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 and the system is said to be in a state of resonance. There are three types of recognized resonance: Phase Resonance, Amplitude Resonance and Natural 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?)
Chord of Resonance
"The chord of resonance of the sphere was the sympathetic etheric chord of E flat, 3rd octave, and is highly sensitive to concordance, against the neutral sevenths of the mass chord, whether that concordance be physical or mechanical or the two combined. When the chord of resonance is harmonized with the mass chord, the highest degree of susceptiveness is manifested to negative antagonism. Antagonistic chords actually move and accelerate the sphere, demonstrating the perfection of Nature's laws governing the sympathetic flow." [HOW THE MUSICAL SPHERE ROTATES]
Hundredth Monkey Effect
Law of Force