Overview

Power spectral density input function, this is a probability distribution, as are the resulting output functions. The shape of the function really depends on how the likely magnitude of the force varies as a function of its frequency and on how probable the loading is for each frequency. If you want the magnitude of the probable load to be the same at all frequencies, then the function should be constant. The Hz in the denominator means that the probability is a density that can be integrated over the frequency range to get a single RMS value.

Following is a link to a document that provides an explanation of how the power spectral density function can be obtained from random vibration time history data using a bandpass filtering method: http://www.linarisrl.com/POWER%20SPECTRAL%20DENSITY%20UNITS.pdf