Porous absorbers

Standing wave in front of a massive wall

Standing wave in front of a massive wall
p: sound pressure
v: sound velocity
λ: wavelength

A porous absorber is any kind of porous or fibrous material such as textiles, fleece, carpets, foams, mineral wool, cotton wool and special acoustic plasters. They all absorb sound energy as they damp the oscillation of the air particles by friction.

Let us take a look at a plane sound wave vertically incident on a rigid wall. It is completely reflected. The incident and the reflected wave superimpose each other according to the principle of superposition to form a standing wave. Directly in front of the wall, which is supposed to be stationary, the air cannot move. Directly on the wall, the sound velocity of the standing wave must be zero. A high sound pressure, however, is very apt to develop at this position. The first maximum of sound velocity is at a distance of a quarter of a wavelength from the wall.

Porous absorbers are most effective in slowing down air particles with a high sound velocity. When mounted directly onto the wall they therefore must be of a certain thickness in order to absorb sound waves down to a certain lower limiting frequency. If, however, the absorber is mounted at a distance from the wall, its thickness can be reduced accordingly. Manufacturers of acoustic ceilings take advantage of this effect. Besides the distance from the wall, the flow resistance of the material also is of high importance.

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 that can be looked through hardly absorb any sound. Their flow resistance is too low.

Most manufacturers of perforated metal, wood, or gypsum board ceilings use special acoustic fleece. This thin fabric offers an optimum flow resistance and fulfils the relevant fire protection requirements. If the perforated portion is high, the panel or board only serves as a mechanic 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.

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.

Foam is often used as upholstery in furniture. Since upholstered furniture is usually distributed over the room the diffusion of the sound field is increased. Foams especially produced for acoustic purposes are used as inserts in metal ceilings, as a filling for panel absorbers, or as edge absorbers. A visually unobtrusive finish can be achieved by covering the foam with a kind of textile wallpaper.

Mineral wool, cotton wool:
Mineral wool in the form of pressed, stiff boards is often used in acoustic ceilings. These boards are inserted into rail systems with different profiles and are available in a variety of colours and finishes. In loose form mineral wool is frequently used as an additional layer on perforated panels and to improve sound insulation in lightweight interior walls. In this form it can be replaced by cotton wool.

Acoustic plaster:
The term acoustic plaster refers to special plasters that are applied in such a way that many interconnected cavities are formed. If only a thin layer of acoustic plaster is applied, it is only effective for high frequencies, similar to carpets. If, however, the plaster is applied to a sound-permeable carrier material mounted at a certain distance from the wall, a good absorption coefficient can be achieved for low frequencies as well. In this way it is possible to install jointless acoustic ceilings that look like a normal plastered ceiling.