When a porous material is exposed to incident sound waves, the
air molecules at the surface of the material and within the pores
of the material are forced to vibrate and, in doing so, lose some of
their original energy. This is because part of the energy of the air
molecules is converted into heat due to thermal and viscous losses
at the walls of the interior pores and tunnels within the material.
At low frequencies, these changes are isothermal, while at high
frequencies, they are adiabatic. In fibrous materials, much of the
energy can also be absorbed by scattering from the fibers and by the
vibration caused in the individual fibers. The fibers of the material
rub together under the influence of the sound waves.8,17