Propagation characteristics of the hottest noise

2022-08-08
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The propagation characteristics of noise

the noise source is always reflected in a certain space, so it is necessary to study the propagation characteristics of sound in space, including attenuation, reflection, refraction, diffraction, interference and other phenomena in the process of sound wave propagation. The space where sound waves propagate is called sound field, which is divided into free sound field, diffusion sound field and semi free sound field. Free sound field is an idealized sound field. Strictly speaking, it does not exist in nature, but we can approximately regard the open field as a free sound field. In acoustic research, in order to overcome the reflected sound and prevent the interference of external environmental noise, we specially create an environment of free sound field, which can be used for hearing experiments, check the axial strain - the direction of force or the strain coaxial with the direction of force, test the noise of various machine products, measure the sound power of sound sources, calibrate some electroacoustic equipment, etc. The diffuse sound field is completely opposite to the free sound field. In the diffusion sound field, the sound wave is close to total reflection, and China is still in the stage of in-depth development of industrialization, informatization, urbanization, marketization and internationalization. For example, in the room, people hear not only the direct sound from the sound source, but also the reflected sound from all indoor surfaces. If the indoor surfaces are very smooth, the sound wave will be completely reflected back when it is transmitted to the wall. If the sound pressure is almost equal everywhere in the room, and the sound energy density is uniform everywhere, then such a sound field is called diffuse sound field (reverberation sound field). In acoustic research, a room with diffuse sound field characteristics, namely reverberation room, can be specially created. It can be used to test the sound power of sound source and speech intelligibility test under different reverberation time. In real life, the most common situation is not a complete free sound field, nor a complete reverberation sound field, but a semi free sound field between the two. According to the different sound absorption ability of the environment, some semi free sound fields are close to the free sound field, some are closer to the diffuse sound field

when the noise emitted by the sound source propagates in the medium, its sound pressure or sound intensity will gradually decay with the increase of propagation distance. High frequency sound waves decay faster than low-frequency sound waves. When the propagation distance is large, the attenuation value is large, so high-frequency sound waves cannot travel far. The strong noise from a long distance, such as the sound of aircraft and artillery, is relatively low, which is the reason why the high-frequency components decay faster in the process of long-distance transmission. In addition to the absorption of sound waves by air, some materials, such as glass, blankets, foamed plastics, etc., are also expected to absorb sound, which are called sound-absorbing materials. When sound waves pass through these porous sound-absorbing materials, due to the internal friction of the material itself and the friction between the air in the material hole and the hole wall, the sound energy is greatly absorbed. The molding concepts company in Stirling heights, Michigan is responsible for the mold and injection molding of water inlet (outlet) parts and attenuation. 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. Because the acoustic properties of these two media are different, some sound waves are reflected back from the surface of the obstacle, while the other part is transmitted into the obstacle. Noise reduction can be achieved by using different characteristic impedance of media. For example, when measuring noise outdoors, the hard ground, highway and building surface are all reflective surfaces. If the reflective surface is paved with sound-absorbing materials, the reflected sound energy will be reduced. Due to the reflection characteristics of sound waves, a certain noise generated indoors will be reflected many times from the wall, ground, ceiling and various indoor objects. The existence of this reflected sound makes the sound pressure level of the noise indoors 10 ~ 15dB higher than that at the same distance in the open air. In order to reduce the impact of indoor reflected sound, a layer of material with good sound absorption performance is covered on the inner surface of the room, which can greatly reduce the reflected sound and weaken the overall noise

when sound waves encounter the interface of different media during propagation, in addition to reflection, refraction will also occur, and the propagation direction will change when sound waves are refracted. In addition, sound waves can also produce diffraction. The diffraction phenomenon is related to the frequency and wavelength of sound waves and the size of obstacles. If the frequency of the sound wave is relatively low and the wavelength is long, and the size of the obstacle is much smaller than the wavelength, then 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 relatively short, and the obstacle is much larger than the wave, then the surrounding phenomenon is not obvious. Behind the obstacles, the sound waves arrive less, forming an obvious shadow area. Diffraction phenomenon has been applied in noise control. Sound barrier is 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 barrier height and the larger the area, the better the noise reduction effect. If the barrier is covered with a layer of sound-absorbing material, the better the effect will be. (end)

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