🔬 Physics · Class 9 · Chapter 10
🔊 Sound Waves: Characteristics and Applications
From vibrating rubber bands to sonar systems — master sound, its properties, and real-world uses!
Production of Sound
Propagation
Wavelength & Frequency
Speed of Sound
Echo & Reverberation
Ultrasound & Infrasound
Sonar
Propagation
Wavelength & Frequency
Speed of Sound
Echo & Reverberation
Ultrasound & Infrasound
Sonar
📚 Contents
- Chapter Overview & Introduction
- Production & Propagation of Sound
- Sound Waves — Compressions & Rarefactions
- Characteristics: Wavelength, Frequency, Speed
- Amplitude, Loudness, Pitch & Human Perception
- Reflection of Sound: Echo & Reverberation
- Ultrasound, Infrasound & Applications (SONAR)
- Quick Revision Summary
- Important Exam Questions
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
Key Idea
Sound is produced by vibration — any back-and-forth motion of an object. No vibration means no sound!
Sound is produced by vibration — any back-and-forth motion of an object. No vibration means no sound!
🎵
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!
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!
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
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.
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.
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.
Tap two spoons underwater — you can hear the sound through water. Sound travels fast in liquids too.
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.
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.
Key Definition — Medium
The material through which sound propagates is called a medium. Sound is a mechanical wave — it requires a material medium to travel.
The material through which sound propagates is called a medium. Sound is a mechanical wave — it requires a material medium to travel.
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!
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 |
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 a 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 a sound wave.
📐 Longitudinal Wave vs Transverse Wave
🔊 Longitudinal Wave (Sound)
Particles vibrate parallel to the direction of wave propagation. Example: Sound waves, slinky waves.
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.
Particles vibrate perpendicular to the direction of wave propagation. Example: Light waves, water ripples.
Important Note
The particles of the medium do NOT travel with the wave. They only vibrate about their mean positions. It is the energy that travels, not the particles!
The particles of the medium do NOT travel with the wave. They only vibrate about their mean positions. It is the energy that 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.
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 |
Exam Trick — Speed of Sound in Air
Speed of sound in dry air = 331 m s⁻¹ at 0°C and 344 m s⁻¹ at 22°C. Speed increases with temperature and humidity. Remember: Hot air = Faster sound!
Speed of sound in dry air = 331 m s⁻¹ at 0°C and 344 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.
─────────────────────────────────
Frequency ν = oscillations / time = 10 / 2 = 5 Hz
Time Period T = 1/ν = 1/5 = 0.2 s
─────────────────────────────────
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
─────────────────────────────────
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
─────────────────────────────────
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
─────────────────────────────────
Distance = v × t = 340 × 5 = 1700 m = 1.7 km
─────────────────────────────────
Distance = v × t = 340 × 5 = 1700 m = 1.7 km
Amplitude, Loudness, Pitch & Human Perception
🔊 Amplitude and Loudness
🔉 Small Amplitude
Less energy → Softer sound (कम आवाज़). Grains barely move on membrane.
Less energy → Softer sound (कम आवाज़). Grains barely move on membrane.
🔊 Large Amplitude
More energy → Louder sound (तेज़ आवाज़). Grains jump higher on membrane.
More energy → Louder sound (तेज़ आवाज़). Grains jump higher on membrane.
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 called intensity. Intensity decreases as we move away from the source.
The amount of sound energy passing through a unit area perpendicular to the direction of propagation of sound wave in unit time is called intensity. 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 |
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)!
Rustling leaves ≈ few dB | Normal conversation ≈ 60 dB | Firecrackers > 100 dB. Prolonged exposure to loud sounds can cause hearing loss (बहरापन)!
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!
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!
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)
What is an Echo?
When sound reflects off a distant hard surface and reaches our ears after the original sound, it is called an echo. For an echo to be heard, the time gap between the original and reflected sound must be at least 0.1 s.
When sound reflects off a distant hard surface and reaches our ears after the original sound, it is called an echo. For an echo to be heard, the time gap between the original and reflected sound must be at least 0.1 s.
📐 Minimum Distance for Echo
Time for echo = 0.1 s | Speed of sound = 340 m s⁻¹
─────────────────────────────────────────
Total distance = v × t = 340 × 0.1 = 34 m (to wall and back)
Minimum distance of wall = 34 / 2 = 17 m
─────────────────────────────────────────
Total distance = v × t = 340 × 0.1 = 34 m (to wall and back)
Minimum distance of wall = 34 / 2 = 17 m
Echo Formula (Exam Favourite!)
Distance = (v × t) / 2 → where t = time taken for echo to return.
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
─────────────────────────────────
Distance = (v × t) / 2 = (340 × 0.5) / 2 = 85 m
─────────────────────────────────
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!
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).
Echo: reflected sound heard ≥ 0.1 s after original (distinguishable). | Reverberation: reflected sound within < 0.05 s — sounds persist but mix (not distinguishable).
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)
SONAR = Sound Navigation And Ranging
Ultrasonic waves are sent into water and reflected waves are analysed to determine the distance, direction, and speed of underwater objects (submarines, shipwrecks, ocean floor).
Ultrasonic waves are sent into water and reflected waves are analysed to determine the distance, direction, and speed of 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
─────────────────────────────────
Time to reach object = 0.90 / 2 = 0.45 s
Distance = 1530 × 0.45 = 688.5 m
─────────────────────────────────
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.
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).
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!
🌏
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!
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.
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.
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.
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.
Final Exam Tip!
Sound chapter has 3–5 numerical questions every year. Master these 3 formulas: v = λν, T = 1/ν, and Distance = (v × t)/2 (for echo/sonar). Always write SI units. Draw a neat diagram for echo problems. Best of luck! 🌟
Sound chapter has 3–5 numerical questions every year. Master these 3 formulas: v = λν, T = 1/ν, and Distance = (v × t)/2 (for echo/sonar). Always write SI units. Draw a neat diagram for echo problems. Best of luck! 🌟
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