  The effect of different possible kinds of motion of the exciting walls
(cyclic, random, ...) is investigated on the dynamics of a granular dissipative
gas. It is shown that the real distribution of speed of the wall which interact
with the balls depend strongly on the real ball speed, due to a screening
effect, so that the transfer of excitation from the walls to the cloud of
particles may drastically depend on the ball speed itself. This makes the
excitation quite non linear, with a change of efficiency law. This may explain
the observation of a biphasic cloud when the mean free path of balls is
slightly smaller than the cell size L, or may predict that the particles in the
cloud is merely at rest as soon as the mean free path is smaller than L/10. It
is obvious that the vibrating cell cannot be considered as a thermostat.
