The movie starts from rest, and after the paddle motion begins, the wave field starts to fill the tank outwards from the paddle. The paddle frequency is much higher than the first sequence, so that the characteristic slope of the rays is much steeper. Waves of a specific frequency can only propagate at a specific angle. Note also that since , the maximum allowed frequency is N, the frequency of the buoyancy oscillation. The much higher frequency makes the characteristic angle much steeper.
Energy travels in nearly vertical paths, and phase propagation is nearly horizontal.
If the paddle frequency were to exceed N, internal waves could not propagate, and the energy would be trapped in the vicinity of the paddle. The buildup of energy would create a lot of local mixing. The dynamics of the upper ocean. Cambridge University Press.