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Tip: Performing a bump test

Vibration Experts

A bump test is used as a quick and easy method to look for natural frequencies (related to resonance) in a structure. A bump test is conducted by hitting the structure with a moderately soft object like a 2×4 in order to make it ring in the same way one would hit a bell to make it ring. When a bell is hit, it rings at its natural frequencies. Another way to state this is to say that the hit excites the bell’s natural frequencies, which can then be measured and identified.

Use a rectangular window (or no window) on your data collector, mount the sensor on the object of interest and randomly hit the object with a 2×4 or other appropriate instrument. There should be a clear transmission path of vibration between where you hit the object and where the sensor is mounted. If your data collector has a negative trigger function, try to set it up so that it triggers just before the hit and captures the entire hit. You should also used a fixed gain setting so the data is not clipped.

When looking at the vibration spectrum from this test, the areas where one finds large humps in the data are possibly natural frequencies. To confirm that they are natural frequencies, one must also collect phase data (see “calibrated hammer test”), but in many cases the bump test provides enough information to get an idea of what is happening.