mechanical phenomenon whereby oscillations occur about an equilibrium point; precisely used to describe mechanical oscillation

Vibration means quickly moving back and forth (or up and down) about a point of equilibrium.[1] The vibration may be periodic (having a pattern) or random. Something that is vibrating may shake at the same time. If it vibrates in a regular way, it may produce a musical note because it can make the air vibrate. This vibration will send sound waves to the ear and to the brain.

In structural engineering including earthquake engineering, vibrations may be bad. They may cause the structure to fail.

The time it takes a vibrating object to go back and forth completely is the period. The number of back-and-forth movements in one second is its frequency, measured in Hertz (Hz). The farthest possible distance from the equilibrium point is the amplitude.

Types of vibration


Free vibration occurs when vibration is set off with a push and allowed to vibrate freely. Examples of this type of vibration are pulling a child back on a swing and then letting go or hitting a tuning fork and letting it ring. The mechanical system will then vibrate at a 'Natural frequency' and gradually wind down.

Forced vibration is when an alternating force or motion is applied to a mechanical system. Examples of this type of vibration include

  • a shaking washing machine due to an imbalance.
  • transportation vibration (caused by a combination of truck engine, springs, road).
  • vibration of a building during an earthquake.

In forced vibration the frequency of the vibration is the frequency of the force or motion applied. The size of the effect depends on the actual mechanical system.



  1. Tongue, Benson 2001. Principles of vibration. Oxford University Press. ISBN 0-195-142462