Devices for absorbing sound waves that are incident upon them. They include sound-absorbing materials and the equipment for mounting them, and sometimes decorative coverings. The most common types of sound-absorbing structures are linings for interior surfaces (ceilings, walls, ventilation ducts, elevator shafts, and so on), individual sound absorbers, and elements of active silencers.

     Sound-absorbing linings are used to reduce the energy of reflected sound waves. It usually consists of a layer of a homogeneous, porous sound-absorbing material (sometimes with a rough-cast layer) or a layer of porous fibrous material and a protective layer in the form of a thin perforated screen or coating. The efficiency of a sound-absorbing lining is given by the sound absorption coefficient in a specified frequency range (an octave or one-third of an octave). The magnitude of the sound absorption coefficient depends on the way in which the structure is mounted on the enclosure and on the physical characteristics of the structure itself, the most important of which is the complex acoustic impedance. An increase in sound absorption at lower frequencies is achieved either by thickening the structure or by the creation of an airspace between the structure and the enclosure. To provide nearly complete sound absorption, sound-absorbing lining in the form of wedges of a sound-absorbing material placed perpendicular to the surface of the enclosure is used.

     Individual sound absorbers are usually used to reduce the noise from industrial equipment in production buildings. They are structures in the form of separate panels, cones, or prisms, which are attached or suspended in areas close to the noise sources. Their efficiency is characterized by the magnitude of the total sound absorption in sq m per sound absorber. Because of the phenomenon of wave diffraction, the individual sound absorbers have a greater sound absorption coefficient than sound-absorbing lining. The walls of these sound absorbers are usually made from a layer of porous fibrous material, with a protective layer in the form of a thin, solid perforated sheet.

     The elements of active silencers (mostly plates or cylinders) reduce the noise caused by the propagation of a stream of air or gas; they are used mainly in the air ducts of aerodynamic installations. The plates may consist of homogeneous, porous sound-absorbing materials or of a layer of porous fibrous material and a protective layer made from a perforated solid sheet (usually metal). Their efficiency is rated by the sound attenuation in decibels per meter of the silencer’s length and depends on the thickness of the plates (or the diameter of the cylinders), the sound absorption coefficient, and the distance between elements.