The contribution of domain walls to the dielectric permittivity of ferroelectrics is customarily explained by the model of locally pinned walls. Here we analyze the complementary case and calculate wall contribution to permittivity of a sample in which 180 degrees domain walls are not pinned locally, moving freely between, "stoppers" representing lattice defects or neighbouring walls. Amplitude dependences and dispersion curves have been calculated for two laws for wall velocity. The model may be pertinent to high quality samples and to systems with freshly formed domain walls.