Notes Class 9 Science Exploration Chapter 10 Sound Waves: Characteristics and Applications

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Chapter Overview: What is Sound?

Every day, we hear birds chirping, mobile phones ringing, thunder clapping, and music playing. But have you ever wondered — what exactly is sound and how does it travel?

You already know from Chapter 7 that sound is a form of energy. In this chapter, we explore how sound is produced, how it travels, and its amazing real-world uses.

Fig: Oscillation — the back-and-forth motion that produces sound

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Key Idea

Sound is produced by vibration — any back-and-forth motion of an object. No vibration means no sound!

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Did You Know? (Kongthong Village)

In Kongthong village near Shillong (Meghalaya), every person has a unique “tune name” that is whistled or sung — called Jingrwai Iawbei. Each mother composes a special lullaby tune for her child at birth!

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Production & Propagation of Sound

🎯 How is Sound Produced?

Sound is produced by vibrating objects. Vibration (कंपन) means the periodic to-and-fro (oscillatory) motion of an object.

Plucking a stretched rubber band — the band vibrates → produces sound
Blowing a bansuri (flute) — air inside the pipe vibrates → sound
Striking a tuning fork — prongs vibrate → sound
Speaking/singing — vocal cords (स्वर तंत्री) inside the larynx vibrate → sound
Grasshoppers & crickets — rub wings or legs → sound

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NCERT Definition — Vibration

Vibration refers to the periodic to and fro motion (oscillations) of an object. The object producing sound is called the‘source’of sound.

📡 How Does Sound Propagate (Travel)?

Sound needs a medium (माध्यम) to travel — it can travel through solids, liquids, and gases.

🪨 Through Solids

Place your ear on a desk — you can hear knocking through the table. Sound travels fastest in solids.

💧 Through Liquids

Tap two spoons underwater — you can hear the sound through water. Sound travels fast in liquids too.

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Sound CANNOT travel in Vacuum!

The vacuum bell jar experiment proves this. As air is pumped out, sound gets fainter. In outer space (near vacuum), astronauts cannot talk directly — they use special communication devices.

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Key Definition — Medium

The material through which sound propagates is called amedium. Sound is amechanical wave— it requires a material medium to travel.

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Sound Waves — Compressions & Rarefactions

🧲 How Does Sound Travel as a Wave?

Think of a slinky stretched on a table. Push and pull one end repeatedly — you see regions where the coils are closer together (compressed) and regions where they are more spread out (rare). This is exactly how sound travels!

Compression and rarefaction in a medium

Fig: Compressions (C) and Rarefactions (R) travelling through a medium

Term	What It Means	Density
Compression (C) — संपीडन	Region where air particles are pushed closer together	Higher than average
Rarefaction (R) — विरलन	Region where air particles spread farther apart	Lower than average

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Definition — Sound Wave

The disturbance consisting of a series of alternating compressions and rarefactions propagating through a medium, without the actual flow of the particles of medium, is called asound wave.

📐 Longitudinal Wave vs Transverse Wave

🔊 Longitudinal Wave (Sound)

Particles vibrate parallel to the direction of wave propagation. Example: Sound waves, slinky waves.

🌊 Transverse Wave (Light)

Particles vibrate perpendicular to the direction of wave propagation. Example: Light waves, water ripples.

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Important Note

The particles of the medium do NOT travel with the wave. They onlyvibrate about their mean positions. It is theenergythat travels, not the particles!

⚡ Sound as Energy

Sound is a form of energy. When a source vibrates, it transfers energy to surrounding medium particles. This is why grains placed on a stretched membrane jump when a loud sound is produced nearby — the sound energy makes the membrane vibrate!

Devices like microphones (convert sound → electrical) and speakers (convert electrical → sound) work because of this energy transfer.

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Characteristics: Wavelength, Frequency, Speed

📊 Key Terms to Describe a Sound Wave

Property	Symbol	Definition	SI Unit
Wavelength (तरंगदैर्ध्य)	λ (lambda)	Distance between two consecutive crests OR two consecutive troughs	metre (m)
Frequency (आवृत्ति)	ν (nu)	Number of density oscillations per unit time at a fixed point	hertz (Hz) = s⁻¹
Time Period (आवर्त काल)	T	Time taken for one complete density oscillation at a fixed point	second (s)
Amplitude (आयाम)	A	Maximum change in density compared to average density	kg/m³
Speed (चाल)	v	Distance travelled by a crest (or trough) per unit time	m s⁻¹

🔢 Important Formulas

ν = 1/T → Frequency = 1 / Time Period (Eq. 10.1)

v = λ × ν → Speed = Wavelength × Frequency (Eq. 10.2)

🌡️ Speed of Sound in Different Media

Sound travels fastest in solids, slower in liquids, and slowest in gases.

