Use a sound source or a place that needs to be quiet, and close it with a sound barrier such as solid wall panels and sealed doors and windows to isolate it from the surrounding environment to reduce noise transmission.
When sound waves propagate in the air, it is generally called a sound insulation by using various energy-absorbing materials to consume the energy of the sound waves so that the sound energy is blocked in the propagation path and cannot pass directly.
Separate the noise source from the receiver with components to isolate the propagation of air noise, thereby reducing the level of noise pollution. With a suitable sound insulation facility, the noise level can be reduced by 20 to 50 decibels. These facilities include partition walls, sound enclosures, sound insulation curtains and sound insulation barriers.
Sound insulation mechanism If a single layer of uniformly dense material member (ignoring the elasticity of the material) is considered to be soft, when it is excited by sound waves, the amplitude of the member is determined by the mass per unit area of ​​the member (called the areal density). Sound pressure and frequency of incident sound waves. The heavier the component, the higher the frequency, the smaller the amplitude of the transmitted wave, and the better the sound insulation effect of the component. It is the law of quality that clarifies this relationship.
The sound insulation (Ro) of the component when the sound wave is incident normally can be calculated by:
Ro=10 lg|pi/pt|2
=10 lg[1+(ωm/2Ïc)2] (dB)
Where pi is the incident sound pressure; pt is the transmitted sound pressure; m is the areal density; ω is the angular frequency (ω = 2Ï€f, f is the frequency); Ï is the air density; c is the speed of sound. This formula is the mass law of normal incident waves. The practical formula is:
Ro=20 lgm·f-42.5
In the case of random incidence, the incident waves in all directions are averaged to obtain the sound insulation amount (R) of the random incident wave. Its formula is:
R=Ro-10 lg (0.23Ro)
The R value is smaller than the Ro value, and the larger the Ro, the larger the difference.
The relationship between the sound insulation of a single wall and the surface density and frequency is shown in Fig. 1.
The above is the ideal situation of ignoring the elasticity of the material. In fact, the sound-insulating member is generally an elastic plate with a certain rigidity, which can reduce the sound insulation due to the anastomosis. Therefore, the sound insulation characteristic curve of the single-layer uniform dense material board should be as shown in Fig. 2. Below the resonance zone in the figure, the sound insulation of the panel is controlled by the elastic stiffness. The valley at the critical frequency generated above the quality control zone is caused by the anastomosis effect.
The phenomenon that the anastomosis effect is caused when the acoustic wave velocity projected on the surface of the member plate coincides with the bending speed of the plate. As shown in Fig. 3, it is assumed that the oblique incident acoustic wave a reaches the point A on the plate at a certain time, and the plate vibrates. After the time t, the bending wave reaches the point B, the wavelength is λB, and the propagation speed is cB. At this time, if the angle θ of the oblique incidence of the acoustic wave is appropriate, the air wave b reaches the point B at the sound velocity c for the same period of time t, that is, λB = λ / sin θ, then the plate is excited at point B to generate a new bending wave. It coincides with the bending wave from point A, thus maximizing the total bending wave amplitude. At this time, the plate will radiate a large amount of sound energy to the other side, and the amount of sound insulation at this frequency will be greatly reduced, and will no longer conform to the "quality law", which is called the "same effect". The anastomosis effect only occurs at the critical frequency fc. The fc is related to the thickness of the plate, the density of the material, and the modulus of elasticity. The frequency range of the influence of noise on people is mainly 100~3150 Hz, and the coincidence effect of this range should be avoided as much as possible. Typically, a hard and thick plate can be used to lower the critical frequency, or a soft, thin plate can be used to increase the critical frequency (Figure 4).
The sound insulation member with complex sound insulation structure is composed of some single-layer members, which has the characteristics of single-layer members in the sound insulation mechanism, and at the same time has the comprehensive characteristics of various single-layer members.
1 Double-layer component: A member with two air layers between two unconnected single-layer members. The air layer acts as a buffering elastic, but can also cause resonance of the two layers of components. Therefore, the amount of sound insulation of the two-layer member is not a superposition of the sound insulation of the two-layer member. If a porous sound absorbing material is added to the air layer, the resonance can be reduced and the sound insulation of the member can be increased. The amount of sound insulation increased by the air layer is proportional to the thickness of the air layer within a certain range. Generally, a double wall can increase the sound insulation by about 5 decibels than a single wall of the same weight.
