Buildings generally use concrete, wood panels or solid bricks as wall materials. Strictly speaking, these materials can achieve the effect of sound insulation, but the depth of the effect depends on the molecular density of different materials. For the audio-visual room that pursues sound effects, the wall material alone is not enough to achieve the effect of sound attenuation. Whether it is a villa or an apartment building, custom private theaters require sound absorption or sound insulation. Acoustic treatment controls the reflection of sound waves in a room, adjusting the reverberation time inside the room, thereby improving the sound quality of watching movies. Let us talk about those things about sound absorption and sound insulation today.

Sound absorption

We briefly mentioned above that some solid materials can weaken the sound wave coefficient during the propagation process. In fact, in addition to being absorbed by the internal material, some materials can also reflect the sound wave and transmit it to the other side. Sound absorption and sound insulation are two completely different sound control methods. Sound absorption refers to the phenomenon of energy loss after sound waves hit the surface of a material, and sound insulation refers to a method of reducing noise in the sound wave propagation path. Simply put, sound absorption removes a certain amount of sound energy, and sound insulation controls the transmission of sound waves to the other side of the wall.

When sound waves travel through the air and touch the surface of the sound-absorbing material, part of it will be reflected and part will be absorbed. Acoustic materials used for sound absorption generally have porous properties, and the common ones are fibers, particles, and foams. The sound wave is incident on the countless tiny voids inside the material, where it causes friction with the air molecules and the void walls, thereby converting the sound energy into heat energy to achieve the sound-absorbing effect. In addition to porous sound-absorbing materials, there are resonant sound-absorbing structures and special sound-absorbing structures. Resonance sound absorption includes single resonator, perforated plate resonance sound absorption mechanism, thin film resonance detail structure and thin plate resonance sound absorption structure. Its principle is also to use the material structure and air to form a resonant system to achieve sound absorption.

Sound absorption coefficient, also known as NRC value, is defined by the American Society for Testing and Materials (ASTM). The NRC numerical calculation method takes the arithmetic mean value of the sound absorption coefficient of the material at four frequencies of 250, 500, 1000 and 2000 Hz. Since the sound absorption coefficient of sound-absorbing materials increases with the increase of frequency, it is difficult for general sound-absorbing materials to absorb some lower-frequency sound waves. Generally, products with a sound absorption coefficient lower than 0.2 are called reflective materials, and materials higher than 0.4 can be regarded as sound-absorbing materials. Consumers need to be cautious in purchasing acoustic materials with a sound absorption coefficient exceeding 1.0. Although it is much higher than the “qualified line” of 0.4, such data values are wrong. The American Society for Experimental Materials, which formulates the NRC (sound absorption coefficient) value, uses the number “0” to represent the sound absorption coefficient of a material that cannot absorb any incident sound energy. In other words, this material can reflect all the sound energy it meets. Conversely, if a material absorbs all sound energy, it is indicated by the number “1”. In fact, any material has the characteristics of reflecting and absorbing sound energy. “0” and “1” are just theoretical limit reference values. The normal sound absorption coefficient of sound-absorbing materials should be between 0 and 1. value. A material with a sound absorption coefficient greater than 1.0 means it absorbs more sound energy than it reflects, which is physically impossible. If consumers encounter such descriptions, they can directly question the accuracy of the data.

Soundproofing

As mentioned earlier, sound insulation and sound absorption are two different types of sound control. In a sense, sound absorption considers more the absorption coefficient of the material itself, while sound insulation considers both the transmission loss of the partition wall and the absorption characteristics of the room during sound transmission. Therefore, it cannot be like the sound absorption coefficient (NRC) that is calculated by taking the average of the values, but need to data (sound transmission loss and frequency characteristics of the graph) and the American Society for Testing and Materials to determine the standards for the comparison of the resulting values for the sound transmission class of the partition wall (STC). Although the calculations vary, the sound transmission loss varies according to frequency, and a partition wall used to control the propagation of sound will generally increase as the frequency increases.

Just as sound can always be absorbed or reflected by a material, it can always pass through a partition wall and transmit sound to the other side of the room. Unlike sound absorption, if the reference limit value 0 is meant to be the case where sound is not projected, then its opposite limit value 1 (where all sound is projected through the partition wall) is possible. This is possible, for example, if we leave the doors and windows open in a room. In an unprotected room (i.e., a single-layer partition wall), any gap in the opening can result in a reduction in sound transmission loss, which I think you all know from your home or office. Many people may think that increasing the thickness of partition walls can increase the sound transmission loss even more deeply, but according to the law of sound insulation quality, even if you double the thickness of the wall, the amount of sound insulation can only increase by 6dB.

Thus, for recording studios, private theaters, and other environments with high requirements for sound transmission, the design of double partition walls is the most common method. The construction of double partition walls is also a delicate matter, for example, between the two walls will be filled with different material layers (such as sound-absorbing materials) to achieve better sound insulation. The material layer must be placed in the air layer, so that the sound waves can be secondary diffusion. Blindly filling the material layers between the two walls will not necessarily achieve the desired sound insulation effect.

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