Text | China Science News reporter Liu Runan
In conference halls, people want speech to be smooth and audible; In the concert hall, people hope that the aftermath will linger for three days; When it comes to theater theaters, people want the sound to be realistic and immersive......
Different buildings require different sound quality designs, and corresponding sound absorption designs are needed to match them. When it comes to sound-absorbing structures, it is inseparable from Ma Dayou, an important pioneer and founder of modern acoustics in China - academician of the Chinese Academy of Sciences and researcher of the Institute of Acoustics of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Acoustics).
In 1966, Ma Dayou proposed the idea of a sound-absorbing structure of micro-perforated plates, and later published his theoretical analysis in the journal Science China. Later, this theory set off a revolution in sound-absorbing materials in the world, and countless researchers have joined the research and application of microperforated plates. This has also become a key step for China's modern acoustics to go international.
Today, in important buildings such as the Great Hall of the People, micro-perforated plate structures can be seen everywhere.
The problem of rocket launch noise needs to be solved urgently
In November, the Gobi Desert is cold and windy, and the sky is full of sand.
At the Jiuquan Satellite Launch Center in Gansu Province, Ma Dayou, who is half a hundred years old, and three young scientific researchers pulled up the frozen cables, carefully laid an outdoor microphone, and placed it 50 meters away from the launch frame.
With the order of "ignition", the rocket took off, and the roar was endless. At the same time, the sound of the entire process of the launch test was recorded by a tape recorder.
In 1965, the plan to develop the mainland's first artificial satellite was approved by the Central Special Committee, called the "651 Project", and the Chinese Academy of Sciences was responsible for the development of satellites and ground tracking systems. Because Ma Dayou had previously participated in the "581 mission" of the Chinese Academy of Sciences' satellite development, he undertook the satellite acoustic environment experiment this time.
The launch of an artificial satellite is inseparable from the launch vehicle, and the noise of the rocket is unavoidable. During launch, the high noise intensity will cause the acoustic fatigue of the rocket skin and damage the instruments and equipment. At that time, there were many accidents caused by this in foreign countries. Ma Dayou and others made this trip to the west precisely to analyze and study the noise of rocket launches and find ways to improve it.
During his work at the launch base, Ma Dayou learned that at that time, the deployment of strategic rockets in developed countries had been transferred from the ground to the underground, and the launch method was changed from ground launch to underground shaft launch, which caused the noise problem to become more serious. For long-term deployment, it is necessary to study the control method of downhole noise in advance.
This is a completely new research task, and there are no ready-made samples to refer to, and no field data to check. The launch of a foreign shaft is itself for concealment, so it is impossible to know what is inside.
"The launch base invited us to participate in the next launch in the near future, but Mr. Kema was anxious to carry out research work, so he decided to return to Beijing immediately after measuring the sound level and making a recording." Zhang Jialuo, a researcher at the Institute of Acoustics who went with Ma Dayou at that time, recalled.
The road to simplicity
At that time, perforated plates and sound-absorbing materials were widely used at home and abroad for noise reduction. People evenly cut some centimeter-level holes on the board, and fix fibrous and porous materials such as glass wool and slag wool behind the plate to form an ordinary perforated plate sound-absorbing structure.
In fact, the perforated plate itself is a resonant sound-absorbing structure, which has a certain sound-absorbing ability, but in practical application, it is often necessary to rely on sound-absorbing materials to enhance the sound-absorbing effect.
Due to the viscous nature of air, when the sound wave passes through the hole of the perforated plate, the air creates resistance and consumes part of the sound wave energy. The rest of the sound waves enter the fibrous material and porous material through the large pores, hit their irregular surfaces, and produce scattering, and the holes and hole walls will also cause multiple reflections and refractions of sound waves, which are propagated and dissipated many times inside the material. At the same time, the sound waves rub against the wall of the hole and the skeleton in the material, converting the sound energy into heat, which again consumes part of the sound wave energy. The combination of the two can effectively reduce the reflection of sound waves, play a role in sound absorption and noise reduction, and improve the acoustic environment.
And in the underground shaft where the rocket was launched, this approach did not work at all. The emission is accompanied by high temperatures, flames, high pressures, high humidity and corrosive gases, which can wipe out these sound-absorbing materials in an instant. In addition, according to the different needs of practical applications, there is no theoretical guidance on how much sound-absorbing materials to add, and it can only be based on trial and error based on experience.
