The instationary convective flow produces pressure fluctuations that travel as sound waves in the stellar interior and transform into shocks in the photosphere. The dashed circle marks the "core" of the model where energy is injected. Note, that the amplitudes of the waves are much smaller in the interior than in the atmosphere: for the movie they have been scaled properly to make them visible everywhere. The waves travel very fast in the center because of the high sound speed due to the high temperature in these region.