A thorough understanding of the acoustical properties of porous materials is becoming increasingly important because of the wide variety of unusual and important applications in which they are finding use. A simple and useful theory is presented for predicting the normal acoustic impedance of isotropic porous materials in terms of three previously described simple constants, namely, flow resistance, porosity, and effective dynamic mass of the enclosed gas. The variation of normal impedance with angle of incidence for spaced‐out or rigidly backed samples is treated. A striking possibility is discussed, namely, thatgaseous expansions are isothermalover most of the audiofrequency range in materials made of finely divided particles. Many isotropic acoustical materials appear to fall in this class. This means that an air cavity becomeslessstiff to dynamic compression when a very porous finely divided material is added to the cavity than when the material is not present. The results are checked against experimental data. A simple method is described for measuring the porosityPof a material, namely, the ratio of the volume of the air spaces in a material to the total volume of the material.