After returning to Beijing from Jiuquan, Ma Dayou was eager to carry out research on the sound-absorbing structure. As soon as he had time, he thought: porous materials are themselves wideband sound-absorbing materials, so why bother adding perforated plates? Perforated panels sometimes just serve as a protective panel. Is it possible to make the perforated plate itself solve the problem of sound absorption?
Ma Dayou (right) conducts a noise measurement test.
Ma Dayou believes that this is completely feasible in theory. If the acoustic impedance of the perforated plate structure matches the acoustic impedance in the atmosphere, there is no need to add additional sound-absorbing materials, and a better sound-absorbing effect can be obtained. At the same time, in order to widen the sound-absorbing frequency band, the sound quality of the perforated plate structure must be reduced as much as possible.
Studies have proved that the larger the aperture of the perforated plate structure, the smaller the acoustic impedance, and the larger the acoustic impedance, its sound quality is roughly only related to the perforation rate. Therefore, by controlling the aperture size and perforation rate, the acoustic impedance and sound quality can be controlled, and then the sound absorption effect of the perforated plate can be controlled.
After repeated theoretical reasoning, Ma Dayou proposed that if the aperture is reduced to the level of silk meters (1 silk meter is equal to 0.1 mm), sufficient acoustic impedance can be obtained, making it a good wideband sound-absorbing structure, and no porous materials need to be added. This is the concept of a microperforated plate.
He judged that making microholes in any plate could achieve the purpose of sound absorption.
"Mr. Ma often told us, 'To deal with problems, we must grasp the essence of the problem' and 'keep the road simple.'" Dai Genhua, a researcher at the Institute of Acoustics who has worked with Ma Dayou for a long time, said, "Although the concept of perforated plates has been around for a long time, the aperture is relatively large and the plates are relatively thick. It may seem that he simply turned the large hole into a small hole, but in reality, it is impossible to think of without a long-term study of the problem of the nature of acoustics and creative thinking. ”
"I didn't want anything in return, this is the tradition of the Chinese Academy of Sciences"
Once the theoretical idea is in place, the next step is to verify it experimentally.
First of all, we drilled holes one by one on each 10-square-meter, about 1-millimeter-thick aluminum plate with sharpened steel nails, and the speed was slow. Later, a metalworking master in the laboratory thought that he could use a sewing machine for shoe repair to tie it line by line, which greatly improved the speed.
Whenever a micro-perforated plate with different aperture and perforation rate is made, Ma Dayou will first measure it in the standing wave tube and then take it to the reverberation chamber for measurement according to different sound environments.
Dai recalled: "When I was doing experiments in the reverberation room, I needed at least 3 to 4 square meters of micro-perforated plate samples. The signal generator is used to generate the acoustic signal, which is amplified by the amplifier to form a reverberant field, and then the acoustic signal is picked up by the capacitor microphone, and then recorded by the recorder, and finally the sound absorption volume and sound absorption coefficient of the sample are calculated. ”
Later, Ma Dayou and his team members dug a simple model well about 4 meters square and 2.5 meters deep in the courtyard of the Institute of Acoustics, solidified the wall of the well with cement, and installed stainless steel micro-perforated plates. The sound absorption coefficient is calculated by measuring the reverberation time before and after the microperforated plate is loaded.
Micro-perforated plates with 0.25 mm pore diameter and different hole spacing.
In this way, Ma Dayou and others tested the sound absorption effect of aluminum plate, cardboard, bakelite board, stainless steel plate and other plates with a hole diameter of 0.75 mm, a thickness of 1 mm and 0.5 mm, and 30,000 or 80,000 holes per square meter.
They eventually found that stainless steel plates with a thickness of 1.5 mm, a hole diameter of 1 mm, and a perforation rate of 1 to 2 percent had the best sound absorption, and could withstand instantaneous high temperatures, humidity, and strong air currents. In order to achieve this "three resistances", they also installed a protective large perforated plate with a hole diameter of 4 mm to 5 mm and a perforation rate of 15% in front of the micro-perforated plate.
