  We experimentally and theoretically investigate the lowest-lying axial
excitation of an atomic Bose-Einstein condensate in a cylindrical box trap. By
tuning the atomic density, we observe how the nature of the mode changes from a
single-particle excitation (in the low-density limit) to a sound wave (in the
high-density limit). Throughout this crossover the measured mode frequency
agrees with Bogoliubov theory. Using approximate low-energy models we show that
the evolution of the mode frequency is directly related to the
interaction-induced shape changes of the condensate and the excitation.
Finally, if we create a large-amplitude excitation, and then let the system
evolve freely, we observe that the mode amplitude decays non-exponentially in
time; this nonlinear behaviour is indicative of interactions between the
elementary excitations, but remains to be quantitatively understood.
