the low-frequency region.The sample with 50 mm cavity thickness shows the broadest absorption band, which shows the potential of the noise reduction material

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International Conference on Applied Physics and Mathematics Journal of Physics: Conference Series 2729 (2024) 012021

Figure 4 .
Figure 4. Absorption coefficient measurement of micro-perforated panel materials