Complete Solutions and Summary of Sound – NCERT Class 9, Science, Chapter 11 – Summary, Questions, Answers, Extra Questions
Detailed summary and explanation of Chapter 11 ‘Sound’ with all question answers, extra questions, and solutions from NCERT Class IX, Science.
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Categories: NCERT, Class IX, Science, Summary, Extra Questions, Sound, Chapter 11
Tags: Sound, Production of Sound, Propagation of Sound, Sound Waves, Longitudinal Waves, Reflection of Sound, Echo, Reverberation, Ultrasound, Infrasound, Human Ear, Class 9, NCERT, Science, Chapter 11, Answers, Extra Questions
Sound
Chapter 11: Science - Complete Study Guide
Chapter Overview
Vibration
Produces Sound
Longitudinal
Wave Type
Wavelength
λ (m)
Frequency
f (Hz)
What You'll Learn
Sound Production
By vibrating objects like tuning fork.
Propagation
Through medium as compressions/rarefactions.
Characteristics
Frequency, amplitude, speed.
Human Ear
Receives and interprets sound.
Key Highlights
Sound is energy produced by vibrations, propagates as longitudinal mechanical waves through mediums like air, characterized by wavelength, frequency, and amplitude. Speed varies with medium; reflection causes echo, used in sonar. Human ear converts vibrations to nerve impulses.
Comprehensive Chapter Summary
1. Introduction to Sound
- Sound from sources like humans, birds, bells, machines, vehicles, TVs, radios; form of energy causing hearing sensation.
- Other energies: Mechanical (previous chapters), light; conservation: Cannot create/destroy, only transform.
- Clapping produces sound using energy; question: Can sound be produced without energy? Which form used?
- Chapter covers production, transmission through medium, reception by ears.
- To double: Everyday sounds vary in pitch/loudness; sound needs medium, unlike light; energy transformation e.g., clapping: Mechanical to sound energy.
Activity 11.1: Tuning Fork Vibration
Strike tuning fork, hear sound near ear; touch prong (feels vibration); touch suspended ball (ball moves away then attracted); discuss vibration causes sound.
Activity 11.2: Tuning Fork in Water
Touch water surface with prong (small ripples); dip both prongs (large waves); vibration transfers energy to water particles.
2. Production of Sound (11.1)
- Sound by vibrating objects: Strike tuning fork, pluck rubber band, flap bird wings, buzz bee wings, vocal cords vibrate for voice.
- Vibration: Rapid to-fro motion; without vibration, no sound.
- Produce by striking, plucking, scratching, rubbing, blowing, shaking.
- Musical instruments: List types, discuss vibrating parts e.g., guitar strings, drum membrane, flute air column.
- To double: Human voice: Air from lungs vibrates vocal cords; bird/bees: Wing beats; rubber band: Tension affects pitch; activities show vibration essential.
Activity 11.3: Musical Instruments
List instruments (guitar, drum, flute); vibrating parts: Strings/membrane/air column produce sound via vibration.
3. Propagation of Sound (11.2)
- Medium: Solid, liquid, gas; sound travels from source to listener via particle vibration.
- Vibrating object displaces adjacent particle, chain reaction; disturbance travels, not particles.
- Wave: Disturbance moving through medium; particles oscillate, disturbance propagates.
- Sound waves: Mechanical, longitudinal; air common medium.
- Compression (C): High pressure/density region (forward motion compresses air).
- Rarefaction (R): Low pressure/density (backward motion rarefies air).
- Propagation: Series of C and R; density/pressure variations.
- To double: Particles oscillate about equilibrium; no net forward movement; speed depends on medium (fastest in solids); vacuum no sound (no medium).
Compression & Rarefaction
- C: High density/pressure.
- R: Low density/pressure.
- Sound as wave of these variations.
Medium Requirement
- Needs particles to propagate.
- No sound in vacuum (moon example).
4. Sound Waves are Longitudinal Waves (11.2.1)
- Activity with slinky: Push-pull creates C (coils close) and R (coils apart); dot moves parallel to propagation.
- Longitudinal: Particles oscillate parallel to wave direction.
- Transverse: Perpendicular oscillation (e.g., water ripples, light); not mechanical for light.
