1. Which sounds do you hear during the recess in the school ?
   2. When there is silence in the classroom, close your eyes and sit quietly. Which sounds in your surroundings can you hear now? Prepare a common list of all these sounds and discuss them. You will see that there is a lot of diversity in the innumerable sounds that we hear. Classify these sounds in two ways – soft/loud and pleasant/unpleasant.

Some sounds are loud and are heard easily while some others are very soft and cannot be heard unless we listen attentively. We like some sounds. On the other hand, we get annoyed by some other sounds.

How are sounds produced?

1. When a song is being played on a radio or a music system in the house, place your hand on its speaker. What do you feel ? Put off the music. What do you feel now?
2. Take a rubber band and stretch it as shown in the picture. Pluck the stretched band. Apart from the movement of the rubber band, what else did you notice ?
3. Spread some sawdust or mustard seeds or sand on the diaphragm of a tabla. Knock on the diaphragm lightly with a finger. What do you see happening so long as the tabla makes a sound? What happens when the sound stops ? What do we understand from these observations ?

The objects that produce sound, that is, the diaphragm of the speaker, the rubber band, the diaphragm of a tabla, show a certain movement. That is, they have a certain kind of motion. They oscillate rapidly. In other words, they vibrate.

The objects that produce sound, that is, the diaphragm of the speaker, the rubber band, the diaphragm of a tabla, show a certain movement. That is, they have a certain kind of motion. They oscillate rapidly. In other words, they vibrate.

The sounds of a sitar, a bell, water that drops from a tap, a saucer that breaks on falling down – what is it that vibrates when these sounds are produced ? The sounds of a sitar, a bell, water that drops from a tap, a saucer that breaks on falling down – what is it that vibrates when these sounds are produced ?

How sound is heard

There is air around a source of sound. As the source of sound begins to vibrate, the layer of air nearest to the source also vibrates. A wave of vibrations of that sound spreads in all the directions from the source of sound. Such a wave is called a sound wave. The sound waves reach our ears. There is a delicate diaphragm or eardrum in the cavity of our ears. It starts vibrating, too. The sensation produced by these vibrations are passed on to the brain through the nerves in the ears and we hear the sound.

Propagation of sound

1. Take two balloons. Fill air in one and water in the other. Press the balloon filled with air against your ear as shown in the picture. Rub a finger on the balloon and listen to the sound. Repeat the same activity with the balloon filled with water. Through which balloon do we hear a clearer sound ?
2. Stand at the end of a big table and make a friend stand at the other end. Ask the friend to knock lightly on the table. You will hear a faint sound. Now you press your ear to the table and ask the friend to repeat the knock. What difference do you notice ?

Sound travels in the form of waves through air, water or through a solid and reaches our ears. Sound is propagated more clearly through a liquid than through air. It is heard most clearly through a solid. Why is this so?

Transmission of sound occurs at a different speed through different mediums. Transmission of sound is faster through a liquid than through a gas, and faster through a solid than through a liquid

Noise pollution

A loud sound is harsh to the ear. Such sounds produce noise.

1. Make a list of all the sounds implied in the picture.
   2. How would this atmosphere affect a person who is not feeling well ?
   3. Would you be able to study in these conditions ?

The situations shown in the two pictures on page 94 are often seen around us. Some of the sounds they depict are pleasant while others are harsh. Very loud or continuous noise has adverse effects on the people of that locality. Their hearing may get impaired which can even lead to deafness. It can also cause restlessness, irritability and mental exhaustion. One cannot work with concentration. This kind of continuous noise which may have ill effects is called noise pollution.

Measures for preventing noise pollution

1. As far as possible, we should avoid blowing the horn.
   2. The volume of the TV or radio in the house should be restricted to those watching the programmes.
   3. Vehicles should be maintained properly to reduce the unnecessary sounds they produce.
   4. Factories, airports, railway stations and bus stands should be located at the proper distance, away from residential areas.