Sonic boom is a common name for the loud noise that is created by the 'shock wave' produced by the air-plane that is traveling at speeds greater than that of sound ( speed of sound is approximately 332 m/s or 1195 km/hr or 717 miles/hour). These speeds are called supersonic speeds, hence this phenomena is sometimes called the supersonic boom.
Normally, for a plane that is going at subsonic speeds (lower than that of sound), the sound of the plane is radiated in all directions. However, the individual sound wavelets are compressed at the front of the plane and further spread at the back of the plane because of the forward speed of the plane. This effect is known as the Doppler effect and accounts for the change of the 'pitch' of the plane's sound as it passes us. When the plane is approaching us it's sound has a higher pitch than if it is going away from us.
Now, if the plane is traveling at the supersonic speeds (greater than that of sound), it is going faster than it's own sound. As a result, a pressure (sound is variation in pressure) wave is produced in the shape of the cone whose vertex is at the nose of the plane, and whose base is behind the plane. The angle opening of the cone depends on the actual speed the plane is traveling at. All of the sound pressure is contained in this cone.
So imagine now this plane in a level flight. Before the plane passes you, you can only see it but you can not hear anything. The pressure cone is trailing behind the plane. Once your ears intersect the edge of this cone, your will hear a very loud sound - the sonic boom. Therefore you will hear the sonic boom once your ears intersect this cone, and not when the plane breaks the sound barrier (as it is commonly misunderstood)
The sonic booms can be sometimes quite loud. For a commercial supersonic transport plane (SST), it can be as loud as 136 decibels, or 120 Pa (in units of pressure).
Answered by: Anton Skorucak, M.S. Physics, PhysLink.com Creator