I am a new SolidWorks Simulation user and after completing a Random Vibration analysis, I ended up with an Acceleration Plot that looks confusing to me. I am hoping that someone can give me an explanation to what I am seeing in the acceleration plots.

In order to easily demonstrate my question, I modeled a 30” stainless steel cube and applied a random base excitation. The input is a 10.0 (g^2)/Hz from 0.0 to 30.0 Hz. The first mode of the cube is at 680Hz, so I was expecting the cube to be “rigid” at 30 Hz and have a uniform acceleration. However when I plot the PSD or RMS acceleration, the base elements of the cube show the input acceleration but everywhere else the acceleration is practically zero.  Considering the cube is practically rigid in the input frequency range, I would think the whole cube will have a uniform acceleration which is the same as the input. I have included images of one PSD and one RMS plots.

I am hoping someone can help me interpret these results, thank you in advance for the help.