  We excite ion acoustic waves in ultracold neutral plasmas by imprinting
density modulations during plasma creation. Laser-induced fluorescence is used
to observe the density and velocity perturbations created by the waves. The
effect of expansion of the plasma on the evolution of the wave amplitude is
described by treating the wave action as an adiabatic invariant. After
accounting for this effect, we determine that the waves are weakly damped, but
the damping is significantly faster than expected for Landau damping.
