In practice it is very difficult to avoid vibration. It usually occurs because of the dynamic effects of manufacturing tolerances, clearances, rolling and rubbing contact between machine parts and out-of-balance forces in rotating and reciprocating members. Often, small insignificant vibrations can excite the resonant frequencies of some other structural parts and be amplified into major vibration and noise sources.

Sometimes though, mechanical vibration performs a useful job. For example, we generate vibration intentionally in component feeders, concrete compactors, ultrasonic cleaning baths, rock drills and pile drivers. Vibration testing machines are used extensively to impart a controlled level of vibration energy to products and sub-assemblies where it is required to examine their physical or functional response and ascertain their resistability to vibration
environments.

A fundamental requirement in all vibration work, whether it is in the design of machines which utilize its energies or in the creation and maintenance of smoothly running mechanical products, is the ability to obtain an accurate description of the vibration by measurement and analysis. (from Bruel & Kjar)