In the plot above, taken from space craft system engineering book, chapter 2.
This is the plot showing the random vibration power spectrum of Ariane 4. The unit is $g^2/Hz$, I understand how the unit came by reading the Wikipedia page of spectral density. But I am unable to comprehend the physical meaning of this graph.
As far as I understand it, this is the power spectrum of the acceleration signal experienced by any payload inside the fairing. How does a payload engineer go about from looking at this graph to testing the payload for such vibration? I mean do they reverse calculate signal required to feed in vibration table having same spectral density? How is this graph practically used and understood?
Edit: I drew question mark in the plot, because I don’t get what acceleration squared per unit frequency means physically.