  For flows of a lossless gas, the stagnation enthalpy obeys a linear convected
wave equation with coefficients which depend on the flow variables. This
equation is self-adjoint and one has a reciprocity relation between source and
observer. It fulfills for subsonic flow a quadratic conservation equation
implying stability. It is taken as basis for an acoustic analogy and is applied
to the sound generation by the collision of a convected vortex and a rigid
cylinder.
