At any point in time, the actual random vibration provided by a test shaker would bear little resemblance to these smooth curves. These profiles are prescribed limits on vibration intensity. It�s convenient to use units of g 2/Hz on these acceleration plots so that the overall grms acceleration is the square root of the total area. However, because the total area under any one curve cannot be determined graphically anyway, it�s the clearly stated slopes and levels that are important.įor random vibration, the total force seen by the device under test (DUT) depends on the bandwidth. The vertical axis above the 0.10-g 2/Hz level has been expanded to more easily distinguish the four profiles from each other. Only for rectangular areas is the calculation straightforward.
On the other hand, finding an area under sloping straight lines requires the use of equation 1. This format accounts for the straight-line slopes labeled as so many dB/octave. One thing immediately noticeable in this example is the odd vertical scale on the psd plot. Adding the two areas, 38.4 + 19.5 = 57.9, and taking the square root result in 7.61 grms, corresponding to the 8 grms on the figure. The area under the downward sloping part of the curve from 1,000 to 2,000 Hz also contributes to the overall grms. The rectangular portion of this curve extends from 15 Hz to 1,000 Hz with a psd of about 0.039 g 2/Hz. Courtesy of NASA Dryden Flight Research CenterĬurve A is the easiest to understand.
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