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, and consequently, the friction with the pore walls dissipates the 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, while this is more significant at low frequencies. For the absorption to be effective, there must be interconnected air paths thought 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. They are materials where sound propagation occurs in a network of interconnected pores in such a way that viscous and thermal effects cause the dissipation of acoustic energy. Porous absorbers are used widely to treat acoustic problems, in cavity walls and noisy environments to reduce noise and in rooms to reduce reverberation.
Textiles
In room acoustics, textiles play a part in the form of curtains or people’s clothes. Curtains with a wall distance of about 10 cm are good absorbers down to a lower frequency of about 125 Hz. This, however, requires that they are not covered by an air-tight synthetic coating. 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 special 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, the absorption coefficient can be increased by an additional layer of mineral wool.
Carpets
As the thickness of carpets 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 for the absorption of lower frequencies.
Mineral wool, cotton wool
People us 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 a variety of colors and finishes. Furthermore, in loose form, people frequently use mineral wool as an additional layer on perforated panels and to improve sound insulation in lightweight interior walls. In this form, cotton wool can replace it.
Source texts
Acoustic Absorbers and Diffusers: Theory, Design and Application, Trevor J. Cox, Peter D’Antoni
Porous absorbers, Dr. Jörg Hunecke
