Comment on “Instability of isolated planar shock waves” [ Phys. Fluids 19, 094102 (2007)]

It is shown that Erpenbeck’s solution of the initial-value problem for small perturbations in the presence of shocks [J. J. Erpenbeck, Phys. Fluids 5, 604 (1962); 5, 1181 (1962)] leads to a straightforward and simple method for analysis of rippled shocks as well. Particularly, the result for the ripple amplitude of a shock is the same as the result of Bates derived from an integral equation for the shock displacement function [J. W. Bates, Phys. Rev. E 69, 056313 (2004); Phys Fluids 19, 094102 (2007)].