2 Light-weight wall: The light wallboard currently used includes paper gypsum board, round hole pearl rock board and aerated concrete board, and the mass per unit area is about ten kilograms to several tens of kilograms. The 240 mm thick brick wall is 530 kg per square meter. According to the law of quality, light wall panels cannot meet the sound insulation requirements. Therefore, it is necessary to isolate the double-layered sheets to form an air layer, or to fill the air layer with sound-absorbing materials, or to use different thickness or stiffness of the sheets to have different matching frequencies to improve the sound insulation of the light walls. Table shows the sound insulation improvement values ​​of different frequency bands of paper gypsum board with different layers in the presence or absence of filler material.
3 Sound insulation doors and windows: The door and window structure is light in weight and has a gap, so the sound insulation ability is not as good as the wall. For doors with high sound insulation requirements (30 to 50 decibels), a reinforced concrete door leaf with a simple structure can be used. But usually a door leaf with a composite structure is used. The impedance change of this structure can improve the sound insulation ability. Sealing gaps are also an important measure to ensure the sound insulation of doors and windows. It is better to use industrial felt as the sealing material than the latex strip, especially for high frequency noise. For windows with high sound insulation requirements, the window glass should have sufficient thickness (6-10 mm) and at least two layers. The two layers of glass should not be parallel to avoid resonance and reduce the sound insulation effect. The gap between the glass and the window frame, the window frame and the wall should be sealed. At the periphery between the two glazing windows, a strong sound absorbing material should be arranged to increase the sound insulation. It is easy to scrub in construction. Fig. 5 is a graph showing the sound insulation characteristics of various soundproof windows. In order to avoid anastomosis between the window panes, the double glazing of the sound insulating window should have different thicknesses, otherwise the sound insulation value will appear low at the critical frequency fc.
4 Acoustic lock: To make the door have high sound insulation ability, you can set “sound lockâ€, that is, arrange strong sound absorbing material in the space between the two doors (door bucket). The sound insulation capability of this measure is sometimes equivalent to the sound insulation of the two doors. In order to facilitate opening and closing, the weight of the door leaf should not be too large.
5 Combination wall: A composite wall is a wall with doors or windows. Its sound insulation is usually lower than the wall without windows. Therefore, it is not possible to simply improve the sound insulation of the wall. In design, it should be carried out according to the design principle of “equal sound insulationâ€, ie Ï„w·Sw=Ï„d·Sd. Where Ï„w and Ï„d are the transmission coefficients of the flat wall and the door, respectively, and Sw and Sd are the area of ​​the wall and the door. therefore
That is, Rw=10 lg(Sw/Sd)×(1/τd)=Rd+10 lg(Sw/Sd) decibels. It can be seen from the above formula that the sound insulation of the wall can be about 10 decibels higher than the door. For convenience, it can be calculated as shown in Fig. 6.
The use of a sound absorbing material inside the construction of the above various sound insulating members is to increase the sound insulation amount of the member by utilizing the characteristics of sound absorption. The essential difference between sound insulation and sound absorption should not be confused. Sound insulation is the propagation of isolated noise. As far as possible, the incident sound waves are reflected back. The sound insulation material is heavier and denser, and the sound insulation performance is better. The sound absorption is to absorb the incident sound waves as much as possible, so that the sound waves penetrate into the material and the sound energy It is consumed and is therefore generally a porous, bulk material.
Sound Insulation Index In recent years, the International Organization for Standardization (ISO) has recommended the use of a single value, the sound insulation index Ia, to evaluate the sound insulation effect of airborne sound. The standard curve in Fig. 7 increases by 9 decibels per octave band between 100 and 400 Hz, and increases by 3 decibels per octave band between 400 and 1250 Hz, and is flat between 1250 and 3150 Hz.
When searching for the sound insulation index, first draw the sound insulation characteristic curve of the component on the coordinate paper, then match the standard curve drawn on the transparent paper with it, and move it up and down in the vertical direction until the following two conditions are met. :1 The difference between the sound insulation of any 1/3 octave band below the standard curve and the standard curve shall not exceed 8 decibels; 2 the sum of the difference between the sound insulation and the standard curve of each 1/3 octave band below the standard curve shall not be More than 32 decibels. The sound insulation amount Ia corresponding to the center frequency of the 1/3 octave band shown in the figure is 500 Hz, which is the reading of the sound insulation index.
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