A Detailed Introduction to Acoustic Treatment Materials

Whether it is a HiFi speaker or a home theater, or a recording studio or cinema, the speaker system will interact with the room. National standards related to construction usually stipulate the sound insulation performance indicators of houses, but they usually do not involve the treatment of reflected sound. Usually if a room does not receive any acoustic treatment, no matter how good the speakers are, they will encounter bottlenecks due to room interference. At this time, acoustic treatment materials can be said to be a necessity.

Acoustic treatment materials can be roughly divided into sound-absorbing materials, diffusion materials and sound-insulating materials according to their functions. Among them, in addition to conventional sound-absorbing panels, the other type of sound-absorbing material is a low-frequency trap that is usually used to specifically absorb low frequencies. However, here I would like to first talk about the difference between sound-absorbing materials and sound-insulating materials. This is also where many people misunderstand. To understand the difference between the two, you first need to understand how sound continues to propagate after it reaches our common walls.

Incident Sound – Reflected Sound = Sound Absorption Coefficient

Incident Sound – Transmitted Sound = Transmission Loss

Some of the sound is absorbed by the wall and turned into heat energy.

From the above relationship, it is not difficult to find that sound insulation only needs to ensure as little transmitted sound as possible, but it does not necessarily have good sound absorption. Here is an extreme example. If the wall of the room is a very smooth double-layer wall, the sound insulation performance is very good because of the double-layer design. However, because the wall is very smooth, the reflected sound, especially the high frequency, is very strong and the sound absorption effect is very poor.

In fact, there are very common similar designs, such as double-layer soundproof glass, which has good sound insulation effect, but the smooth nature of the glass surface will cause the reflected sound to be very strong. Such double-layer soundproof glass has almost no sound absorption effect.

At the same time, common sound-absorbing materials usually have mediocre sound insulation properties and are usually not as good as walls of the same thickness.

Sound absorbing material

Traditional sound-absorbing materials are porous materials, or scientifically known as acoustic resistance sound-absorbing materials. The essence of sound waves is a kind of vibration. To be precise, for the speaker system, it is air vibration. When air vibrations are transmitted to this sound-absorbing material, the vibrations are gradually relieved by the fine porous structure and converted into heat energy.

The thicker the sound-absorbing material, the more such small cavities are in the direction of sound propagation, and the better the absorption effect for sound incident at random or at small angles.

On the other hand, acoustically resistive sound-absorbing materials have a low-frequency cutoff frequency for effective absorption, which is usually related to the thickness of the sound-absorbing material, which requires about 1/4 of the wavelength. Therefore, it is almost impossible for acoustically resistive sound-absorbing materials to be used to absorb low frequencies, and other types of processing materials are required for low frequencies.

At the same time, the sound absorption coefficient of sound-absorbing materials is also related to the incident angle of sound. For example, high-density fiberboard could reflect sound at large angles.

Diffusion material

First, we still need to understand what sound diffusion is or what the role of diffusion materials is.

When sound is incident on a wall, part of the sound will exit along the geometric direction and continue to propagate, but usually this process is not an absolute “specular reflection”. If it is an ideal absolute reflection, the sound should exit exactly in the geometric direction after passing through the surface, and the energy in the exit direction should be consistent with the incident direction. The entire process does not lose energy, which can be understood as no diffusion at all, or more commonly understood as specular reflection in optics.

In ideal diffusion, there is no main emission direction. The sound emits uniformly in all directions in the space after incident on the surface. It can be commonly understood as diffuse reflection in optics.

Of course, all this is also related to factors such as the wavelength of the sound. The actual listening environment not only requires a certain amount of diffusion, but also should try to avoid strong reflections such as the entire floor-to-ceiling glass. But this means that the reflected sound will be more complex and the calculation amount will be greater.

Sometimes we need a certain amount of reflected sound, but if we don’t need particularly strong reflected sound, we can place diffusion materials at corresponding locations. For another example, sometimes if a room is filled with sound-absorbing materials, it will absorb too much sound. Diffusion materials can reduce early reflection sound while ensuring late reflection sound, so that small rooms can also have a certain reverberation effect. For stereo speaker systems, it is recommended to have a certain amount of diffusion material in the room, while for home theater systems, the diffusion material can be appropriately reduced or sometimes a full sound-absorbing design can be adopted.

As we just mentioned, the lowest effective frequency of the diffusion material or the lowest diffusion frequency is related to the protrusion size of the diffuser. The more protrusions of the diffuser, the lower the frequency that can be effectively diffused.

There are also some cases where this type of diffuser is made into a Roman column appearance while also having an aesthetic design feel. And in fact, since the rooms of many audiophiles are not large at present, such a diffuser may not be needed. The effective frequency of a general quadratic remainder diffusion plate is about 700~1000Hz. Sometimes the choice of frequency should also be made on a case-by-case basis.

At the same time, our common HiFi speaker systems and home theaters and other small home listening environments do not need to use diffusers with excessive protrusions. Because the diffusion of sound is usually explained in terms of geometric acoustics, once the frequency is lower than the excessive frequency of the room, it is the category of wave acoustics. Or that diffusion materials suitable for small rooms generally don’t have to be as thick as those for opera houses and concert halls.

Bass trap

As mentioned above, it is almost unrealistic for acoustically resistive sound-absorbing materials to effectively absorb low frequencies. For the absorption of low frequencies, materials that rely on other principles are often needed.

Helmholtz resonator is a very common acoustic structure, often used in applications such as automobile and motorcycle exhaust pipe noise reduction. The principle is to open a small opening in a cavity V and connect 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 will be compressed and the pressure will increase. The inward movement of the air column S is blocked and moves outward instead. After passing the equilibrium position, it continues to move outward due to inertia. Currently, the pressure in the cavity decreases, causing the air column S to stop moving outward and then move inward, repeating the cycle.

Membrane mechanical resonance sound absorbers are another type of sound-absorbing material for low frequencies, usually implemented in architectural designs such as multi-layer wall designs.

Due to factors such as room standing waves, additional gain is usually generated in certain frequency bands, resulting in low-frequency “rumbling” resonance. Low-frequency traps can eliminate peak interference caused by these room modes to a certain extent. Of course, methods such as multiple subwoofers or DSP digital signal processing can also be used.

Sound insulation materials

As explained in detail before, the sound insulation performance and sound absorption performance of materials are different. Sound-absorbing materials often take advantage of the small pore structure within the material. However, this small hole structure usually also results in the transmission of sound waves. In order to prevent further transmission and propagation of sound from the material, it is necessary to reduce the cavity structure as much as possible and increase the density of the material.

Usually, the sound insulation performance of sound insulation materials is related to the density of the material. Purchasing high-density soundproofing materials can further improve the soundproofing properties of a room. However, single-layer sound insulation materials sometimes still have limitations. In this case, double-layer sound insulation treatment can be used, and additional damping materials are added to the two layers of sound insulation materials. However, it should be noted that the two layers of sound insulation materials should be avoided to have the same thickness as much as possible to avoid repetition of the same frequency. If during actual construction and decoration, the whole house should be soundproofed first, and then sound absorption and diffusion treatments should be carried out.

Acoustic treatment is a must for speaker systems. However, acoustic treatment is not a random application. Understanding the relevant basic knowledge is an important prerequisite for correct acoustic treatment. Acoustic treatment design is also closely related to the room, speaker system, etc. There is no fixed solution or answer, and the actual situation should ultimately prevail.

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