What causes a sonic boom?
Air reacts like a fluid, so as an aircraft goes supersonic the air molecules are pushed aside with great force, forming a shock wave much like a boat creates a bow wave.
The shock wave forms a cone of pressurized air molecules which move in all directions forward and back, and extend to the ground.
As the cone spreads across the landscape along the flight path, they create a continuous sonic boom along the full width of the cone's base.
The sharp release of pressure, after the build-up by the shock wave, is heard as the sonic boom.
The change in air pressure associated with a sonic boom is only a few pounds per square foot; it is the rate of change, the sudden onset of the pressure change, that makes the sonic boom audible.
All aircraft generate two cones, at the nose and at the tail.
They are usually of similar strength and the time interval between the two as they reach the ground is primarily dependent on the size of the aircraft and its altitude.
Most people on the ground cannot distinguish between the two, and they are usually heard as a single sonic boom.