The characteristic impedance and propagation constant of porous materials are discussed from a viewpoint of physical realizability, and new regression models based on experimental data by Delany and Bazley are derived. Modifications from their original models are so made that the impedance function satisfies the positive-real property and that the propagation constant written in terms of complex variables becomes a regular function in the right half-plane. The new models are shown to be useful for the prediction of acoustical behavior of porous materials, especially in the double-layer case, even outside the frequency range of validity of the original models.