Ah actually the Wikipedia page explains better - from the perception of someone on the ground it’s a single boom, because the sound is caused by the wave front

Ah actually the Wikipedia page explains better - from the perception of someone on the ground it’s a single boom, because the sound is caused by the wave front of the shockwave cone. But the cone is continuous along the path. https://en.m.wikipedia.org/wiki/Sonic_boom https://en.m.wikipedia.org/wiki/Sonic_boom