Image from hyperphysics.phy-astr.gsu.edu
At supersonic speeds (those greater than the local sound speed), there is no sound heard as an object approaches an observer because the object is traveling faster than the sound it produces. Only after the object has passed will the observer be able to hear the sound waves emitted from the object. These time periods are often referred to as the zone of silence and the zone of action. When the object has passed over the observer, the pressure disturbance waves (Mach waves) radiate toward the ground, causing a sonic boom. The region in which someone can hear the boom is called the boom carpet. The intensity of the boom is greatest directly below the flight path and decreases on either side of it.
Image from aviation.stackexchange.com
And one video: Breaking the sound barrier [Jets Compilation]