I see I was not the only responder. While I am quite sure that the
most important effect was the one I gave, I will relate a story about
surprising atmospheric sound propagation. On a peaceful Sunday
morning a little over 25 years ago, my son and I were quietly fishing
from two separate canoes on Deer Lake here in Burnaby. (He was
teaching me how to catch. I knew how to fish; he knew how to catch.)
Suddenly there was a very loud, very sharp report, as though someone
was blasting a stump in the neighborhood. My wife was eight or nine
kilometers away in a church and she reported a loud sound like a car
running into the building. The sound was heard over a large area
around Vancouver.
It was not, as you have guessed, a discourteous neighbor of mine, but
rather it was the explosion of Mt. St. Helens, hundreds of kilometers
to the south of us. The sound was channeled somehow, and an inverse
square calculation is utterly inappropriate for this phenomenon. Most
amazing of all, the event was inaudible in Seattle, halfway between
us and the mountain.
There is no doubt in my mind that the sound was very loud, though the
environment was quiet. There was no drawn-out rumbling, either; it
was a short, sharp, loud bang.
I didn't catch any fish, however. David got a nice rainbow, as he
always did. Same with supernovae and magnetar explosions.