Development Of Porous Sound Absorbing Materials

1. History of sound absorbing materials

Humans have long realized that fabrics can effectively block sound, so from prehistoric to Egyptian and even Roman times, people have been using fabrics, reeds, hay, etc. as sound-absorbing materials to eliminate echoes and resist noise, making people feel more comfortable. These materials are essentially organic fiber materials, and the tiny gaps between them make them natural sound-absorbing materials, and organic fiber sound-absorbing materials are still playing their role to this day.

In ancient times, many countries and peoples have made great achievements in acoustic research and accumulated a lot of experience, but the research on sound-absorbing materials has always been limited. The scientific research on sound-absorbing materials is closely related to the theory of interior acoustic design. Finally, in 1898, W.C. Sabin founded the theory of reverberation, and pointed out that sound absorption plays a decisive role in the reverberation of a room, and the development of sound-absorbing materials entered a new era.

From 1910 to 1915, he worked with the company, first developing a porous pottery, and later a porous brick, for St. Thomas' Church and Riverside Church in New York. To "maximize the softness and richness of the organ's sound," he suggested using 1-inch-thick felt and wire boards spaced 1/4-inch apart in the music room; to absorb low frequencies, he suggested using a very thin inner lining of wood 1 inch thick space. It can be said that he laid the foundation for modern room acoustics research.

my country's acoustics started in 1929. After Professor Ma Dayou founded the Acoustics Research Laboratory in 1957, my country's acoustics research has made substantial progress. Under the leadership of Professor Ma Dayou, my country proposed the theory of micro-perforated sound-absorbing body, and developed a small-hole muffler to effectively reduce airflow noise. And established the pressure law of airflow noise. It has laid a solid foundation for the development of acoustics in my country.

In the 1950s people began to study the psychology of sound. From the Haas effect in 1951, binaural hearing in the late 1960s

From the concept and indicators such as clarity and binaural correlation coefficient IACC proposed in the 1970s, these theories provide reliable theoretical support for the precise arrangement of sound-absorbing materials. During this period, the research on sound-absorbing materials began to be in-depth and meticulous. The study of sound absorbing materials also involves other properties besides sound performance. Factors such as the mechanical, thermal and optical properties of materials, moisture resistance, fire resistance, maintenance, construction and artistic effects of materials, and the selection and installation location of sound absorbing materials are gradually becoming more precise. In the theatre design, the audience is also studied in detail as a large sound absorber.

According to the sound characteristics of loose and porous materials, people have created sound-absorbing materials, namely porous sound-absorbing materials. Porous materials are the most widely used sound-absorbing materials at present. Its types include inorganic and organic fibrous porous sound-absorbing materials. materials, foam-like porous sound-absorbing materials, granular porous sound-absorbing materials, etc. With the development and innovation of acoustics and technology, the dedicated research of acoustic workers and the development of the market, there will be new porous sound-absorbing materials. At present, porous sound-absorbing materials are widely used in conference halls, concert halls, auditoriums, and other buildings with high sound requirements.

2. The principle of porous sound absorbing materials

The so-called porous sound-absorbing material is that this type of material is composed of solid ribs and micropores or gaps. Porous sound-absorbing materials have a large number of interconnected micropores or gaps, and the pores are small and evenly distributed inside the material, opening outward and going deep into the interior of the material. The sound absorption mechanism is that when the sound wave is incident on the surface of the material, a part of it is reflected on the surface of the material, and the other part penetrates into the interior of the material and propagates forward. Friction occurs on the rib or hole wall, and the sound energy is converted into heat energy and dissipated due to the viscosity and heat conduction effect. After the sound wave is reflected on the rigid wall, when it passes through the material and returns to the surface, part of the sound wave is transmitted into the air, and part of the sound wave is reflected back into the material. Causes the material to absorb some of the sound energy. High-frequency sound waves can speed up the vibration of air particles in the gap, and the heat exchange between the air and the hole wall is also accelerated. This makes the porous material have good high frequency sound absorption properties.

3. Classification of porous sound-absorbing materials

Porous sound-absorbing materials have better sound-absorbing effects and are the most commonly used sound-absorbing materials. Initially, these materials were mainly organic materials such as hemp and cotton, and now they are mainly glass wool and rock wool.

3.1 Fiber Materials

Fiber materials are mainly divided into organic fiber sound-absorbing materials and inorganic fiber sound-absorbing materials according to their physical properties and appearance.

3.1.1 Organic fiber materials

Traditional organic fiber sound-absorbing materials have good sound-absorbing properties in the medium and high frequency range, and are roughly divided into animal fibers and plant fibers. Animal fiber materials mainly include felt and pure wool carpet, which are characterized by good sound absorption performance and gorgeous decoration effect, but are expensive and rarely used. Plant fiber materials, such as organic natural fiber materials such as sugarcane fiberboard, wood fiberboard, cement wood fiberboard, and chemical fiber materials such as acrylonitrile fiber, polyester fiber, melamine, etc., but these materials have poor fire resistance, corrosion resistance, and moisture resistance. , the application is restricted by environmental conditions.

