Abstract Porous diffusers, a distinguished group of silencers, have been developed in the last years, and are used widely in foreign countries. They are preeminent in their small volume, simple structure and high noise reduction. Unfortunately, there are lack of theoretical discussions as yet. In this work, the aerodynamics of flow through porous materials is discused in detail and the authors suggest that the outflow through porous material can be characterized by an effective area and an effective stagnation pressure reduction. Furthermore, the principle of noise reduction of porous diffusers has been studied in some detail, to facilitate the design and the estimation of porous diffusers. Some of the porous diffusers are studied in the light of the present theory.