We report on further experimental studies of the ac field response of transient and permanent gratings written with visible light in both planar and homeotropic, dye-doped liquid crystal cells. It is found that the diffraction efficiency of these gratings can be controlled by the applied ac field. High ac frequencies can switch off diffraction completely in both permanent and transient gratings. The response time to the applied electric field is shown to be in the range of milliseconds. Permanent gratings persist for months and, when heated above the liquid crystal phase transition temperature, they can be only partially erased. Furthermore, on cooling the diffraction efficiency can be restored, indicating a strong anchoring at the boundaries of the cell.