Technical Papers and Presentations

Many of fibrous materials are very good sound absorbers, because the acoustic waves, which propagate in air and penetrate a fibrous layer, interact with the fibers so that the wave energy is dissipated. The dissipation is related to some viscous and thermal effects occurring on the micro-scale level. On the macroscopic level, a fibrous medium can be treated as an effective inviscid fluid, provided that the fibers are stiff. Such a fluid-equivalent approach allows to use the Helmholtz equation for the macroscopic description of sound propagation and absorption [1]. It is applied by the advanced Johnson-Allard models, which require from 5 to 8 parameters related to the micro-geometry of fibrous microstructure. These are the so-called transport parameters in porous media: the open porosity and tortuosity, the permeability and its thermal analogue, two characteristic lengths (for viscous forces and thermal effects), etc. Moreover, some parameters for air (which fills the medium) are also necessary.