A theory is proposed that provides angular momentum conservation through the acoustic drift mechanism for gap and ringlet formation in planetary rings. In this context, a new variant of drift mechanism is found, which is dominant if the collision frequency is of the order of the orbital frequency or less. An angular pseudo-momentum equation is derived for gravitational acoustic waves with dissipation. An explicit expression for angular pseudo-momentum, taking into account vertical wave structure, is obtained. The relation between Eulerian- and Lagrangian-averaged mass fluxes is also derived, and the advantages of the Lagrangian one are emphasized.