- Sound: Longitudinal mechanical wave; particles back-forth, disturbance forward.
- To double: Slinky simulates air particles; in solids/liquids, similar but faster; transverse needs tension (strings), longitudinal compression.
Activity 11.4: Slinky Waves
Stretch slinky, push: Observe C/R; mark dot: Parallel motion confirms longitudinal.
5. Characteristics of a Sound Wave (11.2.2)
- Describe by frequency (f), amplitude (A), speed (v).
- Graphic: Density/pressure vs distance; crest (peak C), trough (valley R).
- Wavelength (\( \lambda \)): Distance between consecutive C or R; unit m.
- Frequency (f): Vibrations per second; unit Hz (hertz); loudness relates to amplitude, pitch to f.
- Amplitude: Max displacement; larger A = louder sound.
- Speed: \( v = f \lambda \); depends on medium temperature.
- To double: Graphs show variations above/below average; high f = shrill (e.g., mosquito), low f = grave (e.g., lion); audible 20Hz-20kHz; ultrasound >20kHz, infrasound <20Hz.
Hertz Biography
Heinrich Hertz confirmed electromagnetic theory, discovered photoelectric effect; unit Hz named after him.
6. Reflection of Sound
- Sound reflects off surfaces; angle incidence = reflection.
- Echo: Reflected sound heard after original; minimum distance 17m for distinction.
- Reverberation: Multiple reflections, prolonged sound.
- To double: Used in halls for acoustics; echo for distance measurement.
7. Sonar and Applications
- Sonar: Sound Navigation Ranging; echoes measure depth/time.
- Time t, speed v: Distance = \( \frac{v t}{2} \).
- Uses: Ocean depth, submarine detection, fish finding.
- To double: Active/passive sonar; medical ultrasound imaging.
8. Human Ear Structure
- Outer: Pinna collects, canal to eardrum.
- Middle: Hammer, anvil, stirrup amplify vibrations to oval window.
- Inner: Cochlea (fluid, hair cells detect frequency), auditory nerve to brain.
- To double: Eardrum vibrates like tuning fork; cochlea separates pitches; hearing range 20-20kHz, decreases with age.
Range of Hearing
Audible: 20Hz-20kHz; ultrasound for bats/medicine; infrasound for elephants.
Key Concepts and Definitions
Vibration
Rapid to-fro motion producing sound.
Medium
Matter for sound propagation.
Longitudinal Wave
Particles parallel to propagation.
Wavelength
\( \lambda \): Distance C to C.
Frequency
f: Cycles per second, Hz.
Speed
\( v = f \lambda \).
Echo
Reflection after 0.1s.
Important Facts
v (air)
330 m/s
Audible
20-20k Hz
Echo Dist
17 m
Sonar
d = vt/2
Cochlea
Frequency Detect
Questions and Answers from Chapter
Short Questions (1 Mark)
Q1. How does sound produced by vibrating object reach ear?
Answer: Through medium particles' vibration.
Q2. Explain sound by school bell.
Answer: Striking vibrates bell.
Q3. Why sound waves mechanical?
Answer: Need medium particles.
Q4. Hear friend on moon?
Answer: No, vacuum.
Q5. What is vibration?
Answer: To-fro motion.
Q6. Medium for sound?
Answer: Solid/liquid/gas.
Q7. What is compression?
Answer: High pressure region.
Q8. What is rarefaction?
Answer: Low pressure region.
Q9. Sound waves type?
Answer: Longitudinal.
Q10. Wavelength symbol?
Answer: \( \lambda \).
Q11. Frequency unit?
Answer: Hz.
Q12. Crest represents?
Answer: Compression.
Q13. Trough represents?
Answer: Rarefaction.
Q14. Speed formula?
Answer: \( v = f \lambda \).
Q15. Transverse wave example?
Answer: Water ripples.
Q16. Sound in vacuum?
Answer: No.
Q17. Amplitude relates to?
Answer: Loudness.
Q18. Frequency relates to?
Answer: Pitch.
Q19. Slinky activity shows?
Answer: Longitudinal wave.
Q20. Human voice produced by?
Answer: Vocal cords vibration.
