How it is created?
If a supersonic plane flies over your house, even very low, won't you hear that sonic boom, won't your window panes shatter? Even if it had reached supersonic soon after take off from Sydney or Singapore?
In this hub, I tried to explain something I know.
Richard Hallion is the master of Aviation History. This book looks closely at the USAF's X-1 program and the US Navy's D-558 program and examines the NACA's roll in shaping the avaition research being undertaken at that time.
What is "sonic boom"?
When you move your hand, the air particles that come in contact with your hand get pushed. These air particles transmit this push to their neighboring air particles, who convey the same to their neighbors, like ripples through water. This transmission of pressure across air is called "Pressure Wave".
When you speak, you create a "Pressure Wave" in the air, which travels (like ripples through water) and reaches the listeners' ears and pushes those ear drums.
Thus, "Pressure waves" travel at speed of sound. Therefore, when you move your hand, the air particles create a pressure wave, and this wave causes the air to move away from the path of the hand, as if the air is "making way" for the hand to move by going out of the path. Had air not moved away from the path of your hand, then air would have got compressed, resulting in increase in pressure and density in that region.
This is very evident in a "wind tunnel" (where they experiment with aircraft models). The air that is blown towards that model separate at a considerable distance before the leading edge of that plane model and flow around the plane in a "streamlined" flow.
How did the air know that it must move away?
Because of the pressure wave that originated from the leading edge.
Now, if the air was blown towards the plane at the speed of sound, then, before the pressure wave could reach them, the air particles would have already collided with the aeroplane model. Before they could generate a pressure wave, the subsequent particles too would have collided with them, AND THERE WOULD BE A GREAT INCREASE IN DENSITY AND PRESSURE OF AIR IN FRONT OF THAT PLANE. THE AIR IN TOUCH WITH THAT PLANE WOULD BE LIKE A WALL, AS COMPARED TO REST OF THE REGION.
This portion of air, in touch with the plane's leading edge, where there is a great increase in density and pressure (and temperature too), down stream of which there is a complete disruption of airflow is called a SHOCK WAVE.
This shock wave causes the sonic boom. It is more severe at lower heights and as altitude increases, the air becomes less dense, more cold (slower speed of sound) and the sonic boom is also less intense.
As I am writing, I remember reading as you accelerate well beyond Mach 1, greater your speed, the shock waves move from your leading edge to the trailing edge, get tilted at an angle (the disruption of airflow is in a direction perpendicular to the shock wave) and therefore less disruptive and somehow not as intense as the one which appears while approaching mach 1. This could be the reason why we don't hear window pane shattering sonic boom from a bullet going at mach 2.