A sonic boom as an impulsive wave similar to thunder caused by an object exceeding the speed of sound, according to Sky-flash.com. The speed of sound is approximately 750 mph at sea level.
As an aircraft flies through the atmosphere, a pressure wave builds up on the nose of the aircraft similar to the wake produced by a boat moving through water. When the aircraft exceeds the speed of sound, these pressure waves collide once they slip past the supersonic aircraft, creating a shock wave. The booming noise is a result of this shock wave, which travels forward along the plane's flight path.