Phase change of longitudinal wave during reflection I know that a transverse wave will undergo a phase change during reflection. I wonder how a longitudinal wave (e.g. sound wave) undergoes the phase change during reflection (or will it happen?)
For a transverse wave, the wave pattern will be reflected along the equilibrium line, i.e. a crest becomes a trough or vice versa. For a longitudinal wave, will the compression become a rarefaction?
 A: Since at the point of reflection on the 100 % reflecting surface the incident and reflected waves must cancel each other--i.e. the combined amplitudes must equal zero. This is because there is no wave beyond the 100 % reflecting surface by definition. It is a boundary condition. 
Then the incident wave compression must become a rarefaction in the reflected wave.
A: Longitudinal wave reflections can have a phase change or not depending on the properties of the surface they are reflecting from. For example, when a sound wave reflects from a hard surface, there is no phase change. The incident and reflected wave pressures add at the surface, since the acoustic impedance of the solid wall is much larger than that of air. The opposite occurs for reflection of acoustic waves traveling from a solid, or water into air, and there is a phase reversal at the interface. 
Also with a resonant tube open at one end, standing waves occur for an odd number of quarter wavelengths, since the reflection at the closed end does not have a phase change, but at the open end phase of the reflection is reversed forming a null.
