Audio-visual room decoration sound absorption, sound insulation materials and structure

Interior decoration has become an independent industry, and large and small decorative decoration companies have sprung up everywhere. Many decoration companies have brought new look to interior architectural decoration with new styles, new materials and new techniques, reaching a new level.

In many cases, interior decoration has certain acoustic requirements. Not only the various theaters, stadiums and dance halls, but also the studios and studios related to acoustics have certain acoustic technical indicators. In public places, it is generally necessary to spread language or music, even for household use. The room now also needs a good music appreciation environment. Therefore, interior decoration projects must pay attention to acoustic requirements. If you ignore this, it is very likely to cause adverse consequences. For example, there is a water fitness entertainment venue. The ground is basically a water surface. The sky is a large glass round item. Because there is no acoustic design, the reverberation time in the hall is particularly long. When there is an entertainment performance, even the report is unclear. If there are some walks or halls, it will be magnificent and magnificent, but even ordinary conversation or background music will continue to pass through in the space, creating annoying interference noise.

The main cause of poor sound quality is the lack of scientific acoustic design. Many decorative engineering companies themselves do not have qualified acoustic designers; some initially invited acoustic experts to do design, and later thought they had "experience", they boldly contracted the design; some were copied from the east and thought they found it. The mystery of others, you make soft bags, I also do soft bags, you use perforated plates, I also do perforated plates, actually do not have real acoustic requirements; do not rule out that some engineering and technical personnel know some acoustic knowledge, but Not inferior to the principles and practices of room acoustics, made an unqualified acoustic decoration design.

The interior acoustic design is a systematic subject, which involves a wide range of topics. This article only briefly introduces the materials and structure knowledge of sound absorption and sound insulation related to interior decoration. It is hoped that the decoration engineering personnel and the owners have the acoustic materials and structures. It is understood that it is possible to understand why the acoustic design is processed in such a way that the decorative engineering achieves perfect integration between aesthetics and acoustic requirements.

Audio-visual room decoration sound absorption, sound insulation materials and structure

1. Basic concepts of sound absorption and sound insulation

First of all, it is necessary to clarify that the sound absorption and sound insulation are completely different from the two acoustic concepts. Sound absorption means that when sound waves propagate to a certain boundary surface, part of the sound energy is reflected (or scattered) by the boundary surface, and part of the sound energy is absorbed by the boundary surface (this is not considered to be absorbed by the medium when propagating in the medium), which includes sound waves. The transformation into thermal energy in the boundary material is consumed or converted into vibration energy that can be transferred along the boundary structure or directly transmitted to the other side of the boundary. For incident sound waves, except for the reflected (scattered) sound energy reflected into the original space, the rest of the energy is considered to be absorbed by the boundary surface. The ratio of the absorbed acoustic energy to the incident acoustic energy over a certain area is called the sound absorption coefficient of the boundary surface. For example, when indoor sound waves are transmitted from an open window to an outdoor room, the window opening area can be approximately considered to "absorb" the sound waves transmitted from the room by 100%, and the sound absorption coefficient is 1. Of course, the sound absorbing materials that we have to consider are mainly based on the sound absorption of the open area, but rely on the acoustic properties of the material itself to absorb the sound waves.

For the interface interlayer between the two spaces, when the sound wave is incident from the chamber to the interface, the sound wave excites the vibration of the spacer, and the vibration radiates the sound wave to the other space, which is the transmitted sound wave. The ratio of the transmitted acoustic wave energy to the incident acoustic wave energy through a certain area is called the transmission coefficient. For open windows, the transmission coefficient can be approximated to 1 (the sound absorption coefficient is also 1), and the sound insulation effect is 0, that is, the sound insulation amount is 0 db. For heavy and thick brick walls or thick steel plates, the mass per unit area is large, and the sound waves can only excite the small vibration of the interlayer when incident, so that the sound energy (transmission sound energy) radiated to another space is small, so The sound volume is large and the sound insulation effect is good. But for the original space, most of the energy is reflected, so the sound absorption coefficient is small.

For a single material (not a specially designed composite), sound absorption and sound insulation are often not compatible. For example, the above brick wall or steel plate can be used as a good sound insulation material, but the sound absorption effect is extremely poor; conversely, if a sound absorbing material (such as glass wool) is used as the sound insulation material, even if the sound wave passes through the material, the sound is sounded. Can be absorbed 99% (this is difficult to achieve), only 1% of the sound energy is transmitted to another space, then the material's sound insulation is only 20db, not a good sound insulation material. It is wrong to misunderstand sound-absorbing materials as “sound insulation materials”. If someone introduces a single material to absorb sound and sound sounds, then he does not understand or is deceiving.

