  In this work we experimentally study the behavior of a freely-rotating
asymmetric probe immersed in a vibrated granular medium. For a wide variety of
vibration conditions the probe exhibits a steady rotation whose direction is
constant with respect to the asymmetry. By changing the vibration amplitude and
by filtering the noise in different frequency bands we show that the velocity
of rotation does not depend only on the RMS acceleration $\Gamma$, but also on
the amount of energy provided to two separate frequency bands which are
revealed to be important for the dynamics of the granular medium: the first
band governs the transfer of energy from the grains to the probe, and the
second affects the dynamics by altering the viscosity of the vibro-fluidized
material.
