Consider a tube with both ends open (or a string
with both ends free to move) .. we can see the evolution of the standing
wave as time goes on. We can select different frequencies of the
oscillation (fundamental and then several harmonics).
The top curve shows the initial wave, the middle curve shows the "reflected" wave returning, and the bottom shows the combined waves thus, what the pressure wave (string shape) would actually be.
Observe : with an open end on the right, watch what happens when the first edge of the wave reaches it .. the wave reflects back "on top" - compare that to the closed end, where the returning wave inverts and come back "upside down".
Credits: Mostly created by Ali Loewy and Andrew Duffy at Boston University - modified partially by Scott Schneider.