I don’t understand either, but read that supersonic objects generate shockwaves at nose, root of wings(if any), and tail. Shockwaves also may reflect off terrains.
Possibly it should always be “BAbababang-baaaang…”, and the first three might be either indistinguishable or ones after the first are too weak, except it becomes noticeably apart on extremely long objects like F9?
In this case the two booms come from the "nose" (the engines, since at this point the booster is flying backwards) and from the grid fins at the far end. Given that the Falcon 9 boosters are about 70m tall, that would mean about a fifth of a second delay between the two booms when the booster is traveling at Mach 1.
Possibly it should always be “BAbababang-baaaang…”, and the first three might be either indistinguishable or ones after the first are too weak, except it becomes noticeably apart on extremely long objects like F9?
1: https://www.nasa.gov/centers/armstrong/multimedia/imagegalle...