This MCQ module is based on: Production and Propagation of Sound
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Production and Propagation of Sound
🎓 Class 9
Science
CBSE
Theory
Ch 10 — Sound Waves: Characteristics and Applications
⏱ ~15 min
🌐 Language: [gtranslate]
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Introduction: A World Full of Sounds
From the moment you wake up, sound surrounds you — the alarm clock buzzing, water running from a tap, the chatter of family members, the chirping of birds, traffic on the road. Sound is a form of energy that lets us communicate, enjoy music, listen to instructions in class, and even sense danger. But what exactly is sound? How is it made and how does it travel from one place to another?
In this chapter we will discover that sound is produced by vibrating objects and that it cannot reach us unless there is a medium like air, water or a solid to carry it. We will also learn the special way in which sound waves move — by squeezing and stretching the medium in a back-and-forth pattern.
Key Idea: Sound is energy produced by vibrations. It needs a material medium for its propagation and travels as a longitudinal wave consisting of compressions and rarefactions.
10.1 Production of Sound
Try a simple test. Place your fingers gently against the front of your throat and say the word "Aaaa" loudly. You will feel a buzzing sensation under your skin. That buzzing is the rapid back-and-forth movement of your vocal cords. Whenever an object moves rapidly to and fro about a fixed position, we say it is in a state of vibration. Sound is always produced by some vibrating body.
Everyday vibrating sources of sound
- Tuning fork: When struck on a rubber pad, the two prongs vibrate sideways. Touch the prongs to a small ball hung by a string — the ball is repeatedly kicked away, proving the prongs are moving.
- Stretched string of a guitar or sitar: Plucking the string makes it oscillate up and down. The vibration is transferred to the air around it.
- Drum or tabla: Striking the membrane causes it to flex in and out. Sprinkle a few rice grains on it and beat — the grains dance because of the vibration.
- Flute or whistle: Blowing forces the air column inside to vibrate as a whole.
- Vocal cords: Air pushed up from the lungs makes the cords flutter; the rate of fluttering decides the pitch of our voice.
10.2 Activity — Catching the Vibration
Activity 10.1 — The Tuning Fork and the Pith BallL3 Apply
Predict first: A small light ball hangs from a thread. You bring a vibrating tuning fork close to it. Will the ball swing, and which prong will it follow?
- Hang a small pith ball (or a thermocol bead) from a thin thread tied to a stand.
- Strike a tuning fork on a rubber pad. The fork now hums softly — its prongs are vibrating.
- Bring one of the prongs gently in contact with the pith ball without holding the fork tightly.
- Observe whether the ball moves and how often it gets pushed.
- Now stop the prongs with your finger, then touch the still fork to the ball.
Observations: When the vibrating prong touches the ball, the ball is repeatedly knocked away — sometimes hundreds of times per second. The fork at the same time produces an audible hum. When the fork is silenced (no vibration), the same prong pressed against the ball produces no kicks and no sound.
Conclusion: A sound-producing object is a vibrating object. The mechanical to-and-fro motion of the prongs is what generates the sound; once vibration stops, sound stops.
Conclusion: A sound-producing object is a vibrating object. The mechanical to-and-fro motion of the prongs is what generates the sound; once vibration stops, sound stops.
10.3 Propagation of Sound
A vibrating object produces sound at one location, yet our ear catches it many metres away. How does the disturbance reach us? It does so by setting the surrounding particles of the medium into oscillation. Each particle nudges the next, and the next, in a chain. The particles themselves do not travel from the source to the listener — they only oscillate around their own positions. What travels is the energy of vibration, in the form of a wave.
Definition. A sound wave is a disturbance produced by a vibrating body that travels through a material medium by causing successive layers of the medium to vibrate.
Sound is a longitudinal wave
When a tuning fork prong moves outward, it pushes the air molecules in front of it close together. This region of higher density (and pressure) is called a compression (C). As the prong returns and moves the other way, it pulls the molecules apart — a region of low density and low pressure forms, called a rarefaction (R). As the fork keeps vibrating, alternate compressions and rarefactions move outward through the air. This is a longitudinal wave — the particles of the medium oscillate parallel to the direction in which the wave travels.
🔊 Wave Walk-through — Click each part of the longitudinal wave L3 Apply
Step through the wave from source to wavelength. Click the tuning fork, a compression (C), a rarefaction (R), and the bracket marked λ to apply what each piece means.
Click the tuning fork, a C, an R, or the λ bracket to step through how a longitudinal sound wave forms.
Why does sound need a medium?
