Dave Looman
Ansys Employee

There are certain structures where the modes in one direction are very high.  For example, a thin flat plate will have low frequency modes in the direction perpendicular to the plate, but only very high frequencies in the plane of the plate.  Even a cantilever beam will have low bending frequencies, but very high axial frequencies.  So it's natural for models to have a low REM in one direction and that doesn't invalidate the PSD analysis, particularly if the direction of the PSD is not in the direction producing the low REM.