Resonance can occur only if external force is applied at the same frequency as that of natural frequency, Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a periodically applied force is equal or close to a natural frequency of the system on which it acts, hence it is forced vibration.

Forced vibration occurs when the object is forced to vibrate at a particular frequency by a periodic input of force. If an object is being forced to vibrate at its natural frequency, resonance will occur and large amplitude vibration can be observed.

Resonance is used to increase the intensity of a sound. The comparatively weak vibrations produced at the end of an organ pipe.