Resonance vs. Forced Vibration
Difference Between Resonance Forced Vibration
An effect called resonance occurs when a number of small repeated pushes cause a large vibration. Resonance increases the amplitude, and thus the intensity, of a sound.
For an example of how resonance occurs, think of a child sitting on a swing. You give the swing a push, and it starts moving in an arc. If you continue giving a small push each time the swing is at its highest point, the swing soon moves through a wide arc. Although each push is small, you have timed the pushes so that their effect adds up, producing the large back-and-forth motion.
The swing has a natural frequency at which it moves back and forth. Your pushes, if timed to this natural frequency, add up to make the wider arc. If you push at a faster or a slower rate, the swing comes almost to a stop. Resonance takes place only when you push at the natural frequency of the swing.
The sounds made by wind instruments in music are reinforced (strengthened) by resonance. The resonance comes from a column of air inside the instrument. Thus, the sound of a trumpet or a bugle is reinforced by resonance of the air inside the instrument.
Sometimes an object can be made to vibrate at a frequency other than its natural frequency. Its back-and-forth motion is then called forced vibration. A common example of forced vibration is in the eardrum. When sound waves strike the eardrum, they cause it to vibrate at the frequency of the received sound wave.
You can use a tuning fork to produce forced vibrations. Strike the tuning fork to make the prongs vibrate, and listen to the sound for a moment. The sound is quite soft. Now place the stem of the fork against the top of a table. The sound at once becomes much louder. The vibrations of the fork force the tabletop to vibrate at the same frequency as the tuning fork. The table’s larger size sets more air in motion than the tuning fork alone does, and you hear a louder sound.
Many musical instruments are designed so that forced vibrations amplify their sounds. In a violin, for example, vibrating strings force the wood to vibrate—thereby increasing the sound made by the strings. The forced vibration of the sounding board of a piano amplifies the sound made when a note is struck.