Linear inertial terms are included in the momentum equation for the unsteady flow. The continuity equation is given in terms of the compressibilities of the constituents involved--solid, gas, and/or liquid. When these two equations are combined, the result is a hyperbolic partial differential equation for pressure. The solution of this equation is used to describe the propagation of pressure waves in the fluid in the porous medium. Theoretical solutions for a harmonic, repetitive pulse, and step input in pressure are obtained. The frequency dependence of the hydraulic conductivity is demonstrated when harmonic or repetitive pulses are considered. The hydraulic conductivity is time dependent when a step variation in pressure is introduced. Experimental measurements were conducted on air dry, liguid saturated, partially liquid saturated, and drained sands. The results of the experiments were found to be in agreement with the theory for all but the liquid saturated case. Methods for determining the static (Darcy) hydraulic conductivity and compressibility using sound wave propagation are developed and applied. (Author)