If a source of waves moves through a medium faster than the speed of waves in that medium, waves are left behind the source in an expanding cone. The slope of the sides of the cone is given by the ratio of the speed of the source to the speed of sound in the medium. At the surface of this cone, waves emitted close to one another in time pile up, producing a conical region of massive constructive interference called a shock wave .
Boats usually travel through water at speeds faster than the speed of water waves.
They generate V-shaped wakes that rise relatively high above the surrounding water. This is an example of a two dimensional shock wave. Airplanes traveling faster than the speed of sound in air produce conical shock waves. As the outside of the cone travels past observers on the ground, they experience this region of constructive interference as a sonic boom .