Take the example of an aeroplane. When it is flying below the speed of sound, any air pressure disturbances that may arise will travel at the speed of sound, right in front of the plane. The airflow gets adjusted and disturbances get scattered.

Below the Speed of Sound

When the aeroplane speed picks up and reaches that of the speed of sound, then a resistance builds up. Instead of scattering or disappearing, the disturbances build up and the aeroplane gets entangled with the pressure waves it is creating (with the forward thrust).
 

At the Speed of Sound

But as the flight gradually increases its speed, the jet squeezes the sound waves and compresses them together to form a shock wave. These powerful waves are formed because airflow does not have sufficient time to get adjusted. This cone shaped wave (shown in the diagram) spreads out and behind the plane. When and if part of this wave reaches the ground, it can be powerful enough to damage buildings. The loud bang that is heard is called a sonic boom.

Higher than the Speed of Sound

Nearly half a century before anyone knew about the 'supersonic' (faster than the speed of sound) phenomenon, an Austrian physicist, Ernst Mach (1838-1916) had discovered the theory behind shock waves. The different levels of aircraft speeds were named in his honor -

• Mach 1 signifies an aircraft flying at the speed of sound (all passenger aircrafts except the Concorde are subsonic - they fly at speeds below that of sound)

• Mach 2 is an aircraft going at twice the speed of sound (Only the Concorde can handle this speed, for which reason it is called Supersonic)

Generally, flights are not allowed to make sonic booms over land, but you may have heard other, less-powerful examples, such as the 'crack' of a speeding rifle bullet, or a crack of a whip. They are all very power-packed yet startling experiences!