Sound absorbing materials are mainly divided into microporous and fibrous types. They are not fundamentally different. The sound absorbing principle is to leave an accessible passage for the sound (the passage of numerous tiny holes that are connected together, or from a myriad of fibers). The crosses are mixed together to form a small number of small gaps, but the sound will not come out once it enters. Because the passage is too messy and too long, the sound is drilled inside and drilled to the left and right, which is gradually consumed in the process. Energy plays a role in sound absorption. All the things with a long, delicate passage that can be used for sound absorption, such as cotton, various fiber gangs, cotton, sponges, and carpets; but things like ordinary foam, egg boxes, and wood boards have no sound absorption. . In addition, the surface of the needle (like a "planar" formed by bundling many needles into a bundle) is the best sound-absorbing material in the theory. The acoustic laboratory is designed in such a way that the sound is constantly after the sound reaches the "needle" surface. The ground reflects inward and never reflects outward. This surface can completely absorb sound (and light) like a black hole. However, this method is expensive, the studio can't afford it, and it doesn't need such a thorough sound absorption, so the recording studio uses the front material.
Need to remind everyone that the sounds of different frequencies are absorbed differently. High-frequency sounds have a short wavelength and are easily absorbed, while low-frequency sounds have long wavelengths that can easily penetrate obstacles. For low-frequency sound, it is not only difficult to sound, but also difficult to absorb. It doesn't crash like a high-frequency sound in a messy, small passage, but it's easy to go around. However, as long as you thicken the sound absorbing material to a certain extent, you can absorb the low frequency above 130Hz. However, in order to absorb lower frequencies, you have to use another method. For details, please see the section on low frequency and standing wave.
The figure below gives an example where the high frequency sound has a short wavelength and is easily absorbed. Low frequency sounds are difficult to absorb.
Sound absorbing materials have to be bought with money. How to use money reasonably is a topic. We don't need to have the entire room not have a single reflection (primary reflection), so we need to make the position of the sound engineer not have a single reflection (primary reflection). So we have to focus on the sounder's position. As shown in the figure, the sound engineer will be disturbed by the primary reflection (main reflection) of the walls on both sides, so we can solve the problem as long as the corresponding position on the wall.
Sound absorbing material Microporous sound absorbing materialThe main village has sound-absorbing sponges and sound-absorbing foam. Please note that this is not a flexible sponge and foam, but a special sound-absorbing sponge and sound-absorbing foam designed for sound absorption. The main feature is that it is filled with countless small passages and holes. Ordinary foam can never be used as sound absorption. use.
Fiber-type sound absorbing materialThere are mainly glass fiber cotton, rock wool, plant fiber cotton (including cotton wool, quilts, etc.).
Glass fiber cotton and rock wool are good sound absorbing materials, and they are cheap and non-flammable, so they are used in buildings for heat insulation and sound insulation. Glass fiber cotton and rock wool are harmful to the human body, and people will be itchy after contact, and this itching is one week. Their fibers are very small and can easily fall off and float in the air and penetrate into human skin. So be sure to wear protective clothing and a mask before touching it. Although there are new types of environmentally friendly fiberglass cotton, it is harmless to the human body, but there are still few in China. The picture below is a foreign environmentally friendly hard fiberglass board. This kind of glass fiber cotton processed into a hard plate is usually called "hard fiberglass board", and the sound absorbing effect is the same as that of soft glass fiber cotton, and it can maintain the shape and is more convenient to use. The cotton wool is made of cotton and is harmless to the human body. It can be directly hanged to absorb sound.
In fact, fiber cotton has a much stronger sound-absorbing ability than sound-absorbing sponges or sound-absorbing foams, but they have a weakness: they are not easily fixed into a shape, so they are not suitable for use as indoor surface materials. If they are to be used, at least on the surface. Then cover a layer of cloth, otherwise it is neither beautiful nor easy to float. Some felts, carpets, wall carpets, etc.; are essentially the same as these fiber cottons, but they are usually very thin, affecting the sound absorption. It is known that the effect of sound absorption is thickness, density, structure, etc., not the fiber itself. The thicker, the more fluffy, the longer the fiber, the more chaotic the material, the better the sound absorption.
