Final answer:
A sonic boom indicates that an aircraft has surpassed the speed of sound, causing a cone-shaped shock wave with auditory manifestations heard once the aircraft is already beyond the observer's sight.
Step-by-step explanation:
Experiencing a sonic boom is indeed evidence that an aircraft has exceeded the speed of sound moments before. A sonic boom is created by constructive interference of sound waves produced by an object moving faster than sound, forming a cone-shaped shock wave due to superposition. This shock wave sweeps along the ground, hence it is not until after the aircraft has surpassed the speed of sound and traveled some distance that the boom is heard.
An aircraft generates two sonic booms, one from its nose and another from its tail, as it travels through the air creating a bow wake similar to that trailing a boat. The distance between the two booms is directly related to the time it takes the aircraft to pass a given point. Observers on the ground often do not see the aircraft because it has already moved past before the shock wave, which carries the sound of the boom, reaches them.