Vertical velocity u_z in 1D test model. The color scale is set to span  ± 30 km s^-1 with black representing upflow and white downflow. The chromosphere adjusts its structure, which initially is not quite in hydrostatic equilibrium, by emitting acoustic waves at the cut-off frequency. These waves are initially strong enough to form shocks, as here during the first 1000 s of the simulation.