Production of sound : Vibrations of an object produces sound

• When the radio or tape recorder is switched on we can feel the vibrations. When they are put off the vibrations stop.
• When the rubber band is stretched it can produce vibrations upon plucking. This forms sound.
• When diaphragm of tabla undergoes vibrations, it produces sound.

There are innumerable sounds that we hear. Some of them are soft while some are loud, some sounds are pleasant while some are unpleasant.

How sound is heard ? :

• When the sound is produced, the sound undergoes vibrations Due to these vibrations, air around the source of sound also undergoes vibrations
• These vibrations spread like a wave in all the directions from the source of the sound.
• This wave is called sound wave. The sound waves eventually reach our ear.
• In the ear cavity there is delicate eardrum or diaphragm. This too starts vibrating
• The vibrations reach the nerves in the internal ear The nerves transmit this message to the brain and therefore we can hear.

Propagation of sound : When sound waves spread in all the directions from the source of sound, it is called propagation of sound.

Medium qf propagation of sound :

The substance around the source of sound is called the medium of propagation of sound.

In different mediums, the sound transmits at different speeds. Transmission of sound is fastest in solid medium, lesser in liquid medium and least in gaseous medium.

• Examples : We can hear the sound of a train which is far off by placing our ear to the railway track. If we knock at the door, the person inside the room can hear it clearly.

Q. If a bell is hung in a vacuum container, will its sound be heard outside ?

Noise pollution : The harsh and unwanted sound that produces discomfort is called noise. Noise pollution occurs when there is one or many noises around us that are harmful. This kind of continuous noise cause ill effects on health.

Effects of noise pollution :

• The hearing capacity of the ears is lessened and it may cause deafness.
• There is mental exhaustion due to constant noise.
• There is restlessness and mental irritability.
• One cannot work with concentration.

Measures for preventing noise pollution :

• We should not blow horns of the vehicles unnecessarily.
• The volume of TV or radio should be kept at minimum, restricted to those who are watching or listening to the programmes.
• Maintenance of vehicles should be done, so that they will not produce noise.
• Factories, airports, railway stations and bus stands should not be located in the residential areas. But they should be away from localities where people stay.