I can't comment on the volcano, per se, but many waves in non-linear media have wavelength-dependent velocities that can "un-disperse" (my terminology) as they propagate -- i.e., pulses actually sharpen up instead of dispersing. In some media -- e.g., long-distance water canals in Victorian Scotland -- there are also *solitons* which bunch up into a characteristic shape which neither disperses nor sharpens. See also "pulse compression", including "chirped pulse amplification"; radar, laser. At 11:18 AM 10/18/2016, Bill Gosper wrote:

https://www.youtube.com/watch?v=3ydnLkVvfZM 2:44 fuzzy bright flash. Commenters offer unconvincing conjectures. Same volcano, https://www.youtube.com/watch?v=BicT13ecUbc 0:19 et al, visible "shock" waves. About as far as possible from Iceland, https://www.youtube.com/watch?v=BUREX8aFbMs 0:12, much larger "shock" wave. Also note ocean impacts ~ 0:34. I'm puzzled by how such a large source can make such a sharp noise. A significant volume of gas has to conspire to come out of solution all at once. The grandaddy was Krakatoa, the loudest noise in recorded history, blowing out sailors' eardrums many miles away. Why "bang" and not "boom"? Was there ear damage to witnesses of thermonuclear atmospheric tests? In the videos, when buildings and stuff suddenly move away from, or get flattened by the explosion, those are actual shock waves-- pistons of air moving faster than sound.

Maybe that explains the "bang". The supersonic trailing gas piles up against the leading edge, which is trying to slow down to Mach 1. Sort of like how tsunamis trip over themselves. --rwg