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

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.