ICSE Class 7 Physics Chapter 3 Sound

Sound

What Is Sound?

In our daily life, we hear different types of sounds like the ringing of a school bell, the sound from a clock, the honking of vehicles, the sound of playing a guitar or tabla, etc. Different people have different types of voice. Similarly, animals also produce different types of sound.

Sound is a form of energy that produces the sensation of hearing in our ears.

Sound Depends On Vibrations

Sound is a form of energy carried by waves of vibrating particles. These waves can travel through solids, liquids and gases but they cannot travel through vacuum because there are no particles present to vibrate. Thus, vibration of a particle is the necessary condition to produce sound.

A rapid back and forth motion of a particle about its mean position is known as vibration. When we speak, our vocal cord is under vibration. When a bell is struck, the bell is under vibration. Similarly, when a guitar is played, its strings are disturbed and they produce vibrations in the instrument.

Teacher's Note

Every sound we hear - from a friend's voice to music from a speaker - is created by something vibrating and sending waves through the air to our ears.

How Is Sound Produced

Take a hack-saw blade and fix its one end on a table or desk as shown in the figure. Bend the other end of the blade and release it. What do you observe? Does the blade vibrate? Do you hear any sound?

You will see that the blade is vibrating. You will also hear a sound, provided the blade vibrates very fast.

Take a rubber string. Hold its one end between your teeth and the other end in your hand and stretch it. Pull the rubber string with the other hand and release it. What happens to the rubber string?

You will see that the rubber string vibrates and as long as it vibrates, a sound is produced.

Take a tuning fork. It is a U-shaped metallic piece with a stem in the middle. Its arms are known as prongs. They are set into vibrations when any one of the prong is struck. Hit the tuning fork hard against a rubber pad. Bring it close to your ear. You will hear a sound. When these vibrations stop, the sound stops as well. Normally vibrations of the prongs are not visible because the vibrations are very fast. But if you touch it with the surface of water, you will notice ripples.

Now suspend a table tennis ball with the help of a thread and tie the other end to a stand. Strike the tuning fork with a rubber pad and bring it just in contact with the suspended ball. You will observe that on touching the prong the ball starts moving. The ball will start moving to and fro (oscillating). It shows that the prongs of the tuning fork are vibrating which set the ball in motion.

From the above observations, it is concluded that the source of sound essentially produces vibrations.

Teacher's Note

When you pluck a guitar string or ring a bell, the vibrations created travel through the air as invisible waves that make our eardrums vibrate, allowing us to hear sound.

Sound Requires A Medium To Propagate

Some forms of energies require a medium to travel. Sound is one of them. When you provide energy, such as mechanical energy, heat energy, or electrical energy, to any kind of matter, the molecules start vibrating about their mean positions transferring the energy from neighbour to neighbour. This constitutes a wave. A wave is a disturbance that carries energy without carrying the material with it. For example, when you beat a drum or a tabla, their membranes start vibrating and constitute sound waves. These sound waves carry sound energy and travel in the air just like ripples in the water.

To show that sound requires a medium to propagate.

Materials required: An electric bell, a big glass jar, vacuum pump.

Procedure: Hang an electric bell inside a jar connected to a vacuum pump. Arrange an electric bell, a glass bell jar, a vacuum pump, a battery and a switch as shown in Fig. 3.5. When the circuit is closed by pressing the switch, we can hear the bell ringing. Also, we can see that the hammer is striking the gong of the bell.

Now remove the air present inside the jar with the help of a vacuum pump. We observe that as the air is taken out, the loudness of the sound gradually decreases and a stage comes when no sound is heard although we can see the hammer striking the gong of the bell. This will happen when the air is completely removed from the jar.

Can sound travel through vacuum? No.

Conclusion: This experiment proves that sound cannot travel through vacuum because a medium is necessary for sound to travel.

Teacher's Note

In space, astronauts cannot hear each other even though they might be standing next to each other, because there is no air or medium to carry sound waves.

Sound Can Propagate Through Solids, Liquids And Gases

Propagation of sound through solids.

You can easily hear the sound of an approaching train by putting your ear on the track well before you could hear its sound in the air. Why? This is because sound travels much faster through steel or iron (a solid) than through the air (a gas).

Place a clock on one end of a table and bring your ear close to that end (or near the clock), Fig. 3.6(a).

