  From pressure and surface dilation measurements, we show that a
solid-liquid-type transition occurs at low excitation frequencies in vertically
vibrated granular layers. This transition precedes subharmonic bifurcations
from flat surface to standing wave patterns, indicating that these waves are in
fact associated with the fluid like behavior of the layer. In the limit of high
excitation frequencies, we show that a new kind of subharmonic waves can be
distinguished. These waves do not involve any lateral transfer of grains within
the layer and correspond to excitations for which the layer slightly bends
alternately in time and space. These bending waves have very low amplitude and
we observe them in a vibrated two-dimensional layer of photoelastic particles.
