Lined ducts containing flow and intense sound have been extensively studied in recent years. Parametric studies have revealed so many variables that design procedures are obscured. Analysis by solution of the convected wave equation has been complicated by uncertainties about the actual modal distribution and by uncertainties about the actual boundary impedance. As a result, when discrepancies occur between experimental and analytical results it is difficult to interpret whether the wave‐equation solution, the modal assumption, the impedance value, or the test procedure is deficient. The simultaneous refinement of all four factors has resulted in a very satisfactory correlation between theory and duct test results and provides a basis for lucid duct design procedures and the evolution of new liner materials. A duct test facility has been built which appears to provide equipartition of energy. The impedance of nonlinear materials has been redefined and measurement methods modified. Contours of equal duct attenuation in the impedance plane have been generated and prove to be simple in shape. From these, a design methodology is deduced which may be applied to any liner material and which eliminates many redundant parameters. The new methodology has led directly to the creation of new liner materials.