NCERT Class 9 Science Chapter 10 Activities — Sound Waves: Characteristics and Applications

NCERT Class 9 Science — Sound Waves: Characteristics and Applications, Activities 10.1 to 10.8 on pages 185 to 198.

Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.

Eight activities — more than any other Class 9 chapter in scope — and they track the chapter’s argument closely:

• Activities 10.1 and 10.2 establish that sound comes from vibration, first with a rubber band and then with a tuning fork.
• Activities 10.3 and 10.4 show that sound travels through solids and through liquids, not only air.
• Activity 10.5 uses a slinky to show what kind of wave it is.
• Activity 10.6 makes the vibration of the medium visible.
• Activities 10.7 and 10.8 use a phone app to measure frequency and to find the limits of hearing.

Key insight. Two facts carry the whole chapter, and the first four activities establish them: sound is produced by a vibrating body, and sound needs a material medium to travel. Everything afterwards — longitudinal waves, frequency, the speed of sound in different materials, ultrasound — is a consequence. If time is short, protect 10.1 and 10.4.

Activity 10.1

Stretch a rubber band across the open side of a cardboard box. Holding the box steady, pluck the band. Do you hear a sound? Watch it — is it vibrating? Wait until it stops — do you still hear anything? Change the tension by stretching or loosening it and pluck again. Finally, remove the band, stretch it between two fingers and pluck it near your ear.

What you should observe. Sound and vibration begin and end together, and the box matters more than you expect.

Why the box is part of the experiment. The band alone moves very little air. Mounted on the box it makes the whole box vibrate, and a larger surface pushes far more air — which is why the final step, plucking it without the box, is so much quieter. It is the same reason guitars and violins have hollow bodies.

Why change the tension. It separates loudness from pitch. Tightening the band changes how fast it vibrates, so the pitch rises; plucking harder changes how far it moves, so the loudness rises. Two different properties, two different causes, and this is the first place the chapter distinguishes them.

What goes wrong. A band so loose it slaps against the box, or so tight it barely moves. It needs enough tension to spring back cleanly. And the box must be held steady — a hand damping the side kills both the vibration and the sound.

Activity 10.2

Hold a tuning fork by its stem and strike one prong gently against a soft rubber pad. Bring it near your ear — do you hear a sound? Now touch a water surface gently with one vibrating prong. Then hold it near your ear in different orientations.

What you should observe. A clean steady note, and ripples spreading on the water.

Why the water is worth the mess. A tuning fork’s prongs move too little and too fast to see. Dipping one into water converts an invisible vibration into a visible splash and spreading ripples — it is the proof that the fork is vibrating rather than simply being a thing that makes a noise.

Why a soft pad and not the bench. Striking a tuning fork on a hard surface damages it and adds a clang of its own. The book says so explicitly.

What goes wrong. Holding the fork by its prongs, which damps them instantly and gives a dull thud. Hold it by the stem, which is why it has one.

Activity 10.3

Stand on one side of a desk with a friend on the other. Have them gently knock or scratch the desk, and listen with your ear in the air. Then press your ear against the desk, block your other ear, and listen again.

What you should observe. The scratch is far clearer through the wood.

Why the other ear must be blocked. Otherwise you hear the airborne sound at the same time and cannot tell which path you are judging. The comparison only works if the two routes are separated.

Why it is louder through the wood. Sound travels faster and with less loss in a solid than in air, because the particles are packed closely and pass the disturbance on efficiently. The chapter returns to this on page 198 with a question about two friends at a steel fence 340 m apart, comparing the sound arriving through the steel with the sound arriving through the air.

What goes wrong. Knocking too hard, which sends plenty of sound through the air as well and flattens the difference. A gentle scratch is much more convincing than a bang.

Activity 10.4

Fill a large tub or bucket with water to the brim and take two metal spoons. Tap them together in the air and listen. Then submerge both spoons — not touching the sides or bottom — and tap them together again.

What you should observe. You can still hear it, muffled, from under the water.

Why the spoons must not touch the sides. If they do, the sound travels through the bucket wall as a solid, and the experiment becomes Activity 10.3 again. Keeping them clear of every surface leaves water as the only path out.

What this activity completes. With 10.3 and 10.4 done, sound has been shown to travel through solids, liquids and gases — and the chapter can then ask the opposite question. Its assertion-and-reason item about a bell ringing in a jar with the air pumped out has the answer that both statements are true and the reason explains the assertion: sound requires a medium, so with no air there is nothing to carry it.

What goes wrong. Filling the bucket too little, so the spoons are close to the surface and much of what you hear went through the air above. Fill it to the brim, as the instruction says, and tap well below the surface.

Activity 10.5

Take a slinky and a marker. Mark one turn, and lay the slinky out along a table or floor. Have a friend hold one end fixed. Push your end sharply towards them and pull it back, and watch both the disturbance and the marked turn.

What you should observe. The disturbance runs to the far end; the marked coil does not.

