It is desirable to normalize data to a common value that will compensate for the difference in analyzer bandwidth, thus eliminating ambiguity. The answer is to incorporate the use of PSD within the testing environment. In the case of PSD, we actually divide the amplitude squared by the selected bandwidth and thus cancel out the effects of filter bandwidth. The table, above, puts this concept into prospective (please refer to the column marked g2/Hz).

So where does the grms fit? In the world of random sine testing, we have become accustomed to working in units of g peak. Since a random event is a non-periodic event, it is necessary to statistically determine the level, thus RMS (root mean squared) is used.

Most vibration controllers and analyzers will also display and/or calculate "total grms." While test specifications may only specify PSD levels in the test profile, the total grms level is important for calculating a shaker's overall capacity to perform the test. The total grms is calculated as follows:

âˆšA12+A22+A32..........

Where: A is the amplitude (in g's) at each filter, or digital line.

While a sine signal was used as an example. PSD is most common in random vibration testing.

### Question

A lot of information is available showing the mathematical development of the power spectral density function (PSD) unit of measure. This article will provide a practical explanation for PSD and its practical applications in testing. Figure 1