3.1.2 Inorganic fiber materials

Inorganic fiber materials mainly include rock wool, glass wool, slag wool and aluminum silicate fiber wool, etc. Due to their good sound absorption performance, light weight, no moth, no rot, no combustion, no aging, etc., they have gradually replaced the traditional Natural fiber sound-absorbing materials have been widely used in acoustic engineering. However, because the fiber is brittle and easy to break, the resulting fiber powder will fly in the air, and the formed dust will itch the skin, pollute the environment, and affect breathing, which is its disadvantage in application. Compared with synthetic organic fibers, glass fibers and natural fibers are not easy to age, which used to be an advantage of glass fiber material performance, but from the perspective of environmental protection, the material is not easy to degrade so that it will eventually become solid waste. secondary pollution of the environment.

3.2 Foam material

Foam materials mainly include foam rice plastic, polyurethane foam plastic, foam glass and aerated concrete. According to the different cell forms of foam materials, it can be divided into closed cells, open cells and semi-open cells. The closed-cell foams are called closed-cell foams, the ones that are connected to each other are called open-cell foams, and the ones that are both connected and closed are semi-open-cell foams.

3.2.1 Closed Cell Foams

The metal foam material with closed-cell structure is represented by closed-cell foamed aluminum. The sound absorption coefficient of closed-cell foamed aluminum is relatively low, because it is difficult for sound waves to reach the inside of the pores and interact with it. There are only some cracks and micropores. It cannot be used as a good sound-absorbing material.

3.2.2 Semi-Open Cell Foams

The semi-open-cell aluminum foam can be prepared by high-pressure infiltration, and during its preparation, the expected pore connectivity can be achieved by controlling the preparation parameters.

3.2.3 Open Cell Foams

The porosity and pore shape can be controlled by controlling the shape and size of the particles, and high-porosity materials can be made. Since the open-cell foam material has a complex channel structure and rough internal voids, it has a high flow resistance, so open-cell foams have high flow resistance. The overall sound absorption performance of cellular aluminum foam is much better than that of closed cells.

3.3 Particulate materials

Granular materials are mainly divided into blocks and plates. Blocks mainly include slag sound-absorbing bricks, expanded perlite sound-absorbing bricks, and terracotta sound-absorbing bricks, which are mostly used for mufflers with large masonry sections. The board mainly includes expanded perlite sound-absorbing decorative board, which is light in weight, non-combustible, heat preservation, heat insulation and low strength. However, the sound absorption effect of granular materials is relatively poor, so they are generally used in occasions with high requirements for moisture resistance and fire protection.

3.4 Metal Materials

Porous sound-absorbing materials produced with metal powder as raw materials are new sound-absorbing materials that have appeared in recent years, such as Carrom metal sound-absorbing panels produced in Japan. Its advantage is that it has the strength of metal, and it can be bent and cut with simple tools. But most of these materials are thin and need to rely on the cavity behind.

4. Future Outlook

Now, after studying the relationship between the surface shape of the sound-absorbing material and the sound absorption coefficient, people have invented the wedge-shaped porous sound-absorbing material, which can improve the sound absorption and transmission loss in a certain band. And fiber sound-absorbing materials have begun to show their strong vitality because of their cheapness, and particle-type sound-absorbing materials have also begun to play their durable advantages in special environments.

In order to replace the mineral fiber sound-absorbing materials that are harmful to health, Germany first developed a micro-perforated plate, a green and efficient resonance sound-absorbing structure. It has broad application prospects in noise control in the industry, especially in extreme environments with high temperature, high sound intensity or in special environments with high cleanliness requirements, so it will also be a research direction that will continue to develop in the future, especially the expansion of noise absorption. Research on acoustic bandwidth and low-frequency noise, use of different materials or micro-perforated derived structures-flexible tube bundle perforated plate sound absorption structure, and the development of micro-perforated plates and structures in different environmental ratios

Such as applications in high temperature, high sound intensity and other environments, it has a wide range of application prospects.

With the progress of society, people have higher and higher requirements for the quality of the sound environment. A single sound-absorbing material can no longer meet the requirements of environmental protection and high-efficiency sound absorption. Among industrial solid wastes, the research on the preparation of sound-absorbing materials from bulk solid wastes such as blast furnace slag, fly ash, and coal gangue should be strengthened, and the possibility of preparing sound-absorbing materials from steel slag, tailings powder, construction waste, etc. as raw materials should be further explored. , so as to realize the low-cost preparation and industrial application of sound-absorbing materials.

In the future, "environmentally friendly" and "safe" acoustic materials will be the focus of development. Eco-environmental protection and sustainable development are the focus of architectural renewal in the new century, and are also the main subject of global research in the 21st century. The production of sound-absorbing materials that are harmless to the human body, can be recycled, and are highly efficient has good application prospects. At the same time, attention should also be paid to the aesthetic design of the product, so that the sound-absorbing material is both practical and ornamental.