We see light ! Where as we hear sound.

Sakshi Education

Sound is a form of energy felt by the sense of hearing.
Sound is classified into Music and Noise.
Music is the sound which is pleasant to hear, where as Noise is the sound which is jarring (unpleasant) to the ear.

Sources of Sound
Pluck the strings of a Veena or violin. Play on Tabala or Mridangam or Drums. Play the flute or sehanai. We hear the strings of violin or veena the membranes of table or drums and the air columns in flute or sehnai vibrating. These vibrations are simple harmonic in nature.
Thus the source of sound is a vibrating body.
In laboratory the source of sound is a tuning fork.

Types of Vibrations

Natural or Free Vibrations
When a body is set into vibrations and then left to itself (pluck the strings of a veena and leave) the vibrations are called ‘free or natural vibrations’.
The amplitude of a vibrating body continuously decreases with time. This process is called ‘Damping’.
Forced Vibrations
Strike a tuning fork and press its stem against the top of a table the sound becomes louder because the table is forced to vibrate since the top of the table has much larger vibrating area than the tuning fork. These vibrations produce louder sound. The vibrations produced in the table are called ‘Forced Vibrations’.
When a body executes vibrations under the action of an external periodic force, then the vibrations of the body are called forced vibrations.
Sympathetic Vibrations: Mount two tuning forks of the same frequency on two hollow sound boxes (without lids) nearer (little away) to eachother. Now if one of the tuning forks is vibrated, the other also begins to vibrate with louder sound. The vibrations produced in the other tuning fork are called sympathetic vibrations and the phenomenon is called ‘Resonance’.

Resonance
If there are two vibrating bodies of nearly equal frequencies nearer to eachother and if one of them is vibrated, the other also set into vibration and vibrate with larger amplitude. The vibrations produced in the other body are called sympathetic vibrations and the phenomenon is called Resonance.

Example: When soldiers are crossing a suspension bridge,they are asked to march out of step. Because if the frequency of vibration of their marching (left, right .....) become equal to the natural frequency of the bridge, the bridge may vibrate and collapse because of ‘Resonance’.