Advances in low power electronics and microsystems design open up the possibility to power small wireless sensor nodes thanks to energy scavenging techniques. Among the potential energy sources, we have focused on mechanical surrounding vibrations. To convert vibrations into electrical power we have chosen mechanical structures based on electrostatic transduction. Thanks to measurements and in agreement with recent studies [1], we have observed that most of surrounding mechanical vibrations occurs at frequencies below 100 Hz. We report here global simulations and designs of mechanical structures able to recover power over a large spectrum below 100 Hz. Contrary to existing structures tuned on a particular frequency [2], we have investigated conversion structures with a high electrical damping. Mathematica analytical models have been performed to determine the mechanical and electrical parameters that maximize the scavenged power for a wide number of applications (Figure 3). Two prototypes of mechanical structures have been designed.