Noise generally refers to the sound of inappropriate or uncomfortable auditory stimuli, and the noise is very harmful to people. Medical research has proved that noise is more harmful to children, especially infants. The hearing systems of children and infants are not yet mature, and the hearing organs are very delicate and fragile, and cannot withstand the violent stimulation of strong noise. We must pay attention. And noise control has become a must-have item to improve social living standards, so what is the principle of noise control? Let me introduce it to you below.
What is the principle of noise control:
When the noise emitted by the sound source propagates in the medium, its sound pressure or sound intensity will gradually attenuate with the increase of the propagation distance. High-frequency sound waves attenuate faster than low-frequency sound waves. When the propagation distance is large, the attenuation value is very large, so high-frequency sound waves cannot travel far. Strong noises from long distances, such as aircraft sounds and gun sounds, are relatively low-pitched, which is why high-frequency components attenuate faster during long-distance transmission.
In addition to the air that can absorb sound waves, some materials such as glass, blankets, foam plastic, etc. can also absorb sound, which is called sound absorbing material. When the sound wave passes through these porous sound-absorbing materials, the sound wave energy is greatly absorbed and attenuated due to the internal friction of the material itself and the friction between the air and the wall of the material pores. This sound-absorbing material can effectively absorb the sound energy incident on it.
Noise sound waves often encounter obstacles in the process of propagation. At this time, sound waves will be incident from one medium (air) to another medium. Due to the different acoustic properties of the two types of coal, some of the sound waves are reflected back from the surface of the obstacle, while the other part of the sound waves are transmitted into the obstacle. The purpose of noise reduction can be achieved by using different characteristic impedances of the medium.
For example, when measuring noise outdoors, hard floors, roads, and building surfaces are all reflective surfaces. If sound-absorbing materials are laid on the reflective surfaces, the reflected sound energy will be reduced. Due to the reflection characteristics of sound waves, a certain noise generated indoors will be reflected multiple times from walls, floors, ceilings and various objects in the room. The existence of this reflected sound makes the sound pressure level of the noise indoors higher than that in the open air. The sound pressure level at the same distance should be increased by 10~15db. In order to reduce the influence of the reflected sound in the room, covering the inner surface of the room with a material with good sound absorption performance can greatly reduce the reflected sound and reduce the overall noise.
When the sound wave encounters the interface of different media during the propagation, in addition to reflection, refraction will also occur, and the propagation direction of the sound wave will change when the sound wave is refracted. In addition, sound waves also produce diffraction. Diffraction is related to the frequency, wavelength and size of obstacles. If the frequency of the sound wave is relatively low and the wavelength is relatively long, and the size of the obstacle is much smaller than the wavelength, the sound wave can bypass the obstacle and continue to propagate behind the obstacle. If the frequency of the sound wave is relatively high, the wavelength is short, and the obstacle is much larger than the wavelength, and the diffraction phenomenon is not obvious at this time.
The sound waves reach less behind the obstacle, forming a distinct shadow area. Diffraction phenomena are used in noise control. Sound insulation screens are often used to reduce the impact of high-frequency noise. Placing a sound barrier between the machine and the staff that radiate noise can reduce high-frequency noise. The higher the height of the barrier and the larger the area, the better the noise reduction effect, and it will be better if another layer of sound-absorbing material is covered on the barrier.
For noise control, mainly from the following aspects:
- The energy of sound in the process of propagation attenuates with the increase of distance, therefore, the purpose of noise reduction can be achieved by staying away from the noise source.
- The radiation of sound is generally directional, and the received sound intensity is also different at the same distance from the sound source but in different directions. Most sound sources are frequency radiated noise with poor directivity, and the directivity increases as the frequency increases. Therefore, controlling the propagation direction of noise (including changing the emission direction of the sound source) is an effective measure to reduce noise, especially high-frequency noise.
- Establish sound insulation barriers or use natural barriers (soil slopes, hills, buildings, etc.) and other sound insulation materials and sound insulation structures to block the spread of noise.
- Apply noise suppression sheet and sound-absorbing structures to convert the propagating noise into heat energy, etc.
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