Simple approximations for the lattice vibrational spectrum allow the regularities of velocity-density plots with structure and composition to be semiquantitatively explained from a simple and unified point of view. It is possible to explicitly separate paths of constant mean atomic weight A where velocities change because of compression or phase changes from paths of constant crystallographic structure where the mean atomic weight is changed. One can approximate the hydrodynamic velocity u _ϕ along the path of constant crystallographic structure by the formula ∂ ln u _ϕ/∂ ln ρ = − ½[1−(∂ ln V _M /∂ ln A)]⁻¹, where ρ is the density and V _M is the molar volume.