On the horizontal axis: distance to the piston. On the vertical axis: time. One period of the initially sinuisoidal wave is shown in its propagation away from the piston and simultaneous distorsion. The shape of the wave is shown at different distances from the wall.

In each little wave shape diagram, the horizontal axis shows the pressure deviation from the background pressure in the wave. We see that in the wave shape most to the right, that the pressure changes here changes very rapidly with time in the middle of the wave: a shock wave is here developing.