When you hear a sonic boom, you often cannot see the plane that made it. Why is that?
If the plane is traveling at the supersonic speeds, it is going faster than the speed of its sound. As a result, a pressure (sound is variation in pressure) wave is produced in the shape of the cone whose vertex is at the nose of the plane, and whose base is behind the plane. The angle opening of the cone depends on the actual speed the plane is traveling at. All of the sound pressure is contained in this cone.
So imagine now this plane in a level flight. Before the plane passes you, you can only see it but you can not hear anything. The pressure cone is trailing behind the plane. Once your ears intersect the edge of this cone, your will hear a very loud sound - the sonic boom. Therefore you will hear the sonic boom once your ears intersect this cone, and not when the plane breaks the sound barrier