It's not. The shock wave is present as long as the plane is in super-sonic flight.

I grew up in fly-over land, and military training flights based out of the Dakotas used to go overhead; we would get the occasional boom.

(Also, once in a while doing my farm-kid chores driving a noisy piece of machinery across a hay field I would get buzzed by a jet sneaking up from behind me flying at a very low altitude. It was always startling because I never heard them coming over the noise of the tractor. I'm sure I made a pretty good surrogate for an enemy tank...)