Porous absorvents are usually materials with some form of holes that trap and absorb sound.
Sound waves travel through this material and, when they're trapped, they're converted into heat energy.
The conversion of sound waves into heat energy results in the sound effectively being absorbed.
In some absorbers with open surfaces, the air movement resulting from sound waves pushes air particles through the narrow passages which in turn generate a viscous loss along with heat.
Perforated materials that trap sound energy in holes or slots of different depths where it is then converted into oscillation energy. Resonators can be customised to reduce a specific set of frequencies. Examples: perforated wood, metal and gypsum board.