Can't Understand Vibration Test Documents?
by Wayne Tustin

Individuals in a wide variety of positions find themselves needing to correctly interpret vibration testing documents such as standards, specifications and reports. Unfortunately, some of the words and phrases make little sense to the uninitiated. An ordinary dictionary is of little use. This short article attempts to present a "crash course" in understanding vibration test documents, supplementing a specialized dictionary.

The italicized words are more formally defined in that dictionary. In case you will be visiting a vibration test lab, this article will help you understand what's going on. In case your customer (or your boss) has handed you a spec, this article will help you to pass the requirements along to your test lab. In case you are a new test engineer or test technician, this article will help you understand your job.

Sinusoidal Vibration Testing 

Sine test specs usually dictate frequency in terms of hertz (abbreviated Hz) and meaning cycles (of vibration) per second. Frequency may be fixed or stepped (rare) or swept between limits. If you visit a lab during such a test, your ears will tell you how the test frequency is varying: fixed or stepped or sliding up and down. One objective of sweeping may be to find the frequency (or frequencies) at which some element on or within the device under test (DUT) responds most strongly (resonates) or at which the DUT misoperates. Sweep rate is usually given in octaves (one octave represents a doubling of test frequency) per minute.

Specs also usually dictate intensity of vibratory motion being applied by the shaker to the DUT (usually through a mounting adapter or fixture. Intensity is usually specified in terms of displacement, velocity, acceleration and/or force. Specs also usually dictate test duration.

Random Vibration Testing

Random testing isn't terribly different. Mainly, a relatively wide range of test frequencies, a continuum, is applied simultaneously. The sound will be much different … sort of a roar or a shhhhhh, something like a jet exhaust or your TV set on a vacant channel with the loudness control set high.

Specs usually dictate intensity of vibratory motion in terms of power spectral density (PSD), sometimes called acceleration spectral density or even auto spectral density ASD). The editor hasn't given me enough space to explain these units. Specs also usually dictate test duration.

Is The Spec Being Met?

Assuming you visit the lab while a sine test is underway, you will observe some instrumentation being used. There probably will be one or more accelerometers with displays showing (if a sine test) the current test frequency and intensity. If a random vibration test is underway, the displays will show the current test spectrum (intensity vs. frequency).

Was The Specified Test Applied?

If you didn't visit the lab while "your" test was underway, you'll probably read a test report. It will contain many of these same words. It will probably also contain graphs of the specified intensity vs. test frequency and other graphs showing the input(s) to the DUT.