The absorption of sound is mainly due to three different physical mechanisms that are all based on the conversion of sound energy into heat.
In porous absorbers the oscillation of the air particles is slowed down by the porous or fibrous structure of the material. As a result, frictional heat is produced. Porous absorbers are textiles, carpets, foams, mineral wool, special acoustic plasters and porous stone materials. Edge absorbers are a special form of the porous absorbers. When arranged at the edges of a room they offer high efficiency at low frequencies.
In Helmholtz resonators the air in the resonator opening is excited to strong oscillations at the resonance frequency. As in porous absorbers, frictional heat is produced as the oscillation of the air particles in the opening is slowed down due to friction. The main difficulty when designing Helmholtz resonators is to adjust the frictional resistance to the ideal value. Micro-perforated absorbers are another form of Helmholtz resonators. They are characterized by a multitude of tiny holes (with a radius of less than 1 mm) and low perforation (less than 4%). Since friction between the air particles and the walls of the holes is sufficiently high it is not necessary to fill the holes with additional porous material. This means that micro-perforated absorbers can also be made from transparent material such as acrylic glass.
Panel absorbers are yet another form of resonant oscillating systems. A panel with a closed surface is fixed in front of a volume of air in such a way that any airborne sound hitting its surface causes the panel to oscillate. At the eigenfrequencies of this oscillating system the amplitudes of the oscillation are particularly high. The oscillation of the panel is damped by the friction between the molecules of the panel material. So here the sound energy is first converted into the oscillation energy of the panel and only then into heat. To achieve optimum efficiency most panel absorbers require additional damping of the air space behind the panel by means of mineral wool or foam material.
To be able to quantify the efficiency of absorbers, the absorption coefficient was defined. It indicates the portion of incident sound energy that is absorbed. The absorption coefficient for normal incidence, which plays an important part in research and development, assumes values between 0 and 1. It is measured in the impedance tube (also called Kundt's tube).
For room-acoustical planning the absorption coefficient for random incidence is needed. It is determined by measuring the sound absorption in a reverberation room according to DIN EN ISO 354, and can assume values greater than 1, both theoretically and metrologically. A simplified explanation of this phenomenon refers to the wave nature of sound: At the edge of the test surface the sound waves are additionally diffracted into the absorber so that the efficiency of the absorber extends beyond its actual surface.
The measurements in the reverberation room are only valid down to a lower limiting frequency of 100 Hz to 125 Hz. When the absorption coefficient of absorbers which are effective below these frequencies is to be determined, due to the wave nature of sound the position of the absorbers within the room is of essential importance. Currently there is no standardized method for the measurement of the absorption coefficient at low frequencies.