1. Fill in the blanks with the proper words.
(a) The propagation of sound does not occur through a vacuum.
(b) Noise pollution is a social issue.
(c) The sound which is disagreeable to the ears is called noise.
(d) Noise has adverse effects on our health.
2. What should we do?
(a) The silencer of a motorcycle is broken.
Ans. If the silencer of a motorcycle is broken, then we should take the motorcycle to a mechanic and get it repaired or replace it.
(b) A factory in the surroundings is producing continuous loud noise.
Ans. If a factory in the surroundings is producing continuous loud noise, then we should file a complaint against the factory with a local government body listing down the problems faced by the residents.
3. Write the answers in your own words.
(a) What is meant by vibration?
Ans. When any body oscillates to and fro, it is known as vibration.
(b) Explain with the help of practical examples how sound is propagated through solids.
Ans. 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 sound. This example shows that sound travels through a solid in the form of waves and reaches our ears.
(c) What is meant by noise pollution?
Ans. Sound that is disagreeable, irregular and loud is called noise pollution.
(d) What measures will you take to control noise pollution?
Ans. The following measures should be taken to control 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.
4. Complete the table.
|Nature of sound||Unpleasant||Pleasant|
|Horns of Vehicles||Unpleasant|
|Rustling of leaves||Pleasant|
|Neighing of a horse||Unpleasant|
|Ticking of a clock||Unpleasant|
Prepare a list of the harsh sounds heard near your house. Write about those sounds which produce noise pollution.
Collect information about places where loud noise is prohibited and discuss why it is so.
Extra Textual questions
Can you tell?
1. Which sounds do you hear during the recess in the school?
Ans. During the recess period we hear the following sounds:
Children talking, playing, fighting, running about, etc.
2. When there is silence in the classroom, close your eyes and sit quietly. Which sounds in your surroundings can you hear now?
Ans. I can hear the sound of the children and teachers from the adjacent classrooms, birds chirping outside the window and the sounds of the traffic from the road.
3. Classify these sounds in two ways – soft/loud and pleasant/unpleasant.
|Sounds we hear||Soft / loud||Pleasant / unpleasant|
|Children running about||loud||unpleasant|
1. (a)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?
Ans. We will feel the speaker vibrating with the sound of the music.
(b) Put off the music. What do you feel now?
Ans. When the radio or music system is put off we do not feel any vibration from the speaker.
2. Take a rubber band and stretch it. Pluck the stretched band. Apart from the movement of the rubber band, what else did you notice?
Ans. We will hear the sound of the rubber band vibrating.
3. Spread some sawdust or mustard seeds or sand on the diaphragm of a tabla. Knock on the diaphragm lightly with a finger.
(a)What do you see happening so long as the tabla makes a sound?
Ans. We see the saw dust/ mustard seeds moving on the diaphragm of the table.
(b)What happens when the sound stops?
Ans. When the sound stops the saw dust/ mustard seeds also stop moving.
(c)What do we understand from these observations?
Ans. We understand that the vibration of an object is necessary for the production of sound. As long as the object vibrates, the sound is heard. When the vibration stops, the sound also stops.
Use your brain power!
When a metal dish falls on the floor, it makes a loud noise.
(a)What do we do to stop the noise?
Ans. We will try to stop the movement of the plate to stop the noise.
(b)What is the effect of that action?
Ans. As soon as we stop the movement of the plate, it will stop vibrating and the sound will stop.
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||the strings of the sitar|
|a bell (electric)||the metal sheet and the magnet|
|water that drops from a tap||the surface on which the water falls|
|a saucer that breaks on falling down||the pieces of the saucer.|
Let’s try this.
Take a pot filled with water. Strike it lightly on its rim.
(a) What do you see?
Ans. We will see waves forming on the surface of the water.
(b) Why are waves formed on the water in the pot?
Ans. 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. That is why we find waves formed on the water in the pot.
Fill in the blanks:
1. The vibration of an object is necessary for the production of sound.
2. The object due to which sound is produced is called the source of that sound.
3. The science of sound, resonance, including the production, propagation and effects of sound is called acoustics.
4. The intensity of sound is measured in a unit called decibel (dB).
5. The vibration of the vocal cords in our larynx or ‘voice box’ also produces sound.
6. The voice box is located in our throat.
7. The quality of the sound produced in the larynx depends upon the tautness of the vocal cords.
8. Sound travels in the form of waves through air, water or solid and reaches our ears.
9. Sound is propagated more clearly through a liquid than through air. It is heard most clearly through a solid.
10. Transmission of sound is faster through a liquid than through a gas, and faster through a solid than through a liquid.
11. Continuous noise which may have ill effects is called noise pollution.
12. Noise pollution occurs when we hear one or many sounds harmful to the ear.
13. A medium is required for the propagation of sound.
14. Sound is propagated through gaseous, liquid and solid mediums.
15. Noise is disagreeable, irregular and loud.
Answer the following questions:
1. What is sound wave?
Ans. 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.
2. How is sound heard by us?
Ans. 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.
3. What is Propagation of sound?
Ans. Sound is said to be propagated when sound waves spread in all directions from a source of sound.
4. What is medium of propagation of sound?
Ans. The substance around a source of sound through which sound waves spread is called the medium of propagation of sound.