Considerable attention in the preceding chapters was devoted to problems associated with nonsteady propellant burning under harmonically oscillating pressure (Sections 4.5 and 4.7). The importance of such considerations stems from the profound role that acoustic waves may play in determining the regimes of operation of rocket motors. The ability of the burning surface to amplify pressure waves upon their reflection dictates the need to consider a wide range of issues associated with the acoustic properties of the surface, such as acoustic admittance. Consequently, burning regimes occurring under harmonically oscillating pressure may be called acoustic regimes.