In rooms where most of the surfaces are exposed brick, plaster, tile, concrete, glass or metal, a long echo is always heard. If there are several signal sources in such a room: musical accompaniment, industrial noise, people's conversations, the direct sound is superimposed on its reflection from the walls.
This results in unintelligible speech and increased noise levels in the room. In most situations, this effect is undesirable. For example, train and airport halls, as well as large supermarkets and subway lobbies, are designed in such a way as to minimize the after-sound time (otherwise called reverberation time), otherwise it is simply impossible to understand the content of announcements. Also, reverberation in theatrical, concert and lecture rooms should be within the specified limits. Increased reverberation time distorts the perception of music and speech. Against,a short time entails the "dryness" of the hall and the lack of sound depth. Sound-absorbing materials and structures are used to reduce or change the reverberation time in interior decoration.
To protect the room from noise, a variety of materials are used that can create an obstacle in its path. Their choice is determined by the task. The task may include both soundproofing and sound absorption. Let's talk about them.
Sound proofing
The purpose of soundproofing is to reflect sound waves in order to prevent them from penetrating through the wall of the room. The special structure of soundproofing materials erects a barrier to the movement of waves, which reflects them. The ability of a structure to soundproof depends primarily on the mass. The more massive and thick the wall is, the more difficult it is for sounds to penetrate the room. To assess the ability of enclosing building structures to sound insulation, a value such as the sound insulation index is used. This parameter is measured in dB and should be in the range of 52-60 dB. Dense materials are considered soundproofing. These include drywall, brick, concrete.
Sound absorption
The purpose of sound absorption is to absorb noise without letting it bounce off the surface back into the room. It is measured by such a parameter as the sound absorption coefficient of materials, which varies in the range from 0 to 1. If the valueof this coefficient is equal to zero, the signal is reflected from the walls in full. When all the noise is completely absorbed, the coefficient is equal to one. The materials with the properties under consideration include those that have a certain level of sound absorption. Their sound absorption coefficients must be more than 0, 4.
Noise absorbers come in the following groups:
- layered structures;
- voluminous;
- porous (including fibrous);
- porous with perforated screens;
- resonant.
The higher the coefficient value, the higher the sound absorption class.
Porous sound absorbers
Porous type sound absorbers are produced in the form of slabs of porous lightweight materials fixed directly on enclosing surfaces or at a distance from them. These materials are produced on the basis of kaolin, pumice, slag, vermiculite, using cement, lime or gypsum as a binder. These materials are durable enough to be used to reduce noise levels in foyers, lobbies, corridors and stairways in public and industrial buildings.
Fiber sound absorbers
In those rooms where the appearance of sound absorbers should be more aesthetic, materials from fibers processed in a special way are used. Mineral wool, glass wool, as well as wood and synthetic fibers are used as raw materials for their manufacture. Suchsound absorbers are in the form of ceiling and wall panels or curved and three-dimensional elements. Sound absorbers are coated with special porous paints that can let air through, or they are covered with special materials or fabrics that also have the property of breathability.
In modern construction, fibrous sound-absorbing panels are most in demand. They have proven themselves acoustically and meet the increasing demands placed on interior decoration.
The nature of sound absorption
Dissipation of the energy of acoustic vibrations in fibrous-type absorbers with heat release (sound absorption of materials) has several reasons. Firstly, due to the viscosity of air, which is quite abundant in the interfiber spaces, the oscillation of air particles in the internal volume of the absorber is accompanied by friction. Secondly, there is air friction against the fibers, which also have a significant total surface area. Next, the fibers rub against each other, and energy is dissipated due to the friction of the fiber crystals with each other. Therefore, particularly effective sound absorption occurs at medium and high frequencies. The sound absorption coefficients of materials are in the range of 0, 4 … 1, 0. At low frequencies, it is more difficult to achieve it.
Note that the sound absorption coefficient is calculated as the ratio of the unabsorbed surface and the signal energy transmitted through it to the total energy that affectssurface. To obtain reference data on the sound absorption of the main building materials, a table of sound absorption coefficients is used. It is shown below.
Table. Sound absorption, sound absorption coefficients
| Material | Noise reduction factor at 1000 Hz |
| Fibreboard board | 0, 40-0, 80 |
| Perforated acoustic sheet | 0, 4-0, 9 |
| Fibrolite | 0, 45-0, 50 |
| Foamglass | 0, 3-0, 5 |
| Concrete wall | 0, 015 |
| Fiberglass | 0, 76-0, 81 |
| Wooden wall | 0, 06-0, 1 |
| Brick wall | 0, 032 |
| Bas alt fiber | 0, 94-0, 95 |
Sound absorbing structures
Sound-absorbing materials of fibrous and porous type are used most often to improve the acoustic properties of theaters, cinemas, concert halls, recording studios. They are also used to reduce noise in kindergartens, hospitals, schools.
In order to increase noise absorption in the low frequency range, the thickness of materials must be increased ora gap of air is planned between the absorber and the sound-reflecting structure.
If the fiber absorbers are unpainted and do not have an outer fabric layer, they can be used with perforated damage protection.
A breathable canvas is placed between the screen and the fiber material to prevent fibrous particles from entering the air. Sound-absorbing structures equipped with a perforated coating make it possible to obtain sound absorption of good quality at all frequencies. Adjustment of the frequency response of sound absorption occurs by selecting materials. And also by varying their thickness, size and shape, the distance between the holes. Sound-absorbing structures equipped with a perforated metal screen are widely used as anti-vandal coatings. One of the modern similar materials is "Shumanet Eco".
The best noise-absorbing materials. Glass wool
Fiberglass-based material with high strength and elasticity. Glass wool is also distinguished by high vibration resistance. The sound absorption of glass wool occurs due to the presence of a large number of voids filled with air in the gaps between the fibers. The advantages of glass wool are fire safety, low weight, high elasticity, lack of hygroscopicity, vapor permeability, chemical passivity. Glass wool serves as an element of acoustic partitions made of rolls or plates, as one of the layers of multilayer sound-absorbingdesigns.
Mineral wool
Mineral wool is a fibrous material, the raw material for which is silicate melts of rocks, metallurgical slags and their mixtures.
Advantages of the material: incombustibility, chemical passivity and, as a result, no corrosion on metals in contact with mineral wool. The quality of sound absorption is realized due to the chaotic arrangement of the fibers.
To obtain a high sound absorption coefficient (from 0.7 to 0.9) across the entire frequency band, resonant-type multilayer structures are used, which consist of several parallel screens having different perforations with air gaps of different thicknesses.
Materials "Shumanet Eco"
Provides a soundproofing layer designed for use in partition walls, plasterboard sheathing or suspended ceiling structures. They are made in the form of hydrophobized fiberglass boards, which are laminated with fiberglass. The material uses an inert acrylic-based binder to make the sound-absorbing panels non-flammable.
Features of large volume rooms
It should be taken into account that in rooms that have a large volume, the effect of reducing the reverberation time due to additional sound absorption structures is not so great. Such rooms regulate the reverberation time due to the shape of the ceilings and walls. For example, the use of not flat, but rounded ceilings and pilasters of various shapes or balconies on the walls increases sound absorption. This form of architectural details makes it possible to obtain a more diffuse acoustic field, which has a positive effect on the acoustic climate in the room.
It should also be noted that the overall sound absorption of the hall increases with the presence of scenery, soft chairs, curtains. This should be taken into account when choosing finishing materials in order to select sound absorption. The sound absorption coefficients in this case will increase.
