Page 53 - Shock and Vibration Overview

Response Spectrums

If a grid of accelerometers are placed around the plate and the same exercise is
repeated, you can begin to start seeing modal shapes at each one of these resonant
frequencies. The first four modes of this representative plate are shown in Figure 38: a
first bending mode, a twisting mode, the second bending mode, and a second twisting
mode. For an interesting video on the modal shapes of a plate, check out this video.

Figure 38: Mode shapes emerge at each resonant frequency which provides more information on
the dynamic characteristics of your structure.

Understanding a structure’s mode shapes help the engineer better design his/her
structure. Performing experimental modal analysis also helps the engineer correlate or
validate a finite element model so that they can have more confidence in their design
and optimize it for a particular operational environment.

Modal analysis is a very powerful tool but it typically requires a fairly expensive and
complex test setup (many wired accelerometers, a shaker and/or impact hammer, large
data acquisition system, and a powerful software package). So it has its limitations and
is traditionally reserved for expensive and/or large structures for testing and design; a
typical design engineer for a smaller or lower volume product/system may not be able
to afford this type of analysis and testing.

For hardware and software, m+p International seem to offer some of the best that
we’ve come across but there are other options too. Typically though, a modal analysis

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