2. Sound absorbing materials

Sound absorbing materials refer to building decoration materials with relatively high sound absorption coefficient. If there are many inter-connected fine spaces inside the material, the air passage formed by the voids can be modeled as a duct structure composed of a plurality of thin tubes or capillaries formed between the solid frames. When the sound wave is introduced, the acoustic vibration velocity in the thin tube near the tube wall and the tube is different, and the internal friction caused by the velocity difference between the media causes the acoustic vibration energy to be converted into heat energy and absorbed. Good sound absorbing materials are mostly fibrous materials, called porous sound absorbing materials, such as glass wool, rock wool, mineral wool, polyester fiber sound absorbing cotton (environmental protection), cotton and rayon cotton, special metal fiber cotton. Etc., also includes foams with voids connected. Sound absorption properties are related to the fiber void structure of the material, such as the thickness of the fiber (between micrometers and tens of micrometers) and the density of the material (determining the equivalent diameter of the "capillary" between the fibers), the volume of air in the material and the volume of the material. The ratio (called void ratio, the porosity of the glass wool is 90% or more), the shape and structure of the voids in the material, and the like. From the point of view of use, regardless of the mechanism of sound absorption, it is sufficient to consult the experimental results of the sound absorption coefficient of the material. Of course, we must pay attention to other requirements such as moisture, fire and decorative.

The porous sound absorbing material has a basic sound absorbing characteristic, that is, a low frequency sound absorption difference, and a high frequency sound absorption sound. The qualitative sound absorption frequency characteristics are shown in Figure 1. When the frequency is high to a certain value, as shown in Figure 1, f0, the sound absorption coefficient α reaches the maximum value. When the frequency continues to increase, the sound absorption coefficient fluctuates at the high end. The position of this f0 is substantially the wavelength corresponding to f0 which is 4 times the thickness t of the material.

When the thickness of the material is increased, the sound absorption characteristics of the low frequency can be improved. In Figure 1, t2 is greater than t1, and the sound absorption coefficient of t2 is greater than the sound absorption coefficient of t1 at the same frequency. If t2=2t1, the frequency corresponding to the same sound absorption coefficient is about f2=f1, that is, the thickness is doubled, and the frequency characteristic of the low frequency sound absorption coefficient is shifted by one octave to the low frequency. However, it is not always possible to increase the thickness to increase the low frequency sound absorption coefficient because the sound wave is damped when propagating in the material gap, so that increasing the thickness to improve the low frequency sound absorption is limited. Different materials have different effective thicknesses. A good sound absorbing material such as glass wool generally has a thickness of about 5 cm, and rarely uses more than 10 cm. For a kind of material such as fiberboard, the gap between the fibers of the material is very small, the damping of sound wave propagation is very large, and the sound absorption coefficient is small, and the effective thickness is also very small.

Generally, the low-frequency sound absorption performance of flat sound-absorbing materials is a common law. An improved method is to cut the entire sound absorbing material into a pointed shape, as shown in Fig. 2. When the sound wave propagates to the pointed material, the ratio of air to material gradually changes from the tip to the base, that is, the sound The impedance gradually changes, and the sound wave propagates beyond the effective thickness of the flat material to reach the base of the material, thereby improving the low frequency sound absorption performance. The sound absorption frequency characteristic is still similar to that of Fig. 1, and the frequency f0 of the maximum sound absorption coefficient corresponds to a wavelength about four times the length t of the sharp sound absorption structure. For example, to make the frequency above 100hz have a high sound absorption coefficient, the length of the sound absorbing tip is about 87cm. Of course, such sound absorbing structures are generally not suitable for interior decoration, and are mainly used in acoustic laboratories or special noise control projects.

3. Resonance sound absorption structure

Different types of resonant sound absorption structures are designed by using different resonance sound absorption mechanisms, so that the absorption peak is selected at the desired frequency position to meet the requirements of different frequency sound absorption, especially to solve the problem of insufficient low frequency sound absorption.

