We performed the experiments with several different wave amplitudes and
      frequencies. The parameters are shown in Figure 4. The Brünt-Väisärä frequency in
      these experiment is 1.4(s-1).
      When the amplitude of the wave is small, mean flow does not occur and the
      wave only propagates downwards. Animation 1 shows a typical movement of fluid visualized
      with floaters in the case of no mean flow. Since the floaters that are located far from
      the wave source move with small amplitudes, it is obvious that the waves are weakened
      during propagation.