Since 1950s, with the development of modern industry and transportation, the problem of noise pollution has become increasingly serious, and it has become one of the four major environmental pollutions in the world (noise pollution, water pollution, air pollution and solid waste pollution), which seriously threatens human physical and mental health and living environment. Some data show that people who are exposed to high noise for a long time will have symptoms such as hearing fatigue (such as noise deafness reported in clinic), fatigue, anxiety and irritability. Noise will make nervous system dysfunction, accelerate heart aging, and even directly lead to some diseases (such as nervous system and cardiovascular system diseases). ). In the industrial field, strong noise will lead to sound fatigue of machines, equipment and some industrial structures, and long-term effects will shorten their service life and even lead to production accidents. In addition, the influence of noise has always been concerned in the military field, and the noise problem will affect the operational performance of some technical weapons. Such as torpedoes, mines, submarines, surface ships and other weapons. Excessive self-noise not only affects the work of the self-god system and reduces its effective range, but also reduces its concealment, which is the most important factor leading to the enemy's attack.
Fang Danqun, Jing Tian, Zhang Bin, Sun Jiaqi and other noise control experts agreed at the Global Chinese Scientists Environment Forum held in Shanghai on May 5-7, 20 10 that noise should be paid attention to in the 12th Five-Year Plan. After repeated discussions, the proposal and appeal of noise control experts at home and abroad on strengthening environmental noise management and control in the Twelfth Five-Year Plan of national environmental protection was drafted. The appeal pointed out that with the development of modern industry and transportation, noise pollution has become increasingly serious and has become one of the important public hazards in modern times. New york, London, Tokyo and other cities have reported that the number of noise complaints ranks first among all kinds of environmental pollution cases every year, and the noise pollution in China is also quite serious. According to the statistical data of environmental letters and visits in 20001-2008 in the Chronology of Environmental Statistics of China in 2008, the number of noise and vibration letters and visits in 20001-2006 ranked first, and it ranked second in 2007-2008, slightly lower than the number of air pollution letters and visits. Disputes, conflicts and mass protests caused by noise pollution occur from time to time, and even lead to casualties. According to the results of "Public Satisfaction Survey on Environmental Status" published in "Announcement on Environmental Status of China" in 2009, "the respondents are most satisfied with the quality of drinking water in urban and rural areas, and the lowest are environmental noise and garbage disposal". Therefore, the noise problem is no longer a simple urban environmental problem, but also a serious problem in rural areas. Noise problem has developed into a social problem that restricts the improvement of people's quality of life and affects the construction of harmonious society.
20 10 12 15 The Ministry of Environmental Protection, the National Development and Reform Commission, the Ministry of Science and Technology, the Ministry of Industry and Information Technology, and the Ministry of Housing and Urban-Rural Development jointly issued the Guiding Opinions on Strengthening the Prevention and Control of Environmental Noise Pollution and Improving the Sound Environment Quality in Urban and Rural Areas. The document pointed out that with the development of economy and society, the influence of environmental noise pollution in China has become increasingly prominent, and environmental noise pollution disputes have occurred frequently. Solving the problem of environmental noise pollution is an inevitable requirement of implementing the concept of scientific and technological development and building an ecological civilization, and it is also an important content of exploring a new road of environmental protection in China. Article 23 of the document emphasizes: strengthen scientific and technological research and development, increase support for the research and development of acoustic environment quality improvement technology, and study noise control technology through scientific and technological plans, relying on the competent departments of the industry, and making full use of the noise and vibration research foundations of relevant scientific research institutions, universities and enterprises.
Therefore, noise control is a research direction worthy of attention in both military and civil fields. How to effectively reduce the noise in the control environment is an urgent problem we are facing.
Passive control method of 1.2 noise
The so-called noise control is to reduce or eliminate the harmful noise influence by using various noise control principles according to the nature, working environment and control requirements of the controlled object. In terms of strategy, noise control can start from three aspects: noise source, noise propagation path and noise receiver. Traditional noise control adopts passive control methods, such as absorption, isolation, damping and structural noise elimination. The mechanism of noise reduction is to attenuate acoustic energy through the interaction between noise sound waves and acoustic materials or structures, which is called passive noise control.
1.2. 1 Sound absorption and noise reduction
Sound absorption and noise reduction is mainly used for indoor noise reduction, which refers to the method of reducing noise intensity by using sound absorption materials or absorbing sound energy. Generally, the sound absorption performance of sound-absorbing materials or structures is defined as the ratio of absorbed sound energy to total incident sound energy, that is, EI- incident sound energy in formula a=E a/E i; EA- sound energy absorbed by a material or structure.
The sound absorption coefficient a is always less than 1. The larger a is, the more sound energy is absorbed, which indicates that the acoustic performance of the material or structure is better.
Common sound-absorbing materials mainly refer to porous sound-absorbing materials, such as glass wool, rock wool and foam plastic. Its sound absorption mechanism is that porous materials have a large number of interconnected tiny pores and holes. When sound waves are incident on porous materials, they can enter the interior of the materials along the pores, causing the vibration of air molecules in the pores. Due to the viscous resistance of air and the friction between air molecules and the hole wall, acoustic energy is converted into frictional heat energy to absorb sound.
For sound absorption structure, the material itself may not have obvious sound absorption characteristics, but after simple mechanical processing and surface treatment such as punching and grooving, the material is made into a structure with sound absorption performance. Such as perforated gypsum board, space sound absorber, sound absorption wedge, etc.
In workshops, factories, airport halls and other occasions, when sound waves propagate indoors, they will be reflected by obstacles such as walls, ceilings and floors, forming a reverberation sound field. By arranging sound-absorbing materials indoors, reverberation sound can be absorbed and indoor noise can be reduced. The maximum noise reduction can reach 10~ 15dB.
