The behavior of high frequency modulated speckles stored in sillenite BSO crystals is analyzed. The diffraction efficiency evaluation of three-dimensional subjective speckle and its dependence on the average speckle size is investigated. The efficiency is analyzed theoretically and experimentally in terms of the input mean modulation, the pupil aperture diameter of the imaging lens and the crystal thickness. The relation among the crystal depth, the speckle volume length and the grating interaction depth as well as the finite extent of the index grating in the speckle volume is discussed. An agreement between the simplified theoretical model and the experimental data is verified.