The absorption of sound mainly results from the dissipation of acoustic energy due to viscosity and heat conductivity of the medium. A number of dissipation mecha­nisms have been proposed by some authors.10,13,28,29 Attenborough and Ver17 cite the friction between the solid body (fiber) of the absorber and the fluid moving in it (air) as the main cause of sound attenuation. Similarly, Cox and D’antonio14 refer to the friction due to viscosity of air as the primary cause and thermal conduction as the secondary cause of the sound energy loss. Fahy13 explains the sound absorption phe­nomena on a molecular level as a combination of viscosity, thermal diffusion and re­laxation processes which take place in the boundary layers next to “pore” surfaces. The next section gives information about sound absorption measuring techniques.