I am starting work on structural durability area for after treatment systems and deal with Random Vibration and PSD profiles quite often. However there are few fundamental questions about PSD profiles that I could not get answer to after a lot of search on internet. So finally decided to write to you. Here are my questions: 1. Why random vibration is represented in Power Spectrum (g^2/Hz) and not amplitude Spectrum (g) ? Both of them represent data in frequency domain and if we assume the random vibration data to be repeatable (which is the case almost always), isnt amplitude spectrum good enough ? 2. What are the steps for calculating PSD from time data ? (Like we do fourier transform on time data to get amplitude specptrum). 3. When we use PSD profiles as input excitation for shaker test, how is spectrum is converted to time data ? When we test the component for X hours with input PSD profile, is the PSD profile repeated certain number of time in that given duration ? If yes how is that done ? (I am trying to correlate to a sine sweep test where we know the time required to cover frequency range and the cycle is repeated a number of times over total duration).