# Shock-waves, Bangs and the Speed of Sound

I was watching this video of an erupting volcano. Some Guys in the comments tried to estimate how far away the volcano is by using the delay until the "shock-wave" hits the camera and the speed of sound in air. However, shock-waves can travel much faster than the speed of sound.

Is there a way to tell (or estimate) if what we are hearing is actually (still) a shock-wave? From this paper on hand clapping and shock-waves I conclude, that not all "bangs" need to be caused by shock-waves. And shock-waves can be only very shortly lasting, especially if there is not much energy involved.

And is the result viable when we calculate the distance in the way described above?

## 1 Answer

The actual shock wave is quite short lived (I think it's visible for less than a second near 0:14 as a white sheet around the smoke/dust cloud) and doesn't propagate very far in this case. When the shock dissipates what's left is a pressure wave, the "bang" or sonic boom, and that propagates at the speed of sound.

So I guess your video uses a reasonable method, but sloppy terminology.