Why does a sound wave reflect at the open end of a pipe?
Let us imagine a pulse of high pressure and therefore high density air travelling down the tube. When it reaches the end of the tube, its momentum carries it out into the open air, where it spreads out in all directions. Now, because it spreads out in all directions its pressure falls very quickly to nearly atmospheric pressure (the air outside is at atmospheric pressure). However, it still has the momentum to travel away from the end of the pipe. Consequently, it creates a little suction: the air following behind it in the tube is sucked out (a little like the air that is sucked behind a speeding truck). Now a suction at the end of the tube draws air from further up the tube, and that draws air from further up the tube and so on. So the result is that a pulse of high pressure air travelling down the tube becomes a pulse of low pressure air travelling up the tube. We say that the pressure wave has been reflected at the open end, with a change in phase of 180.
