Vibration Testing Glossary
Absolute vibration
Vibration of an object relative to a fixed point in space. Seismic sensors (accelerometers and velocity pickups) measure absolute vibration. Contrasts with relative vibration.
Baseline spectrum
A vibration spectrum taken when a machine is in good working condition (new or just overhauled), used as reference for future monitoring or analysis.
Bounce Test
A shaking of unrestrained (loose) cargo. The cargo is repeatedly thrown a short distance into the air and then falls onto the vibrating platform.
Bump Test
A horizontal striking of an impacting surface against cargo, or cargo motion arrested by impacting a stationary object.
Dynamic signal analyzer DSA
Vibration analyzer using digital signal processing and the Fast Fourier Transform (FFT) to display vibration frequency components. May also display the time domain and the phase spectrum. Usually interfaced to a computer.
First order vibration
Rotating machine vibration caused by shaft unbalance. Frequency in hertz (Hz) is calculated by shaft RPM/60. Also called 1x vibration. Additional orders, 2x, 3x .... 36x, etc. are caused by other mechanisms
Fixture
The intermediate structure that attaches a device under test (DUT) to a shaker or shock test machine.
Fragility test
Expensive but highly useful dynamic tests of several samples (to account for variations in tolerances, material properties and manufacturing processes) at potentially destructive frequencies, to determine fragility.
Free vibration
Free vibration occurs without forcing, as after a reed is plucked.
Impact test
A broad frequency range of structural responses is caused by a deliberate impact.
Repetitive shock machine
A platform to which products (to be tested or screened) are attached. Often this platform forms the bottom surface of a thermal test chamber. Pneumatic vibrators are attached to the bottom of the platform, causing it to vibrate, usually simultaneously in several axes.
Seismic
Having to do with earth motion, as earthquakes. A kind of sensor that depends upon the inertia of an internal mass to generate a signal, as an accelerometer or velocity pickup.
Shock machine
Or shock test machine, a device for subjecting a system to controlled and reproducible mechanical shock pulses.
Standing wave
A wave that is characterized by lack of vibration at certain points (nodes), between which are areas of maximum vibration (antinodes). Standing waves are produced at certain forcing frequencies when the resulting resonant vibratory response is confined within boundaries, as in the vibrating string of a musical instrument or the wing of an airplane or the whip antenna on your automobile. Also called "stationary wave".
Steady state vibration
Periodic vibration for which the statistical measurement properties (such as the peak, average, RMS and mean values) are constant.
Vibration
Mechanical oscillation or motion about a reference point of equilibrium.
Vibration exciter or shaker
A device which produces controlled and reproducible mechanical vibration for the vibration testing of mechanical systems, components and structures.
Vibration meter
An apparatus (usually an electronic amplifier, detector and readout meter) for measuring electrical signals from vibration sensors. May display displacement, velocity and/or acceleration.
White Random Vibration
Thatbroadband random vibration in whichthe PSD (ASD) is constant over abroad frequency range.
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