Why the marker is the whole experiment. Without it, a slinky just looks like something wobbling. The marked turn is what shows that the medium stays put while the disturbance moves — which is the single hardest idea about waves and the one this activity exists to make visible.

The contrast the chapter draws. Shake the slinky sideways instead and the coils move perpendicular to the wave’s direction: a transverse wave. Sound is longitudinal; light is transverse — and light, unlike sound, needs no medium, which is why sunlight crosses empty space.

What goes wrong. Pushing too slowly, which gives a lazy bunching rather than a clean pulse travelling the length. One sharp push-and-pull works far better than a gentle shove. A slinky stretched too far also transmits the pulse too fast to follow.

Activity 10.6

Stretch a cellophane or rubber sheet — a piece of balloon works — tightly over the mouth of a wide container and fix it. Sprinkle a few grains on the stretched sheet. Bring a loud sound source close by, such as a metal plate struck with a beater, and watch the grains.

What you should observe. The grains jump, though nothing has touched them.

Why this is the reverse of Activity 10.1. There, a vibration produced sound. Here, sound produces a vibration. Taken together they show the two-way traffic: a vibrating body pushes the air, and the moving air pushes whatever it reaches — which is how an eardrum works, and why this activity is a model of hearing.

Why the sheet must be tight. A slack membrane absorbs the energy instead of responding to it. It needs to be drum-tight, and the grains should be few and light — a scattering of semolina or sand, not a heap.

What goes wrong. A sound source that is not loud enough, or too far away. It needs to be close and genuinely loud; a struck metal plate works because it is both.

Activity 10.7

This is a demonstration. Use a phone app such as Phyphox that identifies frequencies. Make sounds with various objects — a tuning fork, a struck glass, a plucked string, your own voice — and record the frequency the app reports for each.

What you should observe. Every sound has a number, and the number matches the pitch.

Why a marked tuning fork is the first thing to test. It has its frequency stamped on it, so it checks the app before the app is used to measure anything unknown. If the reading matches the stamp, everything afterwards can be trusted.

Why this beats being told the definition. Frequency stops being a symbol and becomes a reading that goes up when the pitch goes up. Whistling slowly upward while watching the number climb does more than a page of explanation.

What goes wrong. A noisy room. The app locks onto whatever is loudest, which may be a fan or a conversation. It also struggles with complex sounds that have several strong frequencies at once — a tuning fork or a whistle gives a far cleaner reading than a spoken word.

Activity 10.8

Also a demonstration. Open an app that generates tones. Set the frequency to about 10 Hz and raise it slowly, noting where the class first hears anything. Then continue upward to find where they stop hearing it.

What you should observe. Hearing switches on and off at two definite frequencies — and not at the same ones for everybody.

Why the upper limit differs around the room. It falls with age. Students will typically hear several kilohertz higher than an adult teacher, and doing this as a class makes that immediate and memorable. It is worth asking for hands at each step rather than a single verdict.

Why the sound does not stop existing at the limit. The vibration is still there; human ears simply stop responding to it. A dog hears well above 20 kHz, bats navigate by ultrasound, and the chapter builds on exactly this for its applications — SONAR and medical ultrasound scans.

What goes wrong. Phone speakers, which reproduce neither very low nor very high frequencies well. A limit found at 15 kHz may be the speaker giving up rather than the ears. Headphones or a decent speaker give a far truer result — and keep the volume moderate, because loud tones near the limits are unpleasant and unsafe.

Common mistakes

• Plucking the rubber band off the box in Activity 10.1 and concluding the box is decorative. The box is what couples the vibration to the air; without it the sound is much fainter.
• Confusing pitch with loudness. Tension changes the pitch; how hard you pluck changes the loudness. Activity 10.1 separates them, and they are conflated constantly.
• Holding the tuning fork by its prongs. That damps the vibration immediately. Hold the stem.
• Letting the spoons touch the bucket in Activity 10.4. The sound then travels through the solid wall and the experiment no longer tests water at all.
• Watching the wave instead of the marked coil in Activity 10.5. The marker is the point: the disturbance travels, the medium does not.
• Thinking the medium travels with the sound. Air does not move from the speaker to your ear; the disturbance does. This is the misconception the slinky exists to correct.
• Trusting a phone speaker’s high-frequency limit in Activity 10.8. What fails first is usually the speaker, not the ear.
• Saying that sound above 20 kHz “does not exist”. It exists; humans just cannot hear it, which is the whole basis of ultrasound.

Practise next

• The end-of-chapter questions, Revise, Reflect, Refine, where the speeds of sound in different media from Activities 10.3 and 10.4 are used numerically — including the steel-fence problem with two friends 340 m apart — and the audible range from Activity 10.8 appears in questions on ultrasound.
• The in-text Pause and Ponder questions of this chapter, thirteen across nine boxes — the most stops of any chapter in the book — including the bell-in-a-jar assertion that Activity 10.4 prepares for.
• The activities of Chapter 4, where the wave speed used here is the same kind of quantity as the velocities defined there.