What is the cause of a sonic boom?

A sonic boom is caused when an object travels through air faster than the speed of sound and displaces air molecules in its path. The molecules become highly pressurized and then release, causing the loud sound. The intensity of the boom depends primarily on the object's altitude.

When an aircraft is traveling at a supersonic speed, it actually creates two booms that are usually so close together they sound like one boom to human ears. This is caused by two cones formed by pressurized air that are displaced as shockwaves, one at the front and one at the back. The pressure change is typically only a few pounds in strength, but it is displaced and released so quickly that it causes a thunderous sound. When an aircraft is at higher altitudes, the cones spread out wider and weaken as they move outward.