This paper is devoted to the determination of the acoustic characteristics of a porous medium saturated by air. The analysis of sound propagation in such a medium is performed using an homogenization technique. This theory is suitable since acoustic wavelengths are much greater than the usual pore size. The macroscopic descriptions involve the effects of air viscosity, inertial forces and heat transfer. The first part of the paper deals with single porosity materials. Two cases are investigated : (i) a medium with large pores in which thermal exchanges are negligible ; (ii) a medium with smaller pores for which thermal exchanges must be accounted for. The second part is concerned with dual porosity media, i.e. when the grains themselves are also porous. Neglecting heat transfer first yields a simplified macroscopic description. This simply dual porosity model is then improved by considering thermal effects. These results show that new porous materials could be evolved by introducing a microporosity structure that would give enhanced absorption properties over a wide range of frequencies.