As a kind of coda to this chapter, here is a small challenge. If you have followed the material so far on simple vibrating systems and on the properties of their frequency response functions, you can test your knowledge, powers of observation and ingenuity now. Figures 1 and 2 show sets of measured frequency response functions for two different vibrating systems — both of them are systems we have looked at in the course of earlier sections. For both sets, the observation point was fixed while three different excitation points were used, just as you might do as part of a scan for the purposes of experimental modal analysis. The curves have been spaced out in the plots, for clarity.


For each set, think about the following questions:
*** Can you guess what the physical system might have been?
*** Can you say anything about the positions of the excitation and observation points?
*** Does anything else strike you about these measurements? Can you make guesses to explain your observations?
In order to address these questions, you might want to consider the pattern of resonance frequencies and the associated peak heights, the pattern of antiresonances and/or shallow valleys, and some of the potential snags and pitfalls of measurement described in section 10.4.2.
The answers are given in the next link.