  By means of a simple model system, the total volume fluctuations of a tapped
granular material in the steady state are studied. In the limit of a system
with a large number of particles, they are found to be Gaussian distributed,
and explicit expressions for the average and the variance are provided.
Experimental and molecular dynamics results are analyzed and qualitatively
compared with the model predictions. The relevance of considering open or
closed systems is discussed, as well as the meaning and properties of the
Edwards compactivity and the effective (configurational) temperature introduced
by some authors. Finally, the linear response to a change in the vibration
intensity is also investigated. A KWW decay of the volume response function is
clearly identified. This seems to confirm some kind of similarity between
externally excited granular systems and structural glasses.