Textiles To put it plainly is cloth. Cloth generally cannot absorb sound, except for thick flannel, but the cloth can be used to wrap fiber cotton and block some ugly sound absorbing materials, so the cloth here is actually an auxiliary material, which only has the function of covering and wrapping other materials.
Note that actually a layer of cloth on the sound-absorbing sponge or fiber cotton does not improve the sound absorption, and may even weaken the sound absorption effect of the medium and high frequency, especially the high frequency. If the cloth is dense, then the mid-high frequency sound is easily blocked and cannot be absorbed into the sound absorbing layer. This is the sound absorption effect curve after the sound absorbing foam is covered with a waterproof cloth. We can see that the absorption effect of high frequency sound is greatly reduced.
So the best is not to be clothed at all. However, if you need to be clothed for aesthetics, you must use a breathable cloth so that the sound can penetrate it and enter the sound absorbing layer. So usually we have to choose this kind of coarse cloth.
Of course, some thicker cloths can absorb sounds themselves, but because they are not thick enough, they can only absorb high-frequency sounds. Therefore, this method is not recommended and can only be used temporarily.
Finally, by the way, the egg box has almost no sound-absorbing effect. Foreign experiments have shown that the egg box only has a little sound-absorbing effect on sounds above 600 Hz.
Sound absorbing device and use examples (1) Use of sound absorbing sponge and sound absorbing foamSound-absorbing sponges and sound-absorbing foams, no matter how thick or thin, are good for high-frequency absorption, but for medium-low frequency absorption, they have a direct and significant relationship with thickness.
The high-frequency sound is very absorbed. Whether it is a thin or thick sound-absorbing sponge, it can absorb high-frequency well, but the low-frequency sound is difficult to absorb. It must be thick and sound-absorbing sponge to get it. The experiment gives the ability of sound-absorbing sponges of different thicknesses to absorb sounds of different frequencies. Only a relatively thick sound-absorbing sponge (more than 5 cm) can absorb the sound of the middle and high frequency bands.
Therefore, we should try to use thick materials. If we can't use all the thick ones, we should at least mix them thickly. If they are all thin, they can only absorb high-frequency sounds, while the low- and medium-frequency sounds will still reflect in the room to reflect. Going, the whole room feels a bit "squeaky".
However, please be aware that sound-absorbing sponges and sound-absorbing foams, no matter how thick, can not effectively absorb low-frequency sound below 200Hz, the sound absorption effect is almost zero. For the low frequency, there must be other methods, and there is a special description in "Low Frequency and Standing Wave".
Many ready-made sound-absorbing panels made of sound-absorbing sponges and sound-absorbing foams are sold in the foreign building materials market. The manufacturers have processed these sound-absorbing sponges and sound-absorbing foam into a certain shape. This is not for the sake of beauty, but it has a purpose, which makes it easier for the sound to "enter" into the sound-absorbing material and avoid being directly reflected back. In addition, some manufacturers apply a layer of glue on the back, which can be directly attached to the wall and ceiling after being bought back, and is easy to install.
Sound absorbing device and use examples (2) Use of fiber cottonGlass fiber cotton and rock wool are good sound absorbing materials, and they are cheap and non-flammable, so they are widely used in buildings to provide heat insulation and sound insulation. Glass fiber and rock wool are harmful to the human body, and people will be itchy after contact, and this itching is one week. Their fibers are very small and can easily fall off and float in the air and penetrate into human skin. So be sure to wear a protective mask before touching it. Although there are new types of environmentally friendly fiberglass cotton, it is harmless to the human body and the price is quite expensive, but it is still rare in China.
