The problem of sound propagation in a duct is investigated. Several results which have been deduced for isentropic duct flow by previous authors are extended to the case where entropy convection is included. For this more general case, a concept is developed to guide the solution of wave‐propagation problems in an optimal manner. Furthermore, it is shown that some new aspects of isentropic wave propagation emerge. To illustrate the methods, the stability of waves behind a shock wave is considered.