Slowly move the clock away from yourself, till you stop hearing the sound of the clock Fig. 3.6(b). Now place one end of a ruler or a metal rod below the clock and the other end very close to your ear, as shown in Fig. 3.6(c). You will again be able to hear the sound of the clock.

Propagation of sound through liquids

To show that sound can travel through liquids.

Materials required: A balloon and a watch.

Procedure: Fill a balloon with water as shown in Fig. 3.7. Hold it near your ear. Now keep the watch gently to the other side of the balloon. Do you hear the sound? Yes!

Conclusion: Sound can travel through liquid.

Propagation of sound through air

Let us take the case of a vibrating tuning fork. The particles of the surrounding medium i.e. air, are pushed against the particles next to them and come back again. Hence, pushing is carried on from particle to particle till it reaches the observer's ear. Fig. 3.8(a) shows the steady or mean position of the metal strip and normal condition of air layers near the strip. When the prong of the vibrating tuning fork moves to the right as shown in Fig. 3.8(b), it compresses the air particles resulting in the formation of compression, shown as C. Similarly, when the prong of the tuning fork moves towards left as shown in Fig. 3.8(c), the air particles are separated again forming rarefaction shown as R in Fig. 3.8(d). Therefore, sound propagates in air or gases through alternate compressions and rarefactions. Now when the strip returns to its mean position, the rarefaction R moves forward and the air layers near the strip come back to their normal position Fig. 3.8(e).

Fig. 3.8(d). Therefore, sound propagates in air or gases through alternate compressions and rarefactions. Now when the strip returns to its mean position, the rarefaction R moves forward and the air layers near the strip come back to their normal position Fig. 3.8(e).

Take two metal rods and strike them against each other. Then immerse the rods into a vessel containing water as shown in the figure. Strike the rods against each other inside the water. Which sound is louder? What do you conclude from this experiment?

We hear the sound louder through liquids than the sound through air.

Teacher's Note

Dolphins use sound to communicate underwater because sound travels efficiently through water, allowing them to "talk" to each other across long distances in the ocean.

Sound Propagates in all Directions

Sound is a kind of energy which propagates in all directions. For example, when you beat a drum or a tabla, their membranes start vibrating and constitute sound waves. These sound waves carry sound energy and travel in the air just like ripples in the water.

When the diaphragm of a loudspeaker vibrates due to music or speech, the vibrations move the molecules of air surrounding it. These air molecules vibrate about their mean positions. Because air particles are all around us, the waves of sound travel outward in all directions.

Hence, we say that sound propagates in all possible directions through a medium.

Speed Of Sound

Sound travels faster if the medium through which it is travelling becomes denser. Sound travels fastest through solids, little slower through liquids and comparatively much slower through gases or air.

Take a long metal rod. Keep it on a flat surface such as a table. Bring your ear close to one end of the rod as shown in Fig. 3.11. Ask your friend to strike the other end of the rod by a hammer.

What do you observe from this activity?

We hear the sound two times. First, through metal rod and second time, through the air.

We conclude that sound travels faster through the solid.

Sound travels through the air at a speed of 332 m/s, through water, it travels at a speed of 1440 m/s and through iron, it travels at a speed of 5000 m/s.

Speed of sound = Distance travelled by the sound / Time taken

Example: A gun is fired in the air at a distance of 660 m from a person. He hears the sound of the gun after 2 seconds. What is the speed of sound?

Solution:

Distance travelled by sound = 660 m

Time taken by the sound = 2 seconds

Speed of sound in air = ?

So, speed of sound = Distance travelled by the sound / Time taken by the sound

Speed of sound = 660 m / 2 s = 330 m/s

Thus, the speed of sound in air is 330 m/s.

Types Of Sound

Sound is classified into two groups - music and noise.

Musical sound: Musical sound is produced by periodic vibrations. It has a regular wave pattern. Musical sound is pleasant to hear and is produced by instruments like the sitar, violin, drum, guitar, piano, etc. Human voice is also considered as a musical sound.

Noise: It is produced by non-periodic vibrations. It has irregular wave patterns. Noise is unpleasant to hear and is generally produced in a factory or by moving vehicles like a bus, train, etc.

Teacher's Note

When you listen to your favorite song, you hear musical sound with regular patterns, but when a car horn blares unexpectedly, that irregular, unpleasant sound is noise.

This is a preview of the first 3 pages. To get the complete book, click below.