Hi, I would like to know about what exactly constitutes a shockwave. Let me write out what I understand of it. There are two explanations I have come across. 1) The shockwave is caused when gas particles move faster than the speed of sound due to some external factor (a plane, an explosion or so). Let's try the plane. When it starts to go faster than the speed of sound, there is a buildup of pressure around the front of the plane (and a corresponding drop around the rear). This outward shockwave travels faster than the speed of sound because the plane pushes the particles faster than the speed of sound. Wikipedia, among others, calls this the shockwave here. 2) The shockwave is just a superposition of regular sound waves. When the plane travels faster than sound, the wavefronts can interfere constructively. This IS the shockwave according to some explanations. Wikipedia's explanation for the sonic boom is that it is a shockwave and then it describes this thing above. You can see that here I have two questions: a) Which one is the "shockwave", 1) or 2)? b) Also, going back to 1), after that region of high pressure gets away from the plane, I believe it propagates as particles moving faster than sound until it degenerates into a normal sound wave. For an observer, will he then hear the wave described in 1), then silence, then the superposed waves described in 2) since the wave in 1) gets ahead due to its superior speed?