Sonic boom

A sonic boom is the audible component of a shock wave in air. The term is commonly used to refer to the air shocks caused by the supersonic flight of military aircraft or (relatively rare) passenger transports such as the Concorde and the Space Shuttle.

The shock wave from a supersonic aircraft is in the form of two long cones centered on the nose and tail of the aircraft, opening to the rear. For smaller aircraft the shocks are close enough together that they sound like one when they pass, but for larger craft the two remain distinct and cause a "double boom".

As the aircraft increases speed the cones grow "tighter" around the craft, and do not become much "louder". At very high speeds and altitudes the cone does not intersect the ground, and no boom will be heard. The power, or volume of the shock wave is dependant on the amount of air that is being sped up, and thus the size of the aircraft.

In the late 1950s when SST designs were being actively pursued it was thought that although the boom would be very large, they could avoid problems by flying higher. This premise was proven false when the North American B-70 Valkyrie started flying and it was found that the boom was a very real problem even ay 70,000ft. A wave of fear-mongering broke out and eventually doomed most SST projects.