3.1 Resonance sound absorption of thin layer porous sound absorbing materials

Thin layer porous sound absorbing materials also include various breathable fabrics such as cotton, hemp, silk, velvet, rayon and the like. As shown in Figure 3a, the material is hung in front of the rigid distance d, then when d = 1/4 (2n + 1) λ (1), λ is the wavelength of the acoustic wave in the air, n is a positive integer, and the fabric is in front of the rigid standing wave. The sound pressure node position, where the sound wave of the sound wave has the highest vibration speed, so that the maximum sound energy is consumed in the fabric to form a resonance sound absorption. In the equation (1) where n is equal to 0, 1, 2, ..., the corresponding resonant sound absorption frequency fn is: fn = (2n + 1) / 4. co / d (2) where co is the acoustic wave in the air The speed of propagation is generally calculated at 340m/s. For example, when the distance between the fabric and the rigid wall is 34 cm, the lowest resonance frequency f0=250hz corresponding to n=0, f1=750hz corresponding to n=1, and f2=1250hz corresponding to n=2. The frequency characteristics of its resonant sound absorption are shown in Figure 3b. The sound absorption peak is related to the fabric performance, generally larger, but the width of the resonance sound absorption peak is not large. In actual use, the curtain is often increased and wrinkled, that is, the distance between the fabric and the rigid surface is continuously changed, and at different distances. Hang more than one layer of fabric to improve the sound absorption frequency characteristics. In addition, a material such as glass wool having a thickness d is installed from the rigid surface d, as shown in Fig. 4, then d→ in the formula (1) becomes d→(d+t) continuously, that is, there are many resonances. The acoustic frequency, and the lowest resonant frequency is f0 = c0 / 4 (d + t).

3.2 Thin film resonance sound absorption structure

If there is a gas-impermeable membrane at d in front of the rigid, see Figure 5, the mass per unit area of ​​the membrane is m, then the membrane and the air layer of thickness d constitute the mass - the resonance system of the spring, the resonance frequency is: fr = co /2 π √ ρ o/md ( 3)

Where ρo is the air density. For example, if the outer surface of the "soft bag" is covered with a gas-impermeable film, the porous sound absorbing material wrapped inside can not play the original sound absorbing function, and the first is the resonance sound absorption of the film and the material is transmitted through the film vibration. The sound absorption inside, and the vibration of the film is restrained by the damping of the material, and the sound absorption performance is limited. If the skin is made of a higher quality material such as artificial leather, such as a seat in a theater, the sound absorption performance is even worse.

3.3 Thin plate resonance sound absorption structure

The thin plate is a two-dimensional vibration system whose resonance frequency is related to the physical constant and geometrical dimensions of the plate and its edge fixation. If a rectangular plate with side lengths of la and lb has a thickness of h, the four sides are firmly clamped, and its resonant frequency fm,n is: fm,n=π/2[eh2/12ρ(1-σ2)] 1/2.[m2/1n2+n2/1b2]1/2 (4)

Where e, ρ and σ are the Young's modulus, density and Poisson's ratio of the plate, respectively, and m and n are positive integers. When n=0, m=1, the lowest resonance frequency is obtained (set la>lb). If the plate is glass, substitute the physical constant of the glass: fm, n = 2.5 × 10h3 (m2 / 1n2 + n2 / 1b2) 1/2 (5)

The length in the formula is in meters. For example, a glass window with a length of 50 cm, a width of 40 cm, and a thickness of 4 mm is fixed on four sides. The lowest resonance frequency of (m, n) is (1, 0) is 20 hz, and the resonance frequency of (m, n) is (0, 1) is 25hz, (m, n) is (1, 1) with a resonance frequency of 32hz. As (m,n) gradually increases, the resonant frequency becomes larger and larger (interval is also getting denser), and there is greater sound absorption and acoustic transmission at these frequencies.

Plates are often used in interior decoration, they all have a certain resonance sound absorption effect, and their resonance frequency is basically as shown in equation (4), which is related to the geometrical size and physical constant of the plate, and is related to the edge fixation condition, for example. How much nails are nailed, how tight they are nailed, whether they are glued, etc. Therefore, such resonance sound absorption is often not actively adopted in the design scheme, and only experienced designers are used with caution. However, it is very important that when the surface is decorated with a thin plate, in order to avoid the excessive resonance frequency, attention should be paid to designing and constructing different spacing dimensions between the wooden ribs of the fixed thin plate to disperse the resonance frequency. . For acoustic spaces where thin plate resonance sound absorption is not desired, the surface treatment uses a thick plate.

3.4 Perforated plate resonance sound absorption structure

The perforated plate resonant sound absorbing structure is often used to compensate for the amount of sound required for low frequencies. The sound absorbing structure of the perforated plate is shown in Figure 6a. The thickness of the plate is t, and the distance from the rigid surface is d. For example, the hole is drilled in the plate (the slit can also be opened), the radius of the hole is a, and the ratio of the perforated area to the area of ​​the plate (perforation The rate is p, then the resonant frequency fr of the perforated resonant structure is fr=co/2 π √ p/(t+16a)d (6)

The equation indicates that the resonant frequency has several parameters that can be adjusted, such as the plate thickness t, the radius a of the hole, the perforation rate p, and the distance d between the plate and the rigid surface. There are now perforated plates with different perforation rates on the market. Different perforation rates can be chosen and the distance d between the plates and the rigid faces can be chosen to obtain the desired resonant frequency.

