Porous Absorbers

When sound propagates in small spaces, such as the interconnected pores of a porous absorber, it loses energy. This is primarily due to viscous boundary layer effects. Air is a viscous fluid; consequently, the friction with the pore walls dissipates sound energy. There is also a loss in momentum due to changes in flow as the sound moves through the irregular pores. The boundary layer in air at audible frequencies is sub-millimeter in size, and consequently, viscous losses occur in a small air layer adjacent to the pore walls. As well as viscous effects, there will be losses due to thermal conduction from the air to the absorber material, which is more significant at low frequencies. For the absorption to be effective, there must be interconnected air paths through the material, so you need an open pore structure. Losses due to vibrations of the material are usually less important than the absorption as sound moves through the pores.

Porous Absorbers

Typical porous absorbers are carpets, acoustic tiles, acoustic (open-cell) foams, curtains, cushions, cotton, and mineral wool, such as fiberglass. Sound propagation occurs in interconnected pores, and viscous and thermal effects cause acoustic energy dissipation. Porous absorbers are widely used to treat acoustic problems, such as reducing noise in cavity walls and noisy environments and reducing reverberation in rooms.  

Textiles

Textiles, such as curtains or people's clothes, play a role in room acoustics. Curtains with a wall distance of about 10 cm are good absorbers to a lower frequency of about 125 Hz. However, this requires that an air-tight synthetic coating cover them. Very thin curtains hardly absorb any sound. Their flow resistance is too low.  

Fleece

Most manufacturers of perforated metal, wood, or gypsum board ceilings use unique acoustic fleece. This thin fabric offers optimum flow resistance and fulfills the relevant fire protection requirements. If the perforated portion is high, the panel or board only serves as a mechanical carrier for the acoustic fleece. If the perforated portion is small, the panel or board and the air volume behind it act as a Helmholtz resonator. With most constructions, an additional layer of mineral wool can increase the absorption coefficient.  

Carpets

As carpets' thickness hardly ever exceeds 1 cm, they only absorb high frequencies from about 1 kHz onwards. Carpets alone are, therefore, not sufficient to achieve good acoustics. Additional measures must be taken to absorb lower frequencies.  

Mineral Wool, Cotton Wool

People use mineral wool in the form of pressed, stiff boards in acoustic ceilings. These boards are inserted into rail systems with different profiles and are available in various colors and finishes. Furthermore, mineral wool is frequently used in loose form as an additional layer on perforated panels and to improve sound insulation in lightweight interior walls. In this form, cotton wool can replace it.