A sonic boom is a shock wave. When an object is going faster than the speed of sound, you aren't going to hear it until the shockwave reaches you. If it temporarily goes supersonic, it will produce a shock wave that continues on without it, which will weaken over time. If it continues flying at the speed of sound, it will travel with the shock wave, maintaining its strength. If it continues going faster than the speed of sound, it will leave a trailing shock wave.
You can visualize it like the bow wave from a boat. The faster the boat goes, the more the wave become conical behind it. When that wave hits, that's the sonic boom. It's the increase in pressure caused by the shock wave that creates the "boom". Depending on the shock strength, it may not be very strong.
A "shock wave" can be of any strength. It's just an instantaneous change (in pressure, temperature, etc.).