1.2.2 Sound insulation and noise reduction
It is called sound insulation to seal the noise sources such as machinery and equipment that produce noise in a small space and isolate them from the surrounding environment to reduce the impact of noise on the environment. Sound barrier and sound shield are two main designs, and other sound insulation structures include sound insulation room, sound insulation wall, sound insulation curtain, sound insulation door and so on.
The sound barrier is mainly used to block the transmission of direct sound. A facility is inserted between the sound source and the receiver, so that the sound wave propagation has a significant additional attenuation, thus weakening the noise influence in a certain area where the receiver is located. Noise barriers are mainly used outdoors. With the increasingly serious traffic noise pollution, some countries have adopted various forms of barriers to reduce traffic noise. The noise barrier facilities next to the railway can reduce the impact of noise generated by trains on residents.
Noise enclosures are used to block noise from noise sources such as machinery and equipment. It can be combined with the shell of the machine, or it can be a cover independent of the machine. Noise enclosures are usually a combination of sound insulation, sound absorption, damping, vibration isolation, ventilation and noise elimination. The sound insulation cover is mainly composed of cover plate, damping paint and sound absorption layer, and its structure can be completely closed, and necessary openings, valves or observation holes can be left. The size of a small sound-proof cover is only a few centimeters, and the largest one can be as high as tens of meters. The noise-proof enclosed facilities in the workshop of the factory enclose the noise-producing machines in a specific space, so as to reduce the hearing damage of the workers engaged in the operation in the workshop caused by machine noise.
1.2.3 muffler noise reduction
Muffler is a device that prevents sound from spreading and allows airflow to pass through, and it is an important measure to eliminate aerodynamic noise. Mufflers are usually installed in the airflow channels of aerodynamic equipment (such as blowers, air compressors, boiler exhaust ports, generators, water pumps and other large equipment) or in the air intake and exhaust systems to reduce noise.
According to the noise elimination mechanism, passive silencers can be divided into resistance silencers, reactive silencers, impedance compound silencers, micro-perforated plate silencers, small hole silencers and so on.
① The resistive muffler mainly uses porous sound-absorbing materials to reduce noise. The sound-absorbing material is fixed on the inner wall of the airflow channel or arranged in the pipeline in a certain way, which constitutes a resistive muffler. When the sound wave enters the resistive muffler, part of the sound energy rubs in the pores of the porous material and is converted into heat energy dissipation, which is weakened by the sound wave of the muffler.
② The reactive muffler is composed of a cavity with abrupt interface, which is similar to an acoustic filter. Each chamber with tubes is a grid of filters and has its own natural frequency. When sound waves with various frequency components enter the first spool, only sound waves with certain frequencies close to the natural frequency of the first mesh can reach the second spool through the mesh, while sound waves with other frequencies cannot pass through the mesh. It can only reflect back and forth in a small room, so we call this structure with the function of filtering sound waves an acoustic filter. By choosing the appropriate tube and cavity, the noise of some frequency components can be filtered and the noise can be eliminated.
③ Impedance compound muffler is composed of resistance structure and reactance structure in a certain way.
(4) Micro-perforated plate silencers are generally made of pure metal plates with a thickness less than 1mm, and are perforated with a drill with a commercial aperture less than 1mm, and the perforation rate is 1%~3%. By choosing different perforation rates and different cavity depths, the spectral performance of the muffler can be controlled, so that it can obtain good noise elimination effect in the required frequency range.
⑤ The structure of the small hole muffler is a straight pipe with one end closed, and many small holes are drilled in the pipe wall. The principle of orifice muffler is based on the frequency spectrum of jet noise. If the total area of the nozzle is kept unchanged and many small nozzles are used instead, when the airflow passes through the nozzle hole, the spectrum of jet noise will move to high frequency or ultra-high frequency, which will obviously reduce the audible sound components in the spectrum, thus reducing the interference and harm to people.
Edge control method of 1.3 noise
Generally speaking, the above passive control methods have good control effect on medium and high frequency noise, but have little effect on low frequency noise, and these methods have shortcomings such as installation and maintenance, heavy equipment and huge volume. Therefore, people began to look for new control methods to make up for the shortcomings of passive control methods, so active noise control (ANC) technology came into being. Theoretically, active noise control (also called active noise control) can achieve high noise reduction in low frequency range. At the same time, it can make the whole system smaller, easy to design and control, and has a great sense of superiority.
1.3. 1 Basic principle of active noise control
The basic principle of active noise control is based on the principle of destructive interference of sound waves, which was first proposed by the German physicist Paul L e u g in 1933, and was patented in Germany and the United States in 1933 N and 1936 respectively.
According to the principle of destructive interference of sound waves, the active control of pipeline noise in L e u g patent uses the artificially added secondary sound source to make the sound waves emitted by it destructively interfere with the sound waves emitted by the original primary noise source, so as to realize noise attenuation. Microphones are used to detect noise and convert it into electrical signals, which are amplified by amplifiers and then excited by speakers. The loudspeaker generates a secondary sound wave with the same amplitude as the primary sound wave but opposite phase. They cancel each other out. In this way, a local silent zone is formed downstream of the pipeline.
In order to obtain good denoising effect, it is necessary to accurately determine the time required for sound waves to propagate from the microphone to the speaker, and the amplifier should have good radiation frequency and phase frequency characteristics. Generally speaking, L e u g system is the earliest feedforward active noise control system, which lays a theoretical foundation for the vigorous development of active noise elimination. However, in the 1930s, the level of electronic technology at that time could not meet the above requirements, so the ideal of L e u g was not realized and was shelved for nearly 20 years.