Medium Questions (3 Marks)
Q1. How sound reaches ear from vibrating object?
Answer: Object displaces medium particle, which displaces next, chain to ear; disturbance propagates.
Q2. Explain school bell sound production.
Answer: Striking vibrates bell metal; vibrations create C/R in air, propagate as sound waves to ear.
Q3. Why sound waves mechanical?
Answer: Produced by vibrating objects; propagate via medium particle collisions; need material medium.
Q4. Why no sound on moon?
Answer: Moon has vacuum; sound needs medium particles to propagate; no air, no vibration transfer.
Q5. Differentiate compression and rarefaction.
Answer: Compression: High density/pressure, particles close; Rarefaction: Low density/pressure, particles apart.
Q6. Why longitudinal waves for sound?
Answer: Medium particles oscillate parallel to propagation; like slinky push-pull creating C/R.
Q7. Define wavelength.
Answer: Distance between consecutive compressions or rarefactions; \( \lambda \), unit metre.
Q8. What is frequency?
Answer: Number of vibrations per second; determines pitch; unit Hz.
Q9. Differentiate longitudinal and transverse waves.
Answer: Longitudinal: Parallel oscillation (sound); Transverse: Perpendicular (water ripples).
Q10. Crest and trough in sound wave?
Answer: Crest: Max compression (high pressure); Trough: Max rarefaction (low pressure).
Q11. Role of medium in propagation.
Answer: Provides particles for disturbance transfer; solids fastest, gases slowest.
Q12. Amplitude effect?
Answer: Larger amplitude: Louder sound; max displacement from equilibrium.
Q13. Speed of sound formula.
Answer: \( v = f \lambda \); product of frequency and wavelength.
Q14. Why no sound in space?
Answer: Vacuum lacks medium; vibrations cannot propagate without particles.
Q15. Slinky activity observation.
Answer: Push creates compression, pull rarefaction; dot moves parallel, showing longitudinal.
Q16. Vocal cords role.
Answer: Vibrate due to air from lungs, produce voice sound waves.
Q17. Density variation in sound.
Answer: High in compression, low in rarefaction; causes pressure changes.
Q18. Audible frequency range.
Answer: 20 Hz to 20,000 Hz for humans.
Q19. Echo condition.
Answer: Reflection after 0.1 s; min distance 17 m in air.
Q20. Sonar principle.
Answer: Echo time measures distance; \( d = \frac{v t}{2} \).
Long Questions (6 Marks)
Q1. Explain sound propagation with compression/rarefaction.
Answer: Vibrating object creates compression (high density/pressure) by pushing air, rarefaction (low) by pulling; series propagates as longitudinal wave; particles oscillate, disturbance travels; visualized as density/pressure variations; speed via \( v = f \lambda \); examples: Tuning fork in air/water shows ripples.
Q2. Differentiate longitudinal and transverse waves with examples.
Answer: Longitudinal: Oscillation parallel to propagation (sound, slinky push); creates C/R. Transverse: Perpendicular (water ripples, string pluck); crests/troughs. Sound mechanical longitudinal, light transverse electromagnetic; slinky activity demonstrates parallel motion for sound.
Q3. Describe sound wave characteristics: Frequency, wavelength, amplitude.
Answer: Wavelength \( \lambda \): Distance C to C, unit m. Frequency f: Vibrations/s, Hz, high f=high pitch. Amplitude A: Max displacement, large A=loud sound. Speed \( v = f \lambda \); graphic shows crests (C), troughs (R); audible 20-20kHz; graphs density/pressure vs distance.
Q4. Why no sound on moon? Explain propagation need for medium.
Answer: Sound mechanical wave needs medium particles for vibration transfer; moon vacuum lacks air/solid path; on earth, air particles chain disturbance from source to ear; solids/liquids faster due closer particles; activities show no ripples in vacuum equivalent.
Q5. Explain production of sound with examples from activities.
Answer: Vibration essential: Tuning fork strike vibrates prongs, sound heard/touched; ball repelled/attracted shows energy transfer; water ripples from touch/dip confirm vibration propagates; rubber band pluck vibrates; voice: Vocal cords; instruments: Strings/membranes; no vibration, no sound.
Q6. Describe slinky activity for longitudinal waves.
