Constant head permeability tests were performed on two soil specimens, S1 and SThe ratio of height of the two specimens (L_S1: L_S2) is 1.5, the ratio of the diameter of specimens (D_S1:D_S2) is 0.5, and the ratio of the constant head (h_s1:h_s2) applied on the specimens is 2.0. If the discharge from both the specimens is equal, the ratio of the permeability of the soil specimens (k_s1:k_s2) is ________