When a vibrating body is held near another body, then the second body starts vibrating. It is called forced vibration. If the natural time period of the body and the time period of the applied force on it are different then the body vibrates in low amplitude and a feeble sound is heard. But if the natural time period of the body and that of the applied force become equal, then the body will be forced to vibrate with large amplitude and the intensity of the sound becomes large. This process is called resonance. So, resonance is a special case of forced vibration. Now we will see what is meant by resonance.
When the natural frequency of a body becomes equal to the frequency of forced vibration, then the body vibrates with maximum amplitude. This type of vibration is called resonance.
It can be said in other words that the frequency of the source and the frequency of vibration of the particles through which the sound produced due to the source becomes equal, then intense sound is produced for which particles vibrate with maximum amplitude. This phenomenon is called resonance.
From the definitions of forced vibration and resonance it is clear that all resonances are forced vibrations but all forced vibration are not resonances.