State	Medium	Speed at 15°C
Solid	Steel	5000 m s⁻¹
Liquid	Water	1500 m s⁻¹
Gas	Air	340 m s⁻¹

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Exam Trick — Speed of Sound in Air

Speed of sound in dry air =331 m s⁻¹ at 0°Cand344 m s⁻¹ at 22°C. Speed increases with temperature and humidity. Remember: Hot air = Faster sound!

🧮 Solved Examples

Example 10.1: 10 density oscillations in 2 seconds.

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Frequency ν = oscillations / time = 10 / 2 = 5 Hz

Time Period T = 1/ν = 1/5 = 0.2 s

Example 10.2: Find λ for 20 Hz and 20 kHz; v = 344 m s⁻¹

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λ = v / ν

For ν = 20 Hz: λ = 344 / 20 = 17.2 m

For ν = 20000 Hz: λ = 344 / 20000 = 0.0172 m = 1.72 cm

Example 10.3: Lightning-thunder delay = 5 s; v = 340 m s⁻¹

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Distance = v × t = 340 × 5 = 1700 m = 1.7 km

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Amplitude, Loudness, Pitch & Human Perception

🔊 Amplitude and Loudness

🔉 Small Amplitude

Less energy → Softer sound (कम आवाज़). Grains barely move on membrane.

🔊 Large Amplitude

More energy → Louder sound (तेज़ आवाज़). Grains jump higher on membrane.

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Intensity of Sound

The amount of sound energy passing through a unit area perpendicular to the direction of propagation of sound wave in unit time is calledintensity. Intensity decreases as we move away from the source.

🎵 Pitch (तारत्व)

How frequency is perceived by humans is called pitch.

High Pitch = High frequency (e.g., whistle, siren, child’s voice)
Low Pitch = Low frequency (e.g., thunder, aircraft rumble, man’s voice)

👂 Human Hearing Range (श्रव्य परास)

Type	Frequency Range	Who Can Detect?
Infrasonic (अवश्रव्य)	Less than 20 Hz	Elephants, whales
Audible (श्रव्य)	20 Hz – 20,000 Hz	Humans
Ultrasonic (पराश्रव्य)	More than 20,000 Hz (20 kHz)	Dogs, cats, bats, dolphins

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Loudness is measured in Decibels (dB)

Rustling leaves ≈ few dB | Normal conversation ≈ 60 dB | Firecrackers > 100 dB. Prolonged exposure to loud sounds can cause hearing loss (बहरापन)!

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Sir C. V. Raman — Indian Scientist of Sound

Sir C. V. Raman won India’s first Nobel Prize in Science (1930) for discovering the Raman Effect in light. He also made important contributions to acoustics by studying Indian percussion instruments like the tabla and mridangam to understand how they produce such rich sounds. The black syaahi patch on the tabla’s membrane is a unique Indian innovation!

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Reflection of Sound: Echo & Reverberation

🔁 Reflection of Sound (ध्वनि का परावर्तन)

Sound bounces off hard surfaces (solids or liquids) — this is called reflection of sound. Sound follows the same laws of reflection as light: angle of incidence = angle of reflection.

🏔️ Echo (प्रतिध्वनि)

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What is an Echo?

When sound reflects off a distant hard surface and reaches our ears after the original sound, it is called anecho. For an echo to be heard, the time gap between the original and reflected sound must beat least 0.1 s.

📐 Minimum Distance for Echo

Time for echo = 0.1 s | Speed of sound = 340 m s⁻¹

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Total distance = v × t = 340 × 0.1 = 34 m (to wall and back)

Minimum distance of wall = 34 / 2 = 17 m

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Echo Formula (Exam Favourite!)

Distance = (v × t) / 2 → where t = time taken for echo to return.

Example 10.5: Echo heard after 0.5 s; v = 340 m s⁻¹

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Distance = (v × t) / 2 = (340 × 0.5) / 2 = 85 m

🎭 Reverberation (अनुरणन)

Reverberation is the persistence of sound due to multiple reflections in a large hall, even after the source has stopped. It occurs when reflected sound arrives within 0.05 s of the original.

Auditoriums use soft, porous materials (curtains, upholstered chairs) to absorb excess reflections
The curved ceilings in concert halls direct sound evenly to the audience
The famous Gol Gumbaz in Bijapur, Karnataka has a remarkable Whispering Gallery design!

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Echo vs Reverberation — Don’t Confuse!

Echo: reflected sound heard ≥ 0.1 s after original (distinguishable). | Reverberation: reflected sound within < 0.05 s — sounds persist but mix (not distinguishable).

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Ultrasound, Infrasound & Applications (SONAR)

🦇 Echolocation (इकोलोकेशन)

Bats (चमगादड़) are nocturnal and fly in complete darkness. They emit short bursts of ultrasonic waves which bounce off obstacles and prey. By sensing these echoes, bats determine the position of objects — this is called echolocation.

Dolphins, whales, and some birds also use echolocation for navigation and hunting.

