Abstract: Empirical equations for porous material to describe the frequency property of the microstructure and predict their sound absorption performance. These empirical equations systematically establish a solid system that relates the porosity, the flow resistance and the air mass to their frequency property and describe how to predict sound absorption coefficient in different thickness. These empirical equations reveal that the microstructure are not uniform across the thickness when the materials are exposed to the sound field. The flow resistance is one of the microstructure and is found to be a step function of the frequency. An interchangeability between the thickness and the frequency was established to predict sound absorption coefficient.