Here are the wave-front models for both:
I am in an introductory physics course. Just learned about this. I am not entirely sure if sonic boom is louder. But from what I saw, it's loudness is certainly being emphasized more than the sound of an object traveling at the speed of sound makes.
My understanding is that for both, their loudness come from the constructive interference of their wavefront overlapping. But from this picture, I see no reason why there has to be more constructive interference for sonic boom than there has for the speed of sound.