In: Physics
Standing waves do not form under just any circumstances. They require that energy be fed into a system at an appropriate frequency. That is, when the driving frequency applied to a system equals its natural frequency. This condition is known as resonance. Standing waves are always associated with resonance.
In our case, for open pipe - the ends must have antinodes for standing waves; for one end closed - the closed end should have a node and the open node an antinode for standing waves. We have calculated the case where the wavelength is maximum possible that satisfies the above constraints.