Sound insulation cover structure
The soundproof cover shell is made of a layer of airtight metal material with a certain weight and rigidity, and it is generally made of 2 to 3 mm thick steel plate, covered with a layer of damping. The damping layer is usually made of fiber fabric or fiber material impregnated with bitumen damping glue (burlap cloth, glass cloth, felt or asbestos wool, etc.) impregnated with bitumen, and some are made of special damping slurry. The additional damping layer of the shell is to avoid the occurrence of plate anastomosis effect and low-frequency resonance of the plate. The shell can also be made of wood or plastic board, and the light sound insulation structure can be made of aluminum plate. The demanding soundproof cover can be made into a double shell, the inner layer is thinner than the outer layer; the distance between the two layers is generally 6-10 cm, filled with porous sound-absorbing materials. A sound-absorbing material is added to the inner side of the cover to receive sound and weaken the noise in the cavity. A layer of perforated protective panel is covered on this layer of sound-absorbing material, and the perforated area occupies about 20-30% of the protective surface area. Between the cover and the machine, the cover and the foundation, the world is filled with rubber cushions to prevent the transmission of vibration. The shutters and observation holes that can be opened must be sealed. Regarding the equipment requiring heat dissipation, the required ventilation ducts should be left on the sound-proof enclosure. This kind of pipe must have a sound-absorbing structure, perhaps with a muffler. When designing a sound-proof enclosure, it is necessary to pay attention to meeting the requirements of process and maintenance, and sometimes to accept methods such as preventing oil, dust and corrosion.
Sound insulation of soundproof enclosures
To measure the noise reduction effect of a hood, the penetration loss and IL are often used to leak the display. It leaks and shows the difference between the sound pressure level before and after the cover at a certain point in the outer space of the cover, which is equivalent to the theoretical noise reduction effect of the cover. The calculation formula for puncture loss is:
Regarding the cover close to the sound source, that is, when the volume of the cover is similar to the volume of the sound source, the space inside the cover is very small, and the cover and the machine itself will become a vibration coupling system. In order to prevent the shell vibration and spatial coupling resonance of the cover, a damping layer and a thicker sound-absorbing material are required. In order to prevent low-frequency resonance, the thickness of the sound-absorbing material should not be less than half of the thickness of the cavity between the cover and the machine.
It is more complicated to calculate the sound leakage from the opening part of the cover. It is related to the size, location, depth of the hole and the frequency of noise. The holes in the edges or corners leak more noise than the holes in the center of the board. When the sound leakage of a hole is generally estimated, the transmission coefficient of the hole can be considered to be 1. With a cover with a total area of S, assuming that the total area of all openings is △S′, the maximum sound insulation R of the cover is:
R =10 lg(S/ΔS′)
Regarding a square cover with six sides, assuming that the opening tooth occupies two sides, its highest average sound insulation is 5 decibels. If it is used to prevent sound from penetrating into the cover from outside the cover (as a sound insulation box), the difference between the external and internal sound pressure levels L1 and L2, that is, the amount of noise reduction NR is:
NR=L1-L2=R +10 lgα
When thinking about low-frequency sound insulation, theoretical analysis and implementation results prove that the low-frequency resonance frequency of the partition and cavity of the cover should be far away from the duty frequency of the equipment.
Many equipment, such as ball mills, air shrinking machines, generators, electric ideas, diesel engines, etc. can use sound-proof enclosures.
The design of sound insulation cover should consider the following points:
(1) Choose the right shape. In order to increase the volume of the soundproof cover and the noise radiation area, its shape should be similar to the outline of the sound source device, and the cover wall should be as close as possible to the shell of the sound source equipment; And its muffler normal obligations. In addition, the curved body should have greater rigidity, which is conducive to sound insulation. Minimize the use of square parallel cover walls to prevent the standing wave effect of airborne sound in the cover and make the sound insulation low.
(2) The wall material of the soundproof cover should have enough transmission loss to drop LTL. The cover wall material can be lead plate, steel plate, aluminum plate, thin-walled, high-density plate, generally 2～3mm steel plate can be used.
(3) Reinforce or apply damping layer on the surface of the metal plate. By stiffening or applying a damping layer, it can suppress and avoid the resonance and coincidence effect of the light structural cover wall such as steel plate, and increase the radiation of sound waves. The thickness of the damping layer should not be less than 2 to 4 times the thickness of the cover wall, and it must be pasted tightly and robustly.
(4) The sound insulation cover should have good sound absorption function. In the hood, 50mm thick porous sound-absorbing materials are used for processing, and the sound absorption coefficient should not be less than 0.5 in general. The composition of the basic components of the sound insulation cover is shown in Figure 10-8. It is a 3mm thick steel plate, robustly coated with a 7mm thick asphalt asbestos wool as a damping layer, and is lined with 50mm thick ultra-fine glass wool (bulk weight 25kg/ m3) As the sound absorption layer, the glass wool protective surface layer is composed of a layer of glass cloth and a layer of perforated steel plate with a perforation rate of 25%. The average transmission loss of this component falls between 34 and 45 dB.
(5) Vibration isolation measures. There can be no rigid connection between the sound insulation cover and the machine. In the world, the flexible connection such as rubber or felt is clamped between the two to directly absorb the vibration, otherwise the vibration of the machine will be directly transmitted to the cover body, making the cover body a noise radiation surface, and then reduce Sound insulation consequences. Vibration isolation methods are also required between the machine and the foundation, and between the soundproof cover and the machine foundation.
(6) How to deal with the holes on the cover. The sound energy density in the soundproof cover is very large, and the small openings or cracks on the soundproof cover can transmit a lot of noise. The study indicated that only need to open 0.01 area holes on the total area of the sound insulation cover, the sound insulation capacity will increase by 20-25dB or less. If you still need to make holes in the cover: ① Add a set of pipes where the drive shaft passes through the opening of the cover. The pipe is lined with sound-absorbing material. The length of the sound-absorbing lining should be greater than that of the drive shaft and the sound-absorbing lining. The cracks between 15 times, this Yang not only avoids the sound bridge, but also reduces the sound leakage through the sound absorption; ②The holes opened for suction and exhaust or ventilation and heat dissipation can be installed with an anechoic box to reduce the sound; ③Cover The joints of body splicing and movable doors, windows, covers and other joints should be padded with soft rubber or other materials. When the cover or door is blocked, it should be fastened with a lock to ensure that the seams are compacted and prevent Leaking sound; ④For the holes of the inlet and outlet, it is common to add a double rubber brush to allow the material to pass through, and the sound is not easy to escape.