Within the random-phase approximation (RPA), we have obtained the average surface-plasmon energy of voids and bubbles in nearly-free-electron metals using energy-weighted moments of the electronic response to operators of type r -(L+1) Y L0 . We have used a local-density approximation of Slater and Wigner type for the exchange and correlation energies, respectively, and the jellium model for the positive ionic background. Compact expressions for the plasmon average energies are given, which allow one to discuss clearly the role played by the kinetic- and Coulomb energy contributions to the restoring force of the L modes