Previous impedance models for grounds whose porosities vary exponentially with depth have been based on the microstructural theory of sound propagation in rigid porous materials. These models were derived by incorporating the spatial dependence of the complex wave number on the porosity into the wave equation. The predicted impedances do not behave as expected and produce nonphysical results for porosities which decrease with depth. In this paper the change in porosity is incorporated into the equivalent complex medium Euler equation and continuity equation and a new wave equation is derived. The solution to this wave equation produces physical meaningful results for exponentially decreasing porosity (the case for most naturally occurring ground surfaces) and produces well‐behaved results for exponentially increasing porosity.