Acoustic wave propagation is considered by transforming the equations of inviscid compressible flow to a coordinate system defined by the wavefront geometry. These equations are linearised and equations for the trajectory of rays are derived in the high frequency limit. The formulation in terms of the new coordinates facilitates a rapid derivation of an expression for the transmission loss associated with propagation along a ray. The form of the equations permits easy and robust calculation of sound propagation through media characterised by a non-uniform sound speed and demonstrates the utility of the coordinate system defined by the natural geometry of the wavefront.
