The variable velocity acoustic wave equation (1) is rigorously valid for use in wave propagation and imaging/inversion applications for the case of a linear isotropic non-viscous fluid medium with constant mass density and spatially variable bulk modulus. However, for reasonably smooth contrasts in density and standard frequency-content data, the validity condition (20) is sufficiently satisfied for the use of (1) in realistic exploration geophysics applications. I hope to extend this analysis in a future work to define the validity conditions for approximating elastic wave propagation with a variable velocity scalar wave equation.