Maharashtra Board Class 6 Science Chapter 13 Sound PDF Download

13. Sound

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 depicted in the pictures below. Have you heard these sounds? Put a tick mark (✓) in the box if you have, and cross (✗) if you have not. What words are used in your locality to denote these sounds? Write them below the pictures.

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?

Can you tell?

Try this.

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?

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.

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 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.

The object due to which sound is produced is called the source of that sound.

Teacher's Note

When a bell rings, it vibrates fast. You can feel this vibration. In India, temple bells vibrate to make sound during prayer time.

Exam Trick

Remember: No vibration = No sound. Just like a stopped fan makes no sound, a stopped bell makes no sound.

Points to Remember

Sound is made by vibration.
Vibration means fast back and forth movement.
When vibration stops, sound stops.
The thing that vibrates is called the source of sound.
We can feel vibration with our hand.

Try this.

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?

Use your brain power!

When a metal dish falls on the floor, it makes a loud noise. What do we do to stop the noise? What is the effect of that action?

Do you know?

Acoustics: The science of sound, resonance, including the production, propagation and effects of sound is called acoustics. The intensity of sound is measured in a unit called decibel (dB).

The vibration of the vocal cords in our larynx or 'voice box' also produces sound. The voice box is located in our throat. The quality of the sound produced in the larynx depends upon the tautness of the vocal cords.

Propagation Of Sound

Let's try this.

Take a pot filled with water. Strike it lightly on its rim. What do you see? Why are waves formed on the water in the pot?

Sound is said to be propagated when sound waves spread in all directions from a source of sound.

The Medium Of Propagation Of Sound

The substance around a source of sound through which sound waves spread is called the medium of propagation of sound.

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.

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.

Let's try this.

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?

Teacher's Note

Sound travels faster in water than in air. In India, dolphins use sound to talk to each other under water.

Exam Trick

Remember: Sound is fastest in solid, slower in liquid, slowest in gas. Solid is like a packed crowd - sound spreads fast. Gas is like empty space - sound spreads slow.

Points to Remember

Sound needs a medium to travel.
Sound travels through air, water, and solids.
Sound travels fastest through solids.
Sound travels slowest through gases.
A medium is any material that sound travels through.

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