It should be noted that the shape of the resonant sound absorption peak of the perforated plate is related to the damping of the resonant structural system. As shown in Fig. 6b, when the damping is small, the formant is sharper, and when the damping is large, the formant is relatively flat. Generally, a relatively gentle sound absorption characteristic can be selected to avoid excessive sound absorption frequency selectivity. The thickness of the plate is small, the diameter is small, and the damping is large. The diameter of the perforation of the micro-perforated plate is about 0-8 to 1 mm, so the damping is large and the absorption peak is relatively gentle. However, it is not suitable for use because it is easy to accumulate ash and is not resistant to corrosion.

Generally, the thickness of the perforated plate is not more than 5 mm, and the diameter of the perforation is about 6 to 10 mm. In this case, the damping is too small. To increase the damping of the resonant structure, it is necessary to add a sound absorbing material near the perforations. Referring to Fig. 6c, when the sound wave propagates through the perforation, the "sound line" is denser in the hole and near the hole like the stream line, where the "flow velocity" is large, that is, the sound velocity of the sound wave is large, and the sound absorbing material generates the maximum damping. effect. It is difficult to fill the sound absorbing material into one hole, so it is often applied to the front or the back of the board with a layer of sound absorbing material (the highest efficiency when the thickness is one hole diameter) to increase the damping of the resonant sound absorbing system and make the absorption peak More gradual. When the sound absorbing material is behind the perforated plate, it only plays the damping role of the resonance sound absorption; if it is placed in front of the perforated plate, it also has the sound absorbing function of the porous sound absorbing material. When the perforation rate p is greater than 0?2, it is generally not a resonant sound absorbing structure, and is merely used as a "guard panel" for porous sound absorbing materials.

4. Sound insulation material

The airtight solid material has sound insulation effect on the sound waves propagating in the air. The most fundamental point of the sound insulation effect depends on the mass per unit area of ​​the material.

Referring to Fig. 7, a very large spacer has a mass per unit area of ​​ms. When the acoustic wave is incident perpendicularly from the left side, the excitation layer is excited as a whole, and the vibration radiates sound waves to the right space. Considering the unit area, the ratio of the acoustic energy transmitted to the right space to the acoustic energy incident on the spacer is called the transmission coefficient τ. Define the transfer loss (also called transmission loss) of an infinite barrier material tl:tl=101g1/ г (7)

In the above simple case, the calculated transfer loss is approximately: tl=20lg ω ms/2 ρ oco (db) (8)

Where ω=2πf is the circular frequency, and ρ0 and c0 are the density of air and the speed of sound wave propagation. The size of tl indicates the sound insulation capability of the material. An important feature of (8) is that the mass per unit area is doubled, and the transmission loss is increased by 6 db. The basic law of this sound insulation is called the "quality law", which means that the sound is separated by weight. Therefore, materials such as brick walls, cement walls or thick steel plates, lead plates, etc., which have a large mass per unit area, have better sound insulation effects.

The formula (8) also shows that the high-frequency sound insulation of the single-layer sound insulation is good and the low frequency is poor. For every doubling of the frequency, the transmission loss is increased by 6db.

It should be noted that the transmission loss tl is the theoretical "sound volume" when the area of ​​the partition is infinite, and as a wall or floor, it has edges connected with other building members, and the "sound volume" and (7) The transfer loss represented by the formula is different. Both the sound insulation ability is increased because the edge is close to being fixed, and the vibration of the plate fixed as the edge has a certain resonance frequency, so that the sound insulation effect at some resonance frequency points is lowered. And as a partition wall or floor between two adjacent rooms, because there are multiple sound transmission (or vibration) channels between the two rooms, the sound insulation between the two rooms (only the sound level difference) can not be The transmission loss of this compartment is represented.

