A Power Spectrum Density (PSD) analysis is just a sophisticated and extremely useful form of modal frequency analysis. Instead of having to interrogate multiple results sets, the PSD approach excites the structure using a broad spectrum acceleration load and then sums up the solutions into one single valued result. One can also think of it as a white noise or broad band excitation where the structure is excited at all frequencies simultaneously.
The approach is statistical and the displacement and stress results are termed 1-σ responses.
The sigma (σ) refers to a Gaussian distribution where 1-σ to 3-σ refers to 68, 95 and 99.7% probability ranges.
At 1-σ you have a 68% chance that the stresses are within this value. For many applications, one is required to use a 3-σ interpolation.
