Vibration is a term that describes oscillation in a mechanical system. It is defined by
the frequency (or frequencies) and amplitude. Either the motion of a physical object
or structure or, alternatively, an oscillating force applied to a mechanical system is
vibration in a generic sense. Conceptually, the time-history of vibration may be considered
to be sinusoidal or simple harmonic in form. The frequency is defined in
terms of cycles per unit time, and the magnitude in terms of amplitude (the maximum
value of a sinusoidal quantity). The vibration encountered in practice often
does not have this regular pattern. It may be a combination of several sinusoidal
quantities, each having a different frequency and amplitude. If each frequency component
is an integral multiple of the lowest frequency, the vibration repeats itself
after a determined interval of time and is called periodic. If there is no integral relation
among the frequency components, there is no periodicity and the vibration is
defined as complex.
Vibration may be described as deterministic or random. If it is deterministic, it
follows an established pattern so that the value of the vibration at any designated
future time is completely predictable from the past history. If the vibration is random,
its future value is unpredictable except on the basis of probability. Random
vibration is defined in statistical terms wherein the probability of occurrence of designated
magnitudes and frequencies can be indicated.The analysis of random vibration
involves certain physical concepts that are different from those applied to the
analysis of deterministic vibration.
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