Later, Ma Dayou formed a summary report of his theoretical analysis and experimental verification, and put forward specific suggestions on the manufacturing process of micro-perforated plates, which were submitted to relevant departments. After the sound-absorbing structure of the micro-perforated plate was put into practical use, the replacement of special plates was also successful.
In 1975, Ma Dayou wrote a paper "Theory and Design of Sound Absorbing Structure of Micro Perforated Plate", which was published in the first issue of Science China that year.
Even after nearly 10 years of postponement before the paper was officially published, the microperforated plate theory is still the world's leading sound absorption theory. The establishment of this theory makes it possible for people not to carry out a lot of experiments and calculations when applying, but only need to master three constants, and through certain formulas, other variables can be calculated. In order to facilitate engineering applications, Ma Dayou also converted these calculation formulas into simple diagrams, which are convenient for design engineers to consult, avoiding the trouble of repeated calculations.
"At that time, we did not have the awareness of patent protection, and after the principle and practice were published, there were many domestic enterprises and factories producing micro-perforated plates." Dai Genhua said.
"When we carried out technical research on the sound-absorbing structure of micro-perforated plates, we didn't think about asking for anything in return, which is the tradition of the Chinese Academy of Sciences." Ma Dayou once said, "For the national strategic needs, we will not only go all out to provide technical support, but also provide application samples free of charge." The fact that the national strategic needs can think of us is the greatest love for us. ”
Renovation of the acoustic design of the Great Hall of the People
After proposing the theory of micro-perforated plates, Ma Dayou did not stop there. In the decades that followed, he continued to explore and deepen its development.
In 1983, Ma Dayou proposed a direct and simple method for measuring the acoustic impedance of microperforated plates, which is simpler and more accurate than the existing methods, and is convenient for people to accurately estimate the working performance of the sound-absorbing structure of microperforated plates. In 1988, he further proposed the perforated acoustic impedance formula to simplify the structural design of microperforated plates.
In high sound intensity environments, the velocity of the particles within the perforation can be quite high compared to the speed of sound, which can affect the acoustic impedance and other characteristics of the microperforated plate. In 1996, Ma Dayou proposed an improvement method based on the application research of microperforated plates in a high sound intensity environment.
When the theory of micro-perforated plate structure was first proposed, in order to break the limitation of the machining conditions of micro-perforated plate in practical application, Ma Dayou made an approximation of the transcendence function in the basic equation. As its application became more and more extensive, in 1997, he further developed the accurate theory and design of the sound-absorbing structure of micro-perforated plates, which further enhanced its application potential.
In 2000, Ma Dayou further developed the theory of microperforated plate sound absorber into the theory of micro-slit sound absorber according to the principle that sound absorbing materials lose energy by friction between sound waves and their solid skeleton through pores, so as to make their structures more diversified.
In 2003, Ma Dayou further discussed the limit of the absorption bandwidth of microperforated plate sound absorbers......
"Mr. Ma published his theory in 1975 and has continued to study it in depth for decades, until he is over 80 years old, and he is still developing and deepening his theoretical research." Li Xiaodong, a student of Ma and a researcher at the Institute of Acoustics, said, "Although Mr. Ma published papers in Chinese back then, his citation rate has been increasing since 2000. The International Congress of Acoustics also set up a special discussion on the theory of microperforated plates. ”
The theory of micro-perforated plates has been applied in various engineering practices in mainland China, among which the Great Hall of the People is the most famous.
In 1999, the Great Hall of the People Administration planned to repair and renovate the Great Hall of the People, in which the architectural acoustic design is an important part of the overall plan. As early as the beginning of the construction of the Great Hall of the People, Ma Dayou was responsible for its sound quality design work, and the acoustic design task of this maintenance and renovation naturally fell on the shoulders of the Institute of Acoustics.
The 10,000-person auditorium not only has a large volume, but also is close to an ellipse, and it is a dome, which makes the reverberation time of the original auditorium long, the echo phenomenon is serious, and the language clarity is low. At that time, the central leading comrades had clear instructions on the maintenance and renovation of the auditorium, requiring that "the renovation of the auditorium must maintain its original architectural style". This means that the volume of the auditorium remains the same, the shape remains the same, and the interior walls and ceiling remain unchanged.
