Neighbors, such as they are, have hauled out the evening’s ordnance. For some folks, the expression is “any excuse for a party”, for these folks it is “any excuse for lots of explosions”.
So, I am outside, and I hear the mortar thump in the next block, and the charge reports right over my head, give or take a football field. There is an echo off the hill to the west, but the report resonates for three or four seconds, oscillating in strength on the order of two or three times a second.
My question is, what drives those repeated oscillations? I understand that a shockwave is supersonic, so that will contribute to one or two of the oscillations, but this seemed to last quite a while. (It is in the range of 25°F/80% humidity.)
I started athread a while back about the same thing, I think.
I’m curious as well.
Now I have a sudden urge to whip out my The Fifth Element DVD. Thanks a lot, eschereal 
A couple of years ago, we were triangulated between three neighbors who had all apparently run up to New Hampshire just before the Fourth and bought the same bargain package. Instead of a few nice rockets and sprays, which are tolerable and shared fun (and also illegal here, but we’re out in the woods)… they all had high-powered aerials, which is short for “aerial burst,” which is code name for “intense anti-aircraft fire.”
Our house shook and windows rattled for an hour and at least two hundred explosions. My dogs were so terrified they were laying on the floor in puddles of their own piss. We could not go outside to shoot off our own feeble 'works because it was literally deafening.
The next year, I had a friend on speed dial, awaiting the first thump. One of his hats is Public Safety Officer. Another is Assistant Fire Marshal. Another is PS liaison to the town. A fourth is deputized trooper.
I did not have to call, that year or the next, which was last year. Maybe even the idiots went deaf and figured out more than a few thrilling thumps was too much.
What you were hearing was echoes - an intense point sound will reflect off many surfaces and come in at varying times.