  We derive the spectrum of low-frequency spin waves in skyrmion crystals
observed recently in noncentrosymmetric ferromagnets. We treat the skyrmion
crystal as a superposition of three helices whose wavevectors form an
equilateral triangle. The low-frequency spin waves are Goldstone modes
associated with displacements of skyrmions. Their dispersion is determined by
the elastic properties of the skyrmion crystal and by the kinetic terms of the
effective Lagrangian, which include both kinetic energy and a Berry-phase term
reflecting a nontrivial topology of magnetization. The Berry-phase term acts
like an effective magnetic field, mixing longitudinal and transverse vibrations
into a gapped cyclotron mode and a twist wave with a quadratic dispersion.
