One probably has seen the above animation, the falling of a slinky. A slinky is a soft spring. In this animation, the down side of the slinky shows peculiar behavior during falling. There has been plenty of videos and articles to explain this. The point I want to emphasis is, this happens when the falling (which is not a free fall) speed is larger than the speed of sound (elastic wave) in the slinky. In other words, the falling induces a shock wave.
Shock wave almost always has a sharp wave front. In the case of blast wave, the shock wave creates a visible wave front through the so-called shadow effect.
|Shock wave from the Trinity Test|