Sound Absorption Mechanism of Acoustic Materials for Diesel Generators

Common sound-absorbing materials for diesel generators, such as glass wool, slag wool, foam plastics, asbestos wool, felt, strawboard, soft fiberboard, and microporous acoustic bricks, are mostly porous materials. The sound absorption mechanism of these materials relies on sound waves entering the pores of the material and interacting with them. Therefore, the interior of porous materials must be porous, with interconnected pores that are also connected to the outside environment.

Some porous materials, such as aerated concrete and foam glass, have most of their bubble pores independent, closed and non-interconnected, resulting in poor sound absorption performance. Certain foam plastics can only be classified as flexible sound-absorbing materials, and their sound absorption coefficients exhibit obvious resonant sound absorption characteristics. Therefore, attention must be paid to this in applications.

In addition, sound-absorbing materials and sound-insulating materials are completely different concepts. Commonly used porous sound-absorbing materials can absorb most human voices, but due to their high porosity, sound can easily pass through them. Therefore, their sound insulation performance is very poor. For example, a sound-absorbing material can absorb 90% of the sound energy. However, if the remaining 10% of the sound energy is completely transmitted, according to sound insulation theory, the sound insulation amount is only 10 dB. Even if 99% of the sound energy is absorbed and 1% is transmitted, the sound insulation amount is only 20 dB. It can be seen that for sound-insulating materials, density is important, while sound-absorbing materials are often air-permeable and porous.

At present, the porous sound-absorbing materials produced in China mainly include inorganic fiber materials, foam plastics, organic fiber materials and acoustic building materials.

(1)Inorganic fiber materials: This category of sound-absorbing materials mainly includes glass silk, glass wool, rock wool, slag wool and their products.

(2)Foam plastics: Foam plastics used as sound-absorbing materials are characterized by low bulk density (10-40 kg/m³), low thermal conductivity, and softness. Their disadvantages are easy aging and poor fire resistance. Urea-formaldehyde foam plastics have poor strength and are easy to break. Currently, polyurethane foam plastics are widely used.

(3)Organic fiber materials: These materials use plant fibers such as cotton and hemp for sound absorption, such as flying flowers from textile factories, cotton and hemp leftovers, cotton wool, straw, seaweed, coconut husk, palm silk and their products. They can also be made into various soft fiberboards by chopping, softening, beating and pressing leftover wood, bagasse, hemp silk, paper pulp, etc. Wood wool boards made by pressing wood wool, cement and water glass also belong to this category. These materials have the characteristics of low price and good sound absorption performance.

Various fiberboards used as sound-absorbing materials are soft fiberboards with a bulk density of generally 200-300 kg/m³. Hard fiberboards with a bulk density of 1000 kg/m³ cannot be used for sound absorption. When using organic fibers, attention should be paid to fire prevention, moth prevention and moisture prevention.

(4) Acoustic building materials: At present, various foam microporous acoustic bricks and foam concrete are often used in buildings to achieve direct sound absorption. Some acoustic bricks have resonant cavities designed for a certain frequency, which do not belong to porous materials.

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