Resonance

"The condition where a forcing frequency coincides with a natural frequency of the system. A resonance is typically identified by a substantial amplitude increase, and related phase shift?." (Field of Rotating Machinery Measurment, Monitoring and Analysis; Bentley Nevada Corporation)

(ACOUSTICS & MECHANICS) "When a mechanical or acoustical system is acted upon by an external periodic driving force whose frequency equals a natural free oscillation frequency of the system, the amplitude of oscillation becomes large (Resonance causes this increase) and the system is said to be in a state of resonance." (McGraw-Hill Concise Encyclopedia of Science & Technology)

Resonance or Co-vibration is the name given to the phenomenon of one vibrating body imparting its vibratory movement to another body, previously at rest. To obtain the maximum resonance two conditions are essential:
1) The two bodies must be in exact unison; that is to say, they must be capable of executing precisely the same number of vibrations in the same time.
2) A certain period of time must be allowed for the exciting body to impress its vibrations on the other. (Hand Book of Acoustics, 5th edition; J. Curwen & Sons, London, 1903?)

See Also

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Sympathetic Association
Sympathetic Oscillation
Sympathetic Vibration

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