Typical porous absorbers are carpets, acoustic tiles, acoustic (open cell) foams, curtains, cushions, cotton, and mineral wools such as fibreglass. These are materials where absorption is mainly caused by the sound propagation that occurs in the network of interconnected pores; this creates viscous and thermal effects that cause acoustic energy to be dissipated. As discussed in Chapter 2, porous materials are widely used to treat acoustic problems, such as in cavity walls to reduce noise transmission and in cacophonous rooms to reduce reverberation. This chapter will detail the physical processes creating the absorption and models for predicting performance.