Page 51 - Shock and Vibration Overview

Response Spectrums

Instead of testing each spring, a shock response spectrum can be used to calculate the
response for a range of natural frequencies, plotted in Figure 35. This plot can be used
by the design engineer to determine frequencies that should be avoided in his/her
design. This example is an academic one but even so, an engineer may be surprised that
for a 1 second half sine input, the worst natural frequency a system could have is
around 0.8 Hz, not the 0.5 Hz of the input. In real world applications you will have real

test data that will have many frequency components. As always with these analyses;
your analysis can only be as good as the raw experimental/input data.

Figure 35: The shock response spectrum is shown for a 1g 1 second half-sine pulse input.

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Tom Irvine’s 23 webinar covers the classical shock pulse and the shock response
spectrum if you are interested in more information. Beyond the more basic shock
response spectrum, a look at the pseudo velocity shock response spectrum may be a
prudent exercise if/when designing a system to survive shock.

Modal Analysis
Modal analysis is the logical next step of vibration testing and looking at response
spectrums; it allows the engineer to determine key dynamic parameters of their

structure:

 Modal resonant frequency
 Modal damping
 Mode shape

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