​The small strain stiffness of soils is studied with experimental micro-mechanical models designed to separate the relative contributions of different internal processes. The role of particle stiffness and contact behavior, fabric and fabric changes, and normal and tangential interparticle forces is elucidated. It is shown that the exponent of stiffness-stress power relations captures contact behavior if no fabric change takes place, otherwise, the exponent also captures the changes in coordination that occurs during fabric changes. The low-pass filtering effect of particulate media is confirmed in all tests.