Answer: Stretch slinky, sharp push: Compression (coils close); pull: Rarefaction (coils apart); alternate: Series C/R; marked dot oscillates parallel to propagation; simulates sound: Particles back-forth, disturbance forward; contrasts transverse perpendicular motion.
Q7. Explain echo and reverberation.
Answer: Echo: Reflection from obstacle after 0.1s (17m air); clear repeated sound. Reverberation: Multiple quick reflections, prolonged blurred sound; controlled in halls for clarity; both due reflection laws (i=r); sonar uses echo for distance.
Q8. Principle and use of sonar.
Answer: Sonar: Pulse sent, echo time t, speed v: Depth \( d = \frac{v t}{2} \); uses: Sea depth, submarine location, fish finding; active (emits), passive (listens); medical ultrasound similar for imaging.
Q9. Structure and function of human ear.
Answer: Outer: Pinna collects, canal to eardrum (vibrates). Middle: Ossicles (hammer/anvil/stirrup) amplify to cochlea. Inner: Cochlea fluid/hair cells detect frequency, nerve to brain; converts vibration to impulse; range 20-20kHz.
Q10. How frequency/amplitude affect sound?
Answer: Frequency: High=shrill pitch (mosquito), low=deep (thunder); vibrations/s. Amplitude: Large=loud, small=soft; max displacement; graphs show wave height for A, cycles for f; human audible 20-20kHz.
Q11. Role of medium particles in propagation.
Answer: Source vibrates, displaces first particle from equilibrium, exerts force on next, chain to ear; particles oscillate, no net travel; disturbance (C/R) propagates; closer particles (solids) faster speed.
Q12. Differentiate audible, ultrasound, infrasound.
Answer: Audible: 20-20kHz (human hearing). Ultrasound: >20kHz (bats echo-location, medical imaging). Infrasound: <20Hz (elephants communicate, earthquakes); all longitudinal, differ in f.
Q13. Explain graphic representation of sound wave.
Answer: Density/pressure vs distance: Upper curve compression (high), lower rarefaction (low); crest max C, trough max R; wavelength between crests; amplitude half peak-trough; shows variations above/below average.
Q14. Why sound faster in solids?
Answer: Solids: Particles closest, quickest force transfer; liquids medium, gases farthest/slowest; v air 330m/s, water 1500, steel 5000; temperature increases speed in gases.
Q15. Describe tuning fork activities.
Answer: Strike: Hear sound, feel vibration; ball: Repels/attracts showing energy; water touch: Small ripples, dip: Large waves; proves vibration transfers to medium, creates waves.
Q16. How musical instruments produce sound?
Answer: String (guitar): Pluck vibrates string. Membrane (drum): Strike vibrates skin. Air (flute): Blow vibrates column; all create C/R in air; pitch by length/tension.
Q17. Explain conservation in sound production.
Answer: Mechanical energy (clap/strike) transforms to sound energy; neither created/destroyed; some lost as heat; total conserved, but sound fraction small.
Q18. Role of cochlea in ear.
Answer: Spiral tube with fluid; vibrations from oval window cause hair cells to bend, detect frequency; different regions for pitches; signals to auditory nerve/brain for perception.
Q19. Conditions for echo.
Answer: Obstacle >17m away; dry surface for clear reflection; time gap >0.1s; used in sonar; multiple echoes cause reverberation if <0.1s.
Q20. Speed of sound variations.
Answer: Depends medium (solid>liquid>gas), temperature (higher=faster in air); formula \( v = f \lambda \); air 330m/s at 0°C, increases 0.6m/s per °C; humidity slight increase.
Interactive Knowledge Quiz
Test your understanding of Sound
Quick Revision Notes
Production & Propagation
- Vibration → C/R in medium
- Longitudinal mechanical wave
Characteristics
- \( \lambda \): C to C
- f: Pitch, Hz
- A: Loudness
- \( v = f \lambda \)
Applications
- Echo: 17m
- Sonar: \( d = vt/2 \)
- Ear: Pinna-cochlea-nerve
Exam Strategy Tips
- Draw wave diagrams
- Explain C/R
- Formulas with units
- Activities detail
- Differentiate wave types
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