Acoustic Materials

subdivided into sound-absorbing and sound-insulating sealing materials.

Sound-absorbing materials are used mainly as sound-absorbing facings for commercial premises and engineering structures where a reduction of the noise level is needed—for instance, industrial shops, typing offices, ventilating and air-conditioning installations, and so on—and also to create optimal conditions for hearing and to improve the acoustic characteristics of such public buildings as theaters, auditoriums, and radio studios. The sound-absorbing property of the materials is due to their porous structure and the presence of a large number of open, interconnected cavities with a maximum diameter that rarely exceeds 2 mm—the total porosity should be no less than 75 percent of the volume. The large specific surface of the materials created by the walls of the open cavities enables the energy of the sound vibrations to be efficiently converted into heat energy as a result of frictional losses. The sound-absorbing efficiency of the materials is assessed by means of a sound-absorption factor a, which is equal to the ratio of the amount of energy absorbed to the total amount of sound-wave energy incident on the material.

Sound-absorbing materials have a fibrous, granular, or cellular structure and may have various degrees of rigidity—soft, semirigid, and hard. Soft sound-absorbing materials are made from mineral wool or glass fiber with a minimum amount of binder (up to 3 percent by mass) or with none. Among these are mats or rolls having a unit weight of up to 70 kg/m³, which are generally used in conjunction with a perforated protective sheet of aluminum, asbestos cement, or rigid polyvinylchloride, or with a porous film covering. The sound-absorption factor of these materials at the middle frequencies (250–1000 Hz) is between 0.7 and 0.85.

Among the semirigid materials are mineral wool or glass fiber panels 500 x 500 x 20 mm in size having a unit weight of 80–130 kg/m³ with a synthetic binder content of 10–15 percent by mass, and also wood fiber panels having a unit weight of 180–300 kg/m³. The surface of the panels is coated with a porous paint or film. The sound-absorption factor of semirigid materials runs from 0.65 to 0.75. In this group are sound-absorbing panels of porous plastics that have a cellular structure—for instance, polyurethane foam and polystyrene foam plastic.

The solid materials of fibrous structure are made in the form of panels—among them are “Akminit” and “Akmigran” in the USSR and “Traverton” in the USA—300 x 300 x 20 mm in size from granular or suspended mineral wool and a colloidal binder such as amylaceous paste or a mortar of carboxymethyl cellulose. The panel’s surface is painted and has a variety of textures; it can be crazed, fluted, or striated. The unit weight is 300–400 kg/m³; the sound absorption factor at the middle frequencies is between 0.6 and 0.7. A variety of hard materials, panels and plastering mortars in particular, includes porous fillers—expanded perlite, vermiculite, pumice—and white or colored Portland cements. Sound-absorbing panels in which wood fiber is combined with cement mortar, so-called acoustic fibrolite, are also used. The choice of material depends on the acoustic conditions, the purpose, and the architectural features of the room.

Sound-insulating sealing materials are used in the form of rolls or panels in the construction of floors between stories, in the interior of walls and partitions, and also as vibration isolating spacers under machines and equipment. They have a low dynamic elastic modulus, as a rule not over 1.2 MN/m² (12 kgf/cm²) under a load of 20 MN/m² (200 kgf/cm²). The elastic properties of the material’s lattice and the presence of air in its cavities causes the impact and vibration energy to be damped out so that structural and impact noise can be reduced. There are layered soundinsulating materials made from fibers of organic or mineral origin—wood fiber panels, mineral wool and glass fiber rolls, and panels from 10 to 40 mm thick having a unit weight of 30–120 kg/m³; there are materials made from elastic gas-expanded plastics—polyurethane foam, expanded polyvinylchloride, synthetic rubber latexes—which are manufactured in the form of panels from 5 to 30 mm thick. The unit weight of the elastic polyurethane foam is 40–70 kg/m³, and the expanded polyvinylchloride is between 70 and 270 kg/m³. In many cases, pieces of molded or sponge rubber are used for sound isolation.