Class 8 Science – Sound

Class 8 Science – Sound

Class 8 Science – Sound (One-Line Notes)

1. Basics

  • Sound: A form of energy that produces hearing sensation.
  • Production of sound: Sound is produced due to vibration.
  • Vibration: To-and-fro motion of an object.
  • Source of sound: Any vibrating object that produces sound.

2. Human Voice

  • Human voice: Sound is produced by vibration of vocal cords.
  • Vocal cords: Two elastic bands present in the voice box.
  • Voice box (larynx): Organ that produces sound in humans.
  • Talking: Air from lungs makes vocal cords vibrate.
  • Whispering: Vocal cords vibrate very little.

Male vs Female Vocal Cords:

FeatureMaleFemale
Vocal cord length17–25 mm12–17 mm
ThicknessThickerThinner
PitchLowHigh
FrequencyFewer vibrations per secondMore vibrations per second

3. Sound Needs a Medium

  • Medium: Substance through which sound travels.
  • Sound in air: Travels fastest in solids, slowest in gases.
  • Vacuum: Sound cannot travel in vacuum; it needs a medium (solid, liquid, or gas) to travel.

4. Characteristics of Sound

  • Amplitude: Maximum displacement during vibration.
  • Loudness: Depends on amplitude of vibration.
  • Louder sound: Produced by larger amplitude.
  • Softer sound: Produced by smaller amplitude.
  • Frequency: Number of vibrations per second.
  • Pitch: Depends on frequency of sound.
  • High pitch: Produced by high frequency.
  • Low pitch: Produced by low frequency.

5. Time Period

  • Time Period (T): Time taken by a vibrating object to complete one full vibration.
  • Relation with frequency (f): T=1fT = \frac{1}{f}
  • Example: If a pendulum swings once in 2 seconds, its time period is 2 s.
QuantitySymbolFormulaUnit
Time PeriodTT=1fT = \frac{1}{f}seconds (s)
Frequencyff=1Tf = \frac{1}{T}Hertz (Hz)

6. Sound Pollution

  • Noise: Unpleasant and unwanted sound.
  • Noise pollution: Excessive noise harmful to living beings.
  • Sources of noise: Vehicles, machines, loudspeakers.
  • Effects of noise: Causes stress, headache, and hearing loss.

Prevention

  • Noise control: Use silencers and soundproofing.
  • Plantation: Trees help reduce noise pollution.
  • Low volume: Loudspeakers should be used at low volume.

MCQs — Sound (Class 8, Copyright-Free)

  1. The part of the human body that produces voice is:
    A. trachea
    B. larynx
    C. lungs
    D. diaphragm
    Answer: B
  2. Sound is a form of:
    A. energy
    B. force
    C. heat
    D. pressure
    Answer: A
  3. Humans can hear sounds in the frequency range:
    A. 1–100 Hz
    B. 20–20,000 Hz
    C. 100–10,000 Hz
    D. 20,000–40,000 Hz
    Answer: B
  4. The pitch of a sound depends on its:
    A. amplitude
    B. frequency
    C. speed
    D. loudness
    Answer: B
  5. Sounds with frequency below 20 Hz are called:
    A. infrasonic
    B. ultrasonic
    C. audible
    D. musical
    Answer: A
  6. Loudness of a sound depends on:
    A. frequency
    B. amplitude
    C. wavelength
    D. speed
    Answer: B
  7. Sound cannot travel through:
    A. solids
    B. liquids
    C. gases
    D. vacuum
    Answer: D
  8. Sounds with frequency above 20,000 Hz are called:
    A. ultrasonic
    B. infrasonic
    C. audible
    D. sonic
    Answer: A
  9. The part of the ear that receives sound vibrations is:
    A. cochlea
    B. eardrum
    C. ossicles
    D. pinna
    Answer: A
  10. Number of vibrations per second is called:
    A. amplitude
    B. frequency
    C. pitch
    D. loudness
    Answer: B
  11. Sound travels fastest in:
    A. air
    B. water
    C. solids
    D. vacuum
    Answer: C
  12. Device that uses ultrasonic sound waves:
    A. stethoscope
    B. sonar
    C. telescope
    D. prism
    Answer: B

Short Questions & Answers

Q1. What is sound?
A: Sound is energy produced by vibrating objects.

Q2. How is sound produced?
A: Sound is produced when an object vibrates, causing surrounding particles to vibrate.

Q3. Can sound travel without a medium?
A: No, sound needs a material medium (solid, liquid, or gas) to travel.

Q4. What is vibration?
A: Vibration is the back-and-forth motion of an object that produces sound.

Q5. What is frequency?
A: Frequency is the number of vibrations per second.

Q6. What is pitch?
A: Pitch is how high or low a sound seems; it depends on frequency.

Q7. What determines loudness of a sound?
A: Loudness depends on the amplitude of vibration.

Q8. Give audible range of human hearing.
A: Humans can hear sounds from 20 Hz to 20,000 Hz.

Q9. What is noise?
A: Noise is unwanted or unpleasant sound.

Q10. What is ultrasonic sound?
A: Sound with frequency above 20,000 Hz, beyond human hearing.

Q11. Why does sound travel fastest in solids?
A: Particles in solids are closer together, allowing vibrations to pass faster.

Q12. What part of the ear senses vibrations?
A: The cochlea and inner ear structures.

Q13. What is loudness measured in?
A: Loudness is measured in decibels (dB).

Q14. How does amplitude affect loudness?
A: Larger amplitude produces louder sound.

Q15. Give an example of ultrasonic application.
A: Sonar used in submarines or for detecting objects underwater.

Important One-Word / Very Short Answers for Exams

  1. Source of sound – Vibrations
  2. To-and-fro motion – Vibration
  3. Sound is produced by – Vibrating objects
  4. Medium required for sound travel – Medium
  5. Sound cannot travel in – Vacuum
  6. Fastest medium for sound – Solid
  7. Slowest medium for sound – Gas
  8. Device producing sound in humans – Larynx
  9. Voice box – Larynx
  10. Thin membranes in larynx – Vocal cords
  11. Sound produced by tight vocal cords – High-pitched
  12. Sound produced by loose vocal cords – Low-pitched
  13. Loudness depends on – Amplitude
  14. Pitch depends on – Frequency
  15. Number of vibrations per second – Frequency
  16. Unit of frequency – Hertz
  17. Loudness is measured in – Decibel
  18. High frequency sound – Shrill
  19. Low frequency sound – Deep
  20. Larger amplitude sound – Loud
  21. Smaller amplitude sound – Soft
  22. Musical sound – Pleasant
  23. Noise – Unpleasant sound
  24. Unwanted sound – Noise
  25. Sound below hearing range – Infrasonic
  26. Sound above hearing range – Ultrasonic
  27. Hearing range of humans – 20–20000 Hz
  28. Organ of hearing – Ear
  29. Part of ear collecting sound – Pinna
  30. Tube connecting ear to throat – Eustachian tube
  31. Vibrating membrane in ear – Eardrum
  32. Tiny bones in middle ear – Ossicles
  33. Fluid-filled inner ear part – Cochlea
  34. Conversion of sound to signals – Cochlea
  35. Excessive noise pollution causes – Deafness
  36. Device to reduce noise – Silencer
  37. Reflection of sound – Echo
  38. Repetition of sound – Echo
  39. Time gap for echo – 0.1 second
  40. Multiple reflection used in – Megaphone
  41. Device amplifying sound – Megaphone
  42. Device used in hospitals to hear heartbeat – Stethoscope
  43. Sound produced by flute – Vibrations
  44. Sound produced by tabla – Vibrations
  45. Sound travels as – Waves