  We study the vibrational spectrum and the low energy modes of a three
dimensional colloidal crystal using confocal microscopy. This is done in a
two-dimensional cut through a three-dimensional crystal. We find that the
observed density of states is incompatible with the standard Debye form in
either two or three dimensions. These results are confirmed by numerical
simulations. We show that an effective theory for the projections of the modes
onto the two-dimensional cut describes the experimental and simulation data in
a satisfactory way.
