Page 52 - Shock and Vibration Overview

Response Spectrums

Picture a flat plate supported in the center with an accelerometer positioned in one of
the corners. Then excite the plate with a sinusoidal sweep of different frequencies but
fixed amplitude and measure the response from the accelerometer. If you do this an
accelerometer may measure the type of response shown in Figure 36. You may expect
that the output from the accelerometer would also be fixed because of the fixed input
levels; but this is the whole beauty of modal analysis! The response amplifies as the

force is applied with a rate of oscillation closer to the resonant frequencies in the
system.

Figure 36: Response from accelerometer on a plate measuring acceleration as the plate is excited
with a range of different frequencies, but a fixed force amplitude.

If a frequency response function is applied to the data the resulting plot is a function of
frequency shown in Figure 37. This allows the engineer to directly see where the
resonant frequencies lie but with curve fitting, you can also determine the damping
characteristics of each mode.

Figure 37: Frequency response function from a modal analysis test.

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