Sound Absorber, Sound Absorption

The absorption coefficient is a measure of the efficiency of a material in absorbing sound. If 70% of the sound energy is absorbed, the absorption coefficient is 0.70. The absorption coefficient of 1.0 is assumed an open window, there all passing through sound energy never will return into the room again, resulting in a 100% absorption.

Typical Absorption Coefficients

 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz
brick, unglazed 0.03 0.03 0.03 0.04 0.05 0.07
carpet, heavy, on concrete 0.02 0.06 0.14 0.37 0.60 0.65
concrete floor or terrazzo 0.01 0.01 0.015 0.02 0.02 0.02
chairs, metal or wood, unoccupied 0.15 0.19 0.22 0.39 0.38 0.30
seats fully occupied 0.5 0.64 0.76 0.86 0.86 0.76

Typical Graphics (Absorption Coefficient over Frequency)

Sound absorption should be considered not only in one direction rectangular to the material's surface but from all possible directions as sound is reflected in a room to all directions and will hit the material from all different directions. For this reason the measurements of the absorption coefficient are taken in a special reverberation chamber that simulates real conditions during the test phase. A (relatively large) sample of the tested material will be put into the (also large) chamber. The chamber itself should have a very long reverberation time. From the measured difference between the empty chamber and the chamber with the sample material, the absorption can be calculated.

Types of Absorbers

Porous Absorbers
are materials with an open pore structure such as mineral wool, glass fibre, cellulose fibre or plastic foams. The sound energy is converted to heat energy. This small amounts of heat result from frictions and the resistance of materials to movement and deformation. Porous absorbers are mostly effective for high frequencies.


Membrane Absorbers
are flexible materials stretched over supports or rigid panels. They are mounted at some distance from the hard surface. Due to rapid flexing and the resistance of the enclosed air energy will be converted to heat.

Cavity Absorbers,
or Helmholtz resonators, are air containers with a narrow opening. The air within the cavity has a spring-like effect at the resonance frequency of the enclosed air volume.
At the mouth the pressure is near zero and this generates a 'vacuum cleaner' effect that 'sucks in' sound energy from the surrounding areas too.

Perforated Panel Absorbers,
mounted with distance to the hard surface combine all three of these types.
The panel itself acts as a membrane absorber, material behind the panel works as porous absorber and the perforation holes act with the air space behind them as multiple cavity resonators.
Most broad spectrum acoustic absorber materials fall into this category.

Specified Types of Mounting of Absorbers

A material directly on the hard surface
B material cemented to plasterboard
C, D and E mounted with space:
C-20 perforated material with 20mm (3/4") space to the hard surface
C-40 perforated material with 40mm (1 1/2")space to the hard surface
D-20 material with 20mm (3/4") space to the hard surface
E-405 material with 405mm (16") space to the hard surface

Absorbing material is most effective by positioning in the corners or near the corners of the room because most normal modes have pressure maximums at the room corners.

Recommended Positions for typical Porous Absorbers

Porous absorbers should be positioned in a way that they don't diminish useful reflections on side walls and ceilings.