The following is a wide-screen sound absorbing body designed by Beijing Jingmeikang Acoustics Technology Co., Ltd., with high-density imported environmentally-friendly sound-absorbing cotton, which is 4 to 5 times denser than the general porous sound absorbing body, and the internal cavity structure, which brings lower suction. Lower acoustic limit and flatter sound absorption curve. The outer bread has a layer of sound-permeable cloth. The high-end finish not only balances the absorption of high frequencies, but also creates a very decorative look.
Plant fiber cotton is a sound-absorbing material that is completely harmless to the human body, but it is not fireproof and expensive. However, it is also quite cheap in China. It is also tens of dollars to buy a cotton wool in a small shop. The use of plant fiber cotton is the same as that of fiberglass wool and rock wool, but it is more casual to use and can be completely exposed to the air. In addition, since every household has ready-made cotton wool and mattresses and quilts, they are also the best temporary sound absorbing materials.
Using a quilt (cotton) to absorb sound, the effect is good and save money, it is ugly.
Felt, carpet, wall carpet, etc., also have a certain sound absorption. However, since they are usually relatively thin, the absorption energy absorbs high frequencies.
Carpets are used in a more versatile way, and the sound absorption on the ground can only rely on it. I believe everyone has this experience, the room with carpet on the floor will be quieter than the room without carpet. So for small sheds, carpets are almost necessary.
Sound absorbing device and use examples (3) Corner sound absorptionIn every corner such as a corner, it is usually easy to gather sound energy, so placing a large sound-absorbing device in the corner can do more with less. These devices are also sound absorbing sponges, but they have a huge volume and thickness. The following is a corner sound absorbing device designed by Beijing Jingmeikang Acoustics Technology Co., Ltd. The shape of the corner sound absorbing device is triangular prism, which can absorb the maximum amount in the smallest space. The triangular design can be easily placed in a corner or wall, filled with flame retardant inside, and passed the British BS5856 fire certification.
The low frequencies are concentrated at the corners of 90 degrees, the corners of the walls, floors, and ceilings. Installed in these locations, it is very effective in absorbing low frequencies. It is capable of absorbing frequencies as low as 125 Hz or more.
Such devices are capable of absorbing lower frequencies, so they are often referred to as bass traps. The large bass trap device absorbs all sounds above 125 Hz and is actually a full-band sound absorbing device. However, they are also powerless for low frequencies below 125 Hz. For our personal studios, the use of such corner devices is a wise choice.
Furniture with fiber cotton is also a good absorbent material, such as soft large pillows, cotton wraps, some cloth sofas, large plush toys, and so on.
appendix Why do you rarely see sound absorbing devices in large studios?I said at the beginning that many people misunderstood sound absorption and over-exaggerated the effect of sound absorption. In fact, the importance of sound absorption is much lower than the anti-standing wave and diffuse reflection. So, when you see a sound-absorbing device rarely seen in a professional studio, there is no need to be surprised.
In addition, the professional studio space is large, and it is also a reason that does not need to absorb sound. When sound is transmitted through the air, it loses energy. In the larger space, the sound needs to take a longer path to reflect back, and the reflection is less obvious. In a small space, the sound is easily reflected. In small studios and personal studios, reflections are much more harmful than large studios, so small sheds must be more absorbing than sheds, not copying sheds. For small studios, especially for personal studios in small-sized homes, my advice is to do a sound-absorbing process throughout the room, without leaving any smooth surfaces, including tables.
Professional large studios do not do complete sound absorption because they have the ability to prepare temporary sound-absorbing panels that are used when needed. Small studios and personal studios don't have this ability, so they must do a complete sound absorption process. As shown in the figure, the two professional studios below can flexibly place some active sound absorbing devices and sound absorbing panels during recording.
Sound absorption coefficient table for various materials and objectsThis table provides the absorption of sound from various frequencies for various materials.
The sound absorption coefficient is from 0 to 1, 0 means no sound absorption, the sound will be completely reflected back, 1 means complete sound absorption, the larger the sound absorption coefficient, the stronger the sound absorption capability.
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