So a standing wave is one which looks to be standing still at certain harmonic frequencies. At these frequencies, when the wave reflects off the end, it will undergo a phase change of 180 degrees. This creates constructive interference as shown below: In the case of a wave on a string, since that would constitute a transverse wave, when the wave hits a barrier, much like with thin film interference, the reflected wave is shifted by 180 degrees. in the case of a longitudinal wave, such as with an open-closed or open open air pressure system, the phase change occurs at the open end(s) but not at the closed ends. What I dont understand is, if the reflected wave is constructive, and in phase with the incident wave, then why are standing wave diagrams always drawn with the reflected wave out of phase/having destructive interference? Am i misinterpreting the diagrams? See below for standing waves: See below for Longitudinal waves: (Open Open = phase change at both ends) Open Closed = Phase change only at open end. Also otherwise, is my understanding of what's going on (with the reflections and phase changes) accurate? Thanks guys!