之前有人跟我討論過這個問題，我發現我不是非常了解，於是做了一些Study
Wikipedia:
A sonic boom is the sound associated with the shock waves created by the supersonic flight of an aircraft. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion.
NASA:
A sonic boom is the thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound or supersonic.
Air reacts like a fluid to supersonic objects. As objects travel through the air, the air molecules are pushed aside with great force and this forms a shock wave much like a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces.
NASA的解釋似乎比較詳細："對超音速的物體來說，空氣就像是液體；當這個物體在空氣中移動，空氣分子會被強大的力量推開，而這會造成一股震波如同船再水面上行進時產生Bow Wave；而飛行器如果越大越重則越多的空氣會被推開"
Bow Wave:
這樣產生了一個疑問，為什麼會在突破音障才會發生Sonic Boom?
When an object passes through the air, it creates a series of pressure waves in front of it and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, and as the speed of the object increases, the waves are forced together, or compressed, because they cannot "get out of the way" of each other, eventually merging into a single shock wave at the speed of sound. This critical speed is known as Mach 1 and is approximately 1,225 kilometers per hour (761 mph) at sea level at room temperature. In smooth flight, the shock wave starts at the nose of the aircraft and ends at the tail. Because directions around the aircraft's direction of travel are equivalent, the shock forms a Mach cone with the aircraft at its tip.
The shock wave forms a cone of pressurized air molecules which move outward and rearward in all directions and extend to the ground. As the cone spreads across the landscape along the flight path, they create a continuous sonic boom along the full width of the cone's base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom.
Q:什麼時候聽得到Sonic Boom?
其實只要在一個超音速移動的物體附近就可以聽到Sonic Boom，例如戰機在超音速飛行時飛過你的頭頂...
Q:These waves travel at the speed of sound, and as the speed of the object increases, the waves are forced together, or compressed, because they cannot "get out of the way" of each other, eventually merging into a single shock wave at the speed of sound. 這句似乎還是解釋不清？
我想這還要牽扯到Speed of Sound還有Longitudinal wave，這時候已經有點頭昏眼花，改天再研究....
最後我覺得這段非常有趣....
Some common whips such as the bullwhip or sparewhip are able to move faster than sound: the tip of the whip breaks the sound barrier and causes a sharp crack—literally a sonic boom. Firearms since the 19th century have generally had a supersonic muzzle velocity. However, the sound barrier may have been first breached some 150 million years prior to the inventions of these implements. Some paleobiologists report that, based on computer models of theirbiomechanical capabilities, certain long-tailed dinosaurs such as apatosaurus and diplodocus may have possessed the ability to flick their tails at supersonic velocities, possibly used to generate an intimidating booming sound. This finding is theoretical and disputed by others in the field.
大意是當牛仔甩牛鞭的時候，牛鞭尖端移動速度會比音速還快，這產生一個尖銳的聲音，這聲音其實就是音爆。"有古生物學家研究指出，根據電腦模型推算，有一些尾巴很長的恐龍如雷龍(apatosaurus)和梁龍(diplodocus)，能以超音速甩動他們的尾巴，以產生驚嚇敵人的聲音。
附上Bullwhip影片
Reference:
http://en.wikipedia.org/wiki/Sonic_boom
http://www.nasa.gov/centers/dryden/news/FactSheets/FS-016-DFRC.html