Due to the requirements of fire protection, the materials used in the transformation are also clearly stipulated, that is, the surface material must be metal, and the inner material must be first-class fireproof. At the same time, in order to ensure that "water and sky are the same", the perforation rate of the metal perforated plate as the surface layer of the sound-absorbing structure must be consistent.
"This required us to choose materials with fire safety at the center and sound quality to improve compared to what we had before the retrofit. This undoubtedly brings considerable difficulty to the design of sound quality. Li Xiaodong said.
Ma Dayou (right) supervises his PhD student Tian Jing in noise analysis.
On the basis of the theory of Ma Dayou's micro-perforated plate, his students, Tian Jing and Li Xiaodong, researchers of the Institute of Acoustics, formally determined the sound quality design plan for the renovation of the 10,000-person auditorium after more than a year of sound field simulation, sound-absorbing material and structure testing, and theoretical calculations.
Later, after more than three years of testing such as meetings and large-scale theatrical performances, the headquarters of the Great Hall of the People maintenance and renovation project believed that the sound quality of the renovated 10,000-person auditorium had been greatly improved, the original excessively long reverberation time had become shorter, the serious echo phenomenon had been basically eliminated, and the language clarity had been greatly improved.
Great Hall of the People 10,000 people auditorium. Courtesy of the Acoustics
Save the halls of the German parliament
Ma Dayou did not expect that 17 years after his theoretical design of micro-perforated plates was proposed, he helped Germany save an important project, which triggered a boom in the research and application of micro-perforated plates abroad.
It was December 1992, after German reunification, and a new parliament building was built. In order to fully reflect the transparency of the meeting, the building is surrounded by transparent glass. From a distance, the building's central council chamber resembles a huge cylindrical glass dome.
"Ladies and gentlemen!" At the first meeting, the speaker had just said a word when the loudspeaker in the chamber went silent. The meeting was broadcast live throughout the country, and 662 parliamentarians were seen angrily leaving their seats and returning to their original parliamentary chambers to continue the meeting.
You know, the new parliament building cost 270 million marks (about 1.35 billion yuan), and the maintenance personnel did not find any problems with the power supply, audio equipment, and microphones. This shook the German engineering community and became a scandal in Germany for a time.
It turned out that this glass-enclosed rotunda had a serious acoustic focusing phenomenon. When speaking in the council chamber, the sound is constantly bounced back by the dense and smooth glass walls around it, and is concentrated in the center of the hall - where the podium, microphones, and speakers are set up, and the computer-controlled sound reinforcement system automatically locks.
Later, the relevant departments approached the Fraunhofer Institute for Building Physics in Germany. At that time, it coincided with the visit of several Chinese scholars such as Zha Xueqin to the institute. Zha Xueqin thought of Ma Dayou's paper on the sound-absorbing structure of microperforated plates, and immediately put the theory into practice, drilling holes in aluminum plates for experiments, and finally found that the measured data were consistent with the theoretical calculations given by Ma Dayou's formula.
After several weeks of research and measurement work, with the support of a small local factory, Zha Xueqin et al. took out samples - 30,000 microporous holes with a pore size of 0.8 mm were punched into 5 millimeters of plexiglass with a thickness of 5 millimeters per square meter to form a transparent microperforated plate. They finally solved the acoustic problem in the parliament hall, which was widely reported in the German engineering community and featured in the German media such as the German newspaper Bild.
Later, in recognition of Ma Dayou's achievements in establishing the design theory of microperforated plate sound-absorbing structures, the German Fraunhofer Society awarded Ma Dayou the Gold Medal, and the Fraunhofer Institute for Building Physics awarded the ALFA Prize and a prize of 10,000 marks.
In 2021, the International Conference on Noise Control Engineering selected 7 milestones in the development history of noise control in the past century, and the theory and design of microperforated plate structures is one of them.
Nowadays, the researchers of the Institute of Acoustics, standing on the shoulders of Ma Dayou and other predecessors, are concentrating on research and innovation, and constantly expanding the theoretical and application boundaries of micro-perforated plate structure.
Li Xiaodong said: "We have made a number of achievements in the theoretical research and engineering design of micro-perforated plates, and we are still stimulating its application potential, and I believe that more and more original breakthroughs will be made in the future!" ”