ELI5: Does a sonic boom only occur at the moment you break the sound barrier or is it constant and if it is does it get louder the faster you go?

It is constant.

A sonic boom is a shock wave that travels outward from the plane. It follows the plane wherever it goes. When the plane flies by a person on the ground, the shock wave will hit that person (and be heard as a “boom”) shortly after. Since the boom moves at the speed of sound, and the plane is moving faster than that, a cone shape develops, with the plane at the point of the cone. Here’s an example image.

As for speed vs. loudness, it’s actually the opposite of what you’d expect. The faster the plane, the skinnier and pointier the cone. As a result, the energy of the shock wave gets spread out over a longer distance. Additionally, super-fast aircraft tend to fly at much higher altitudes, meaning the boom travels through a lot more air before it reaches the ground, dissipating it. So sonic booms actually get quieter the faster the plane flies.

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