A sonic boom is a loud noise which is produced when an object displaces air so quickly that it forms a shock wave. Some examples of things which can cause sonic booms include: lightning, the space shuttle, bull whips, and extremely fast aircraft. People hear sonic booms as low booming sounds, with the boom often sounding a double beat. Sonic booms are of scientific interest, and they are also a concern, because they can cause damage or generate noise pollution; many militaries have dealt with a number of civilian complaints as a result of sonic booms generated by their aircraft, for example.
The science behind a sonic boom can be easily visualized by thinking of air as a medium like water. When something moves through water, it causes a visible ripple effect, which can turn into a wake if the object moves so quickly that the ripples start to overlap each other. The same thing happens in the air, even if it can't be seen. Sonic booms occur when the ripples of air displaced by a moving object become compressed, resulting in the formation of a shock wave. This shock wave becomes a very strong sound wave which will be audible to observers when the compressed air reaches them.
The shape of the wake of air is known as a Mach Cone. The width of a Mach Cone varies, depending on the speed of the object as it moves through the air. Sometimes, part of the Mach Cone becomes visible as water vapor in the air gets trapped between pressure waves and it briefly condenses, causing a cloud of condensed vapor to form a halo around the object.
Sonic booms occur when aircraft “break the sound barrier,” meaning that they are traveling faster than the speed of sound, and they also continue as long as the aircraft moves at high speed, moving behind the aircraft just like a wake of water behind a boat. The closer a plane is to the ground, the more intense the sonic boom will feel, because it will have traveled a shorter distance, retaining much more of its energy. Sonic booms generated by lightning are better known as thunder, and the booms created by bull whips are responsible for the distinctive cracking noise of a bull whip in action.
In response to complaints about sonic booms caused by aircraft, several nations have laws which explicitly forbid aircraft from breaking the sound barrier, unless they are over water, where they will be less likely to cause upset. People often find the noise of a sonic boom irritating, especially if they live in high traffic areas where many aircraft generating sonic booms may fly overhead, and the pressure wave of a sonic boom can also break windows and cause other types of damage.