The spacer material has a certain physical elasticity, and when the sound wave is incident, the vibration is excited to propagate in the interlayer. When the sound wave is not perpendicular to the incident, but is incident at an angle θ with the interlayer, the acoustic wavefront reaches the surface of the interlayer in turn, and the curved vibration wave in the acoustic excitation barrier of the first layer propagates laterally along the interlayer, if the bending wave When the speed of propagation coincides with the speed at which the sound waves in the air gradually reach the surface of the partition, the sound waves enhance the vibration of the bending waves. This phenomenon is called an anastomosis effect. At this time, the vibration of the bending wave is particularly large, and the energy of radiating the sound wave to the other side of the air is also particularly large, thereby reducing the sound insulation effect. The frequency fc at which the anastomosis effect is generated is: fc = co2/2 π sin2 θ [12 ρ (1- σ 2) / eh2] 1/2 (9)

Where ρ , σ , and e are the density, Poisson's ratio, and Young's modulus of the barrier material, respectively, and h is the thickness of the barrier. The arbitrary matching frequency fc is related to the incident angle θ of the acoustic wave. In most rooms, the sound field is close to the reverberant sound field, and the angle of incidence to the barrier is from 0° to 90°, so the coincidence frequency appears at a frequency from fc0 of grazing incidence (θ=90°). The range, that is to say the anastomosis effect, makes the sound insulation effect in a certain frequency range worse. Generally this frequency range occurs at medium to high frequencies. It is known from the law of mass that the medium and high frequency sound insulation is large, and the phenomenon of lowering the medium and high frequency sound insulation due to the anastomotic effect does not cause much trouble, except for the metal plate with little internal damping.

5. Double layer sound insulation structure

According to the law of mass, the frequency is reduced by half, and the transmission loss is reduced by 6 db. When the sound insulation effect is improved, the mass is doubled and the transmission loss is increased by 6 db. Under the control of this law, if the sound insulation ability is to be significantly improved, it is obviously impossible to increase the quality of the partition, such as increasing the thickness of the wall, sometimes it is impossible, such as the sound insulation structure on the aircraft. . At this time, the solution is mainly to use a double layer or even a multilayer sound insulation structure.

The model of the double-layer sound insulation structure is shown in Fig. 8. The mass per unit area is m1 and m2, and the thickness of the middle air layer is l. The transmission loss of the two-layer structure can be theoretically calculated, and the results are more complicated. Different simplified representations can be obtained in different frequency ranges, which are only qualitatively introduced here.

The two compartments form a resonance system with the intermediate air layer. The resonance frequency is fr (m is the unit of kg/m2, and the unit of l is m): fr=60/√m1m2l/(m1+m2) (10)

Near this resonance frequency, the sound insulation effect is greatly reduced. However, for heavy walls, this frequency is already below the audible frequency range. For example, m1 is a half brick wall 250kg/m2, m2 is a brick wall 500kg/m2, and the air layer thickness is 0?5m. At this time, the resonance frequency is about 7hz.

For light-structure double-layer sound insulation, the resonance frequency may fall within the audible frequency range. For example, two layers of aluminum plates are 5?2kg/m2 and 2?6kg/m2 respectively, and the middle air layer is 5cm. The resonance frequency can be calculated to be about 200hz. . At this time, the damping material should be packed between the two plates to suppress the vibration of the plate. Generally, if a thin steel plate is used as a double-layer sound insulation structure, a damping layer is coated on the steel plate to suppress the vibration of the steel plate.

Below the resonance frequency fr, the effect of double-layer sound insulation is like the sound insulation effect of one layer (m1+m2) without air layer; in the frequency range above fr, the double-layer sound insulation effect is close to two single-layer sound insulation. The sum of the transmission losses; at a higher frequency, when the thickness l of the air layer is an odd multiple of a quarter wavelength, the double layer sound insulation effect is equivalent to the sum of the transmission losses of the two single layers plus 6 db, l is the wavelength When the even multiple is used, the double-layer sound insulation effect is equivalent to the transfer loss of the two single layers combined and increased by 6db. At other frequencies, the sound transmission loss is between these two values. Therefore, in general, when the frequency is greater than fr, the double-layer sound insulation structure significantly improves the sound insulation performance.

Generally, two layers of double-layer sound insulation structure do not use the same material of the same thickness to avoid the same frequency of matching between the two layers.

Special care must be taken during design and construction to ensure that there is no rigid connection between the two layers. Destroyed the solid-air-solid two-layer structure, the two layers of solid barriers are connected by rigid members, so that the vibration of the two compartments are connected together, and the sound insulation is greatly reduced. Especially for double-layer light structure sound insulation, when they must support or connect each other, they must be supported or suspended by elastic members, and at the same time, note that there is no seam or hole communication between the two spaces that need to be divided. The “leakage” will leak sound, which is the actual problem of sound insulation. One-stop navigation for home theater entry: http://

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