The disturbance can propagate only if there are particles to pass it on. In empty space (a vacuum) there are no air molecules, no liquid molecules and no atoms — so a vibrating body has nothing to push or pull. No compression or rarefaction can be set up, and no sound can travel. This is why the surface of the Moon, where there is hardly any atmosphere, is silent. Astronauts on the Moon must use radio transmitters even when standing right next to one another.
Bell-jar experiment
The classical proof that sound needs a medium is the bell-jar experiment. An electric bell is suspended inside an airtight glass jar that is connected to a vacuum pump. When air is present, switching the bell on produces a clear ringing. As the pump gradually removes the air from the jar, the ringing becomes fainter and fainter, even though the hammer can still be seen striking the gong. When almost all the air has been pumped out, no sound reaches the ear at all. Letting the air back in restores the sound. The conclusion: sound cannot travel through a vacuum.
10.4 Speed of Sound in Different Media
Sound travels at very different speeds in different materials. The denser the bonds between particles, the faster vibrations are passed along. As a rule, sound moves fastest in solids, slower in liquids, and slowest in gases. The speed also depends on temperature — sound becomes a little faster as the medium warms up.
| Medium | Approximate speed of sound (m/s) at 25 °C |
|---|---|
| Air (dry) | 346 |
| Hydrogen gas | 1284 |
| Water (distilled) | 1498 |
| Sea water | 1531 |
| Aluminium | 5100 |
| Iron | 5130 |
| Glass | 5170 |
Important: Solids > Liquids > Gases for the speed of sound. This is exactly the opposite of what happens with light, which travels fastest in a vacuum.
An everyday demonstration: place your ear against one end of a long iron railing while a friend taps the other end. You will hear two distinct taps — first the one that travelled through the iron (very fast), then the slower one through the air.
Worked example — distance from speed and time
Q: A girl shouts and her echo from a distant cliff returns after 3 s. If the speed of sound in air is 340 m/s, how far is the cliff?
A: Total distance travelled by sound = \(v \times t = 340 \times 3 = 1020\) m. The sound has covered the cliff distance twice (going and returning), so distance to cliff \(= 1020/2 = 510\) m.
A: Total distance travelled by sound = \(v \times t = 340 \times 3 = 1020\) m. The sound has covered the cliff distance twice (going and returning), so distance to cliff \(= 1020/2 = 510\) m.
Competency-Based Questions
During a science exhibition, Aman demonstrates a vibrating tuning fork that knocks a hanging pith ball repeatedly. He then puts an electric bell inside a sealed bell jar connected to a vacuum pump and switches it on. As his classmate slowly removes air from the jar, the sound becomes feeble and finally inaudible, although the hammer is still striking the gong.
Q1. Why does the pith ball get repeatedly pushed back when the vibrating prong touches it? L2
(b) The prong moves to and fro about its rest position. On each outward swing it kicks the pith ball, proving the prong is vibrating — and that vibration is the source of the sound.
Q2. As the air is pumped out of the bell jar, the ringing fades. What does this tell us about sound? L3
Sound requires a material medium for propagation. With less air, fewer particles are available to pass on compressions and rarefactions, so the wave cannot reach the listener. In a complete vacuum, no sound can travel.
Q3. State whether True or False: "Sound is a transverse wave in air." Justify briefly. L1
False. Sound is a longitudinal wave. Air particles oscillate parallel to the direction of propagation, alternately producing compressions and rarefactions.
Q4. A boy hears two sharp taps when a metal rod is struck at the far end. Explain. L4
The first tap travels through the metal rod (a solid) and reaches the ear quickly because sound moves much faster in solids (~5000 m/s). The second tap arrives a moment later through the surrounding air (~346 m/s). The two paths produce two clearly separated sounds.
Q5. A thunder strike occurs 1.7 km away. After how many seconds will the sound reach you? Take v = 340 m/s. L3
\(t = d/v = 1700/340 = 5\) s. You will hear the thunder five seconds after seeing the lightning flash.
Assertion–Reason Questions
Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.
A: Astronauts working on the Moon's surface cannot speak directly to one another even when standing close together.
R: The Moon has practically no atmosphere, so there is no medium to carry sound waves.
(A) Both statements are true and R is the correct explanation. They use radios because radio (electromagnetic) waves do not need a medium.
A: Sound travels faster in iron than in air.
R: The particles of iron are tightly bonded and pass vibrations on more rapidly than gas molecules.
(A) True and the reason correctly explains the assertion. Solids transmit sound several times faster than gases.
A: A tuning fork still produces sound when its prongs are firmly held by the hand.
R: Sound is produced only when an object is vibrating.
(D) Assertion is false — gripping the prongs damps the vibration and silences the fork. Reason is true and is the basic principle.
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