* Sonic booms are caused by a pressure wave: When a plane travels faster than the speed of sound, it pushes the air out of its way. This creates a cone-shaped pressure wave that travels behind the plane at the speed of sound.
* The pressure wave is momentary: The sonic boom is the moment this pressure wave passes a stationary observer on the ground. It's a single, sharp sound, not a continuous one.
* The pressure wave exists continuously: While the sonic boom is a single event, the pressure wave itself exists continuously as long as the plane is traveling faster than sound. However, a stationary observer only hears the boom once when the wave passes.
Think of it like this: Imagine throwing a stone into a pond. The ripples in the water spread out from the point of impact, but you only hear a "splash" when the ripples hit the edge of the pond. Similarly, the pressure wave from a supersonic plane is continuous, but the sonic boom is the "splash" you hear when the wave reaches your location.
