Sound absorbing material

Traditional sound absorbing materials are porous materials, or sound resistance materials. The essence of sound wave is a vibration, specifically in the case of a speaker system, air vibration. When air vibrations are transferred to the sound-absorbing material, they are gradually mitigated by tiny pores and converted into heat.

Generally speaking, the thicker the sound-absorbing material, the more such holes in the direction of sound propagation, the better the absorption effect of random incident or small Angle incident sound. We can usually buy the sound absorption material in the market mainly 30mm thickness and 60mm thickness two kinds. My advice is, don't buy 30mm if you can buy 60mm.

On the other hand, acoustic resistance materials have low frequency cutoff frequencies that are effectively absorbed, which is usually related to the thickness of the acoustic material and requires about 1/4 of the wavelength. So it is almost unlikely that acoustic-resistant materials will be used to absorb low frequencies, for which other types of processing materials are needed. At the same time, the absorption coefficient of sound absorbing material is also related to the incidence Angle of sound. High-density fiberboard, for example, is reflective of sound at large angles.

Diffused material

First of all, we need to understand what sound diffusion is or what the purpose of the diffusion material is.

When sound strikes a wall, some of it will continue to travel in a geometric direction, but usually this process is not absolutely "specular reflection". If it is an ideal absolute reflection, the sound passing through the surface should be emitted in exactly the geometric direction, and the energy in the exit direction is the same as that in the incident direction. There is no loss of energy in the whole process, which can be understood as no diffusion at all, or more commonly understood as specular reflection in optics. The ideal diffusion, however, is that there is no major exit direction, and the sound uniformly exits in all directions in space after the incident surface, which can be popularly understood as diffuse reflection in optics.

Of course, it all has to do with things like the wavelength of the sound. The actual listening environment not only needs a certain diffusion, but also should try to avoid strong reflection such as the whole floor glass. But that means reflections are more complex and computationally demanding.

There are times when you need a certain amount of reflection, but you don't need a particularly strong reflection to put the diffusion material in place. For example, sometimes if the room is full of sound absorbing material, it will absorb too much sound. Diffusion material can reduce the early reflection sound, while ensuring the reflection sound in the later stage, so that the small room also has a certain reverberation effect. Generally speaking, for stereo speaker systems, it is recommended to have some diffusion material in the room, and for home theater systems, you can appropriately reduce the diffusion material or sometimes you can use full sound absorption design.

Diffusion material on the improvement of sound reflection, this is compared to those meaningless fever wire and blind replacement of power amplifier, shock absorbers, equipment frame and so on obviously completely different.

As we mentioned earlier, the lowest effective frequency or the lowest diffusion frequency of the diffused material is related to the size of the bulge of the diffuser. Generally speaking, the more bumps the diffuser has, the lower the frequency that can be effectively diffused. It is not difficult to see from the figure below that a single 30cm cylindrical diffuser already has good diffusion characteristics at very low frequencies, but multiple diffusers placed in parallel will lead to loss of diffusion performance.

There are also some examples of these diffusers being made to look like Roman Columns, but also aesthetically designed. And in fact, since many current audiophiles' rooms are small, such diffusers may not be needed. The effective frequency of the general quadratic remainder diffuser plate is about 700~1000Hz. Sometimes the choice of frequency should be discretionary. At the same time, our common HiFi speaker systems and home theater and other small room listening environment also don't need to use a raised diffuser. Because the diffusion of sound is usually explained in terms of geometric acoustics, once the frequency falls below the excess frequency of the room, it is in the category of wave acoustics. Or the diffusion material used in small rooms usually doesn't have to be as thick as in opera houses and concert halls.

Low frequency trap

As mentioned above, it is almost impossible for acoustic resistance absorbing materials to effectively absorb low frequencies. For low frequency absorption, materials that rely on other principles are often needed.

Helmholtz resonator is a very common acoustic structure, often used in automobile and motorcycle exhaust pipe noise reduction applications. The principle is to make a small opening in a cavity V, connected to a short tube, to form the simplest device, the Helmholtz resonator. If the air column S in the short tube is disturbed and moves into the cavity, the gas in the cavity is compressed and the pressure is increased. The inward motion of the air column S is blocked, and it moves outward instead. After passing the equilibrium position, it continues to move outward due to the inertia effect. The pressure in the chamber decreases, causing the air column S to stop moving outward and move inward again, and so on.

When the frequency of sound is the same as the resonant frequency, a strong mechanical wave is produced. When choosing low frequency trap, it is necessary to pay attention to the corresponding resonance frequency. Generally, merchants will clearly mark it, and it can also be calculated by the above principles.

Membrane type mechanical resonance sound absorption body is another kind of low-frequency sound absorption material, usually achieved by architectural design such as multi-layer wall design, for the vast majority of ordinary players is of little significance, unless the Tuhao specially built a house to fever.

There is usually extra gain in certain frequency bands due to factors such as room standing waves, resulting in a low-frequency "rumble" resonance. Low frequency traps can eliminate peak interference from these room patterns to some extent. Of course, can also use multiple subwoofer or DSP digital signal processing and other methods.

Sound insulation material

As has been explained in detail above, the sound insulation properties of materials and sound absorption properties are different. Sound absorbing materials often make use of small holes in the material. However, this pore structure usually causes sound waves to propagate through the air. To prevent further transmission of sound from the material, it is necessary to reduce the cavity structure as much as possible and improve the density of the material.

Usually the sound insulation performance of sound insulation materials is related to the density of the material. The purchase of high density sound insulation materials can further improve the sound insulation performance of the room. However, a single layer of sound insulation is still limited in some cases, where a double layer of sound insulation can be applied and additional damping material can be added to the two layers of sound insulation. However, it is important to note that as far as possible to avoid two layers of sound insulation material to take the same thickness, in order to avoid repeated anastomotic frequency. If in the actual construction and decoration, should first do the whole house sound insulation treatment, sound absorption and diffusion treatment.

Acoustic processing is necessary for a speaker system. However, acoustic processing is not a random application like "jumping god", understanding the relevant basic knowledge is an important prerequisite for correct acoustic processing. Acoustic treatment design is also closely related to the room, the speaker system, there is no absolute fixed solution or answer, ultimately should be based on the actual situation.