🚢 SONAR (सोनार)

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SONAR = Sound Navigation And Ranging

Ultrasonic waves are sent into water and reflected waves are analysed to determine thedistance, direction, and speedof underwater objects (submarines, shipwrecks, ocean floor).

Distance = (v × t) / 2 → where v = speed of sound in water, t = echo return time

Example 10.6 (SONAR): Signal returns in 0.90 s; v = 1530 m s⁻¹

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Time to reach object = 0.90 / 2 = 0.45 s

Distance = 1530 × 0.45 = 688.5 m

🏥 Applications of Ultrasound in Medicine & Industry

🏥 Medical Uses

Ultrasonography — imaging internal organs without surgery. Breaking kidney stones (lithotripsy) into small pieces.

🏭 Industrial Uses

Detecting defects inside metal blocks. Ultrasonic welding. Cleaning delicate machine parts (watches, electronic components).

🌍 Applications of Infrasound

Detecting earthquakes and volcanic eruptions
Detecting severe storms (infrasound travels long distances through Earth)
Elephants communicate over hundreds of kilometres using infrasound!

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Sound Exploring Space & Earth!

Space probes have recorded the first sounds from Mars. Scientists time distant earthquake sounds to measure tiny changes in ocean temperature. Biologists use mosquito buzz patterns to identify disease-carrying mosquitoes!

⚡ Quick Revision Summary

Production of SoundSound is produced by vibrating objects. No vibration = No sound.

Sound WaveA longitudinal mechanical wave — alternating compressions (C) and rarefactions (R).

Wave Formulav = λ × ν | T = 1/ν | Speed in solid > liquid > gas

Human Hearing Range20 Hz to 20,000 Hz. Infrasound < 20 Hz. Ultrasound > 20 kHz.

Echo & ReverberationEcho: gap ≥ 0.1 s, min distance = 17 m. Reverberation: multiple reflections in hall.

SONARUses ultrasound to find underwater objects. Distance = (v × t) / 2.

Pitch & LoudnessPitch ∝ Frequency. Loudness ∝ Amplitude. Sound energy decreases with distance.

Particles Don’t Move!In sound propagation, only energy travels. Particles only vibrate about mean positions.

Speed in Air331 m s⁻¹ at 0°C. 344 m s⁻¹ at 22°C. Increases with temperature & humidity.

EcholocationBats, dolphins & whales use ultrasound echoes to navigate and hunt in darkness.

Sound Needs MediumCannot travel in vacuum. Proved by the vacuum bell jar experiment.

Noise PollutionSounds above safe limits cause hearing loss. Measured in decibels (dB).

📝 Important Exam Questions

Q1. What is the relationship between frequency and time period of a sound wave? If a sound wave has a frequency of 250 Hz, find its time period. (CBSE Pattern / 2 Marks)

Ans: Frequency (ν) and Time Period (T) are inversely related: T = 1/ν.

For ν = 250 Hz: T = 1/250 = 0.004 s.

Q2. What is an echo? What is the minimum distance required between the source and the reflecting surface to hear an echo? (CBSE Pattern / 3 Marks)

Ans: An echo is the repetition of sound caused by reflection off a distant hard surface. For an echo to be heard, the time gap must be ≥ 0.1 s. Distance = (v × t)/2 = (340 × 0.1)/2 = 17 m minimum.

Q3. Give the full form of SONAR. How is it used to find the depth of the ocean? A sonar signal takes 4 s to return. Speed of sound in seawater = 1500 m s⁻¹. Find the depth. (CBSE Pattern / 3 Marks)

Ans: SONAR = Sound Navigation And Ranging. Ultrasonic waves are sent into water; reflected waves are detected to find underwater objects.

Depth = (v × t)/2 = (1500 × 4)/2 = 3000 m.

Q4. What are ultrasonic waves? Give three applications of ultrasound. (CBSE Pattern / 3 Marks)

Ans: Ultrasonic waves have frequency above 20,000 Hz (20 kHz). Applications: (i) Ultrasonography for imaging internal organs. (ii) Breaking kidney stones (lithotripsy). (iii) Detecting defects inside metal blocks. (iv) Echolocation by bats and dolphins.

Q5. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m s⁻¹. Also find its time period. (CBSE Pattern / 2 Marks)

Ans: Using v = λ × ν → λ = v/ν = 440/220 = 2 m.

Time Period T = 1/ν = 1/220 ≈ 0.0045 s.

Q6. Why can’t astronauts doing spacewalks hear each other directly? What do they use instead? Explain with the concept of sound propagation. (CBSE Pattern / 2 Marks)

Ans: Outer space is a near vacuum — it has no material medium for sound to propagate through. Sound is a mechanical wave that needs a medium. So astronauts cannot hear each other directly and use special communication devices fitted into their spacesuits.

Notes Class 9 Science Exploration Chapter 10 Sound Waves: Characteristics and Applications

Amresh Academy

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