Sound absorption: sound absorption coefficients. Sound absorption of materials: table

In rooms where a large proportion of surfaces are made of open bricks, plaster, tiles, concrete, glass or metal, there is always a long echo. If there are several sources of signal in such a room: musical accompaniment, production noises, people's conversations - there is a superimposition of direct sound on its reflection from the walls.

This entails an illegibility of speech and an increase in the level of noise in the room. In most situations, this effect is undesirable. For example, the halls of railway stations and airports, as well as large supermarkets and metro vestibules are designed in such a way as to minimize the time of the sound (otherwise it is called reverberation time), otherwise it is simply impossible to understand the content of the ads. Reverberation should also be within the given limits in theatrical, concert and lecture rooms. The increased reverberation time distorts the perception of music and speech. On the contrary, a short time entails the "dryness" of the hall and the lack of depth of sound. To reduce or change the reverberation time in the decoration of rooms, sound absorbing materials and structures are used.

To protect the room from noise, a variety of materials are used that can create an obstacle in its path. The choice is determined by the task. The task can include sound insulation, and sound absorption. About them and talk.

Soundproofing

The goal that faces soundproofing is the reflection of the noise waves, so as not to let them penetrate the wall of the room. A special structure of noise insulation materials creates an obstacle in the way of wave propagation, which reflects them. The ability of a structure to soundproof depends primarily on the mass. The more massive and thick the wall is, the harder it is to penetrate the room. To assess the ability of enclosing building structures for noise insulation use such a value as the index of sound insulation. This parameter is measured in dB and must be within 52-60 dB. Soundproof materials are dense. Among them, drywall, brick, concrete.

Sound absorption

The purpose of sound absorption is to absorb noise, not allowing it to reflect from the surface back into the room. It is measured by a parameter such as the sound absorption coefficient of materials, varying from 0 to 1. If the value of this coefficient is zero, the signal is reflected from the walls in full. When all noise is completely absorbed, the coefficient is one. The materials with the properties considered are those that have a certain level of sound absorption. Sound absorption coefficients for them should be more than 0.4.

Sound absorbers come in the following groups:

• Layered structures;
• Three-dimensional;
• Porous (including fibrous);
• Porous with the presence of perforated screens;
• Resonant.

The higher the value of the coefficient, the higher the sound absorption class.

Porous sound absorbers

Sound absorbers of porous type are produced in the form of slabs of porous light materials fixed directly on the 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 have sufficient strength, and they can be used to reduce noise in the foyer, vestibules, corridors and staircases of public buildings and industrial buildings.

Fibrous sound absorbers

In those rooms where the appearance of the sound absorbers should be more aesthetic, materials from fibers, processed in a special way, are used. As a raw material for their production, mineral wool, glass wool, as well as wood and synthetic fibers are used. Such sound absorbers have the appearance of ceiling and wall panels or curved and volumetric elements. Special porous paints are applied to the surface of the sound absorbers, which can pass the air, or they are covered with special materials or fabrics, which also have the property of air permeability.

In modern construction, fibrous sound-absorbing panels are most in demand. They proved to be effective in terms of acoustics and satisfy the increased requirements that are required for interior decoration.

Nature of sound absorption

The dissipation of the energy of acoustic vibrations in fibrous absorbers with the release of heat (sound absorption of materials) has several causes. Firstly, because of the viscosity of the air, which contains a lot of interfiber interstices, the vibration of air particles in the internal volume of the absorber is accompanied by friction. Secondly, there is friction of the air about the fibers, which also have a significant total surface area. Then, the fibers rub against one another, and the energy is dissipated due to the friction of the crystal fibers with each other. Therefore, at medium and high frequencies there is a particularly effective sound absorption. Sound absorption coefficients of materials are in the range of 0.4 ... 1.0. At low frequencies it is more difficult to achieve.

We note that the coefficient of sound absorption is calculated as the ratio of the signal that is not absorbed by the surface and transmitted through it to the total energy that acts on the surface. To obtain reference data on the sound absorption of the main building materials, a table of the sound absorption coefficients is used. It is given below.

Table. Sound absorption, sound absorption coefficients

Sound-absorbing structures

Sound-absorbing materials of fibrous and porous type are used most often to improve the acoustic properties of the premises of theaters, cinemas, concert halls, recording studios. They are also used to make less noise in kindergartens, hospitals, schools.

To increase the noise absorption in the low frequency range, the thickness of the materials must be increased or a space between the absorber and the sound-reflecting structure is planned.

If the fibrous sound absorbers are not dyed and there is no outer layer of fabric on them, they can be used with damage protection based on perforated material.

In the gap between the screen and the fiber material, an air-permeable web is placed to avoid the entry of fibrous particles into the air. Sound-absorbing structures, equipped with a perforated coating, make it possible to obtain sound attenuation 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

Material made on the basis of fiberglass, which has high strength and elasticity. Glass wool is also distinguished by high vibration resistance. Absorption of sound glasswool occurs due to the presence in the gaps between the fibers of a large number of voids filled with air. Advantages of glass wool are fire safety, low weight, high elasticity, absence of hygroscopicity, vapor permeability, chemical passivity. Glass wool serves as an element of acoustic partitions from rolls or plates, as one of the layers of multi-layer sound-absorbing structures.

Mineral wool

Mineral cotton wool is a fibrous material, raw materials for which are silicate melts of rocks, metallurgical slags and their mixtures.

Advantages of the material: incombustibility, chemical passivity and, as a consequence, the absence of 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) in the entire frequency band, multilayer structures of resonant type are used, which consist of several parallel screens having different perforations with air gaps of different thicknesses.

Materials of "Shumanet Eco"

They represent a soundproof layer intended for use in frame partitions, plasterboard sheet linings or suspended ceilings. Are made in the form of hydrophobized plates of fiberglass, which is laminated with fiberglass. The material uses an inert acrylic-based binder that makes the sound-absorbing panels non-combustible.

Features of large rooms

It should be noted that in rooms that have a large volume, the effect of reducing the reverberation time due to the additional sound absorption structures is not so great. Such rooms regulate the reverberation time due to the shape of ceilings and walls. For example, the use of not flat but rounded ceilings and pilasters of various shapes or balconies on the walls makes great sound absorption. This form of architectural details makes it possible to obtain an acoustic field of greater diffusion, which positively affects the acoustic climate in the room.

It should also be noted that the overall noise absorption of the hall increases with the presence of scenery, soft armchairs, curtains. This should be taken into account when choosing the finishing materials in order to choose sound absorption. The sound absorption coefficients in this case will increase.