NCERT Class 9 Science Exploration Chapter 10 Sound Waves Characteristics and Applications PDF Download

Chapter 10: Sound Waves: Characteristics and Applications

Sound is an everyday sensory experience that helps us become aware of our surroundings. Every day, we hear a variety of sounds in our surroundings, such as human voices, birds chirping, waves crashing on the seashore, leaves rustling, mobile phones ringing, vehicles honking, music, and the claps of thunder. You have learnt in Chapter 7 that sound is a form of energy. You have also learnt that energy can neither be created nor be destroyed. It only changes from one form to another. Which form of energy gets converted to sound energy? How is sound produced and how does it reach our ears? In this chapter, we will try to find the answers to these questions.

Think It Over

• Two astronauts are repairing the arm of a space station together during a spacewalk. Can they talk to each other and hear the sounds of metal clanking as they do on the Earth?
• How do most bats use sound to locate their prey in the dark at night?

10.1 Production of Sound

You learnt a few ways to produce sound in your textbooks for arts in the earlier grades.

Do you also remember learning about sonority earlier, a property of some metals where sound is produced when struck with an object (Fig. 10.1)?

[Figure 10.1: Taal a musical instrument, See in your textbook]

Activity 10.1: Let us explore

1. Take a cardboard box with one side open and a rubber band.
2. Stretch the rubber band across the open side of the box (Fig. 10.2).
3. Holding the box steady with one hand, pluck the rubber band with a finger. Do you hear any sound?
4. Pluck the rubber band again and watch it carefully. Is it vibrating?
5. Wait till the rubber band stops vibrating. Do you still hear the sound?
6. Change the tension in the rubber band by stretching it more or loosening it slightly and plucking it each time. Does the sound change? What changes do you notice?
7. Remove the rubber band from the box. Stretch it between two fingers and pluck it near your ear. Is the sound still produced? Is it as loud as before?

You may have noticed that as long as the rubber band is stretched and vibrating, sound is produced. Once the vibration stops, so does the sound. From this activity, we conclude that sound is produced by vibrations. Vibration refers to the periodic to and fro motion (oscillations) of an object. Pulling a stretched string or striking a metal object makes them vibrate, which produces sound. Likewise, when you blow through a bansuri (flute), vibration of the air inside the hollow pipe produces sound. You have learnt earlier that some of these methods are used in a variety of musical instruments. Sound can be produced by vibrating strings membranes, air columns, and many other vibrating objects. In most musical instruments, more than one vibrating part is involved in producing sound. The object that produces sound is called the 'source' of the sound.

Teacher's Note

When you see a vibration producing sound, remember that the vibration itself is not the sound \u2014 the vibration creates waves that travel to your ear. If something vibrates in a perfect vacuum with no air around it, you would not hear anything, even though it is vibrating. Sound needs a medium like air, water or a solid to travel through.

How do humans and animals create sound? While talking or singing, gently touch your throat. Do you feel vibrations anywhere? In humans and some other animals, sound is produced by the vibration of vocal cords, which are tightly stretched muscular flaps located inside the voice box or larynx, in the throat (Fig. 10.3). The tongue, lips, mouth and nasal cavity in humans help in converting sound into speech or music. Some animals produce sound by striking or rubbing certain body parts. For example, grasshoppers and crickets rub their wings or legs to produce sound.

[Figure 10.3: Vocal cords in humans, See in your textbook]

10.1.1 Tuning fork

An instrument that is often used for experiments with sound is a tuning fork. A tuning fork is a U-shaped metal bar with a stem. It is usually made of steel or aluminium. The sides of the 'U' are called prongs or tines (Fig. 10.4a) which are struck on a pad to make them vibrate.

Activity 10.2: Let us explore

1. Take a tuning fork and a soft rubber pad.
2. Hold the tuning fork by its stem.
3. Strike one of the prongs of the tuning fork gently against the rubber pad (Fig. 10.4b) and bring it close to your ear. Do you hear a sound? (Take care not to strike the tuning fork against a hard surface).
4. Now, gently touch a water surface with one of the vibrating prongs of the tuning fork. Do you see waves forming on the surface of water?
5. Repeat step 3 a few times while bringing the prongs of the tuning fork near your ear in different orientations. Do you hear the sound?

When you bring a vibrating tuning fork near your ear you hear the sound that is produced. When the prong touches the surface of water, waves form on the surface of water, indicating that the prongs of the tuning fork are indeed vibrating (Fig. 10.4c). These observations support the idea that sound is produced by vibrating objects.

Pause and Ponder

1. Explore various ways of producing sound.
2. Make a list of different types of musical instruments and identify their vibrating parts which produce sound.

Threads of Curiosity

What if your name were a tune instead of a word? In Kongthong, a village near Shillong in Meghalaya also known as the Whistling Village every person has a 'tune name' that can be sung or whistled. This unique tradition, called Jingrwai Iawbei, begins at birth when a mother composes a lullaby-like tune for her child.

Teacher's Note

In Activity 10.2, when you see the water rippling as the tuning fork prong touches it, you are seeing direct evidence that vibration and sound are connected. The water waves show that the prongs are moving back and forth, and it is this rapid motion that creates the sound you hear. This is a good moment to sketch in your notebook what the prong's motion might look like.

10.2 Propagation of Sound

You have seen that sound is produced by vibrating objects. How does sound reach your ear from the source? Sound travels through air but does it also travel through solids and liquids?

Activity 10.3: Let us investigate

1. You and your friend stand on opposite sides of a desk in the classroom. Let your friend gently knock or scratch on the desk. Listen carefully to the sound produced with your ear in the air.

Key Points

• Sound is produced when objects vibrate. Vibration is the to and fro motion of an object, and when it stops, the sound stops.
• Many sources can produce sound: vibrating strings in musical instruments, vocal cords in humans and animals, and vibrating air columns in instruments like flutes.
• A tuning fork is a U-shaped metal instrument with prongs that vibrate when struck, and it is used to study sound in experiments.

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