Science S Chand Solutions Class 8th CBSE Chapter Sound

Exploring the Science of Sound - Class 8 Questions and Answers

Welcome to our comprehensive guide on the science of sound, tailored for Class 8 students! In this post, we’ll dive into the fascinating world of sound, answering a series of questions from a science textbook. Each answer is explained in a clear and engaging way to help you understand the concepts better. Let’s explore how sound is produced, how it travels, and its properties, all while making learning fun!

Very Short Answer Type Questions

1. How is sound produced?

Sound is created when an object vibrates, causing the surrounding air particles to move back and forth. These vibrations create pressure waves in the air, which our ears perceive as sound. For example, when you pluck a guitar string, it vibrates and produces sound waves.

2. What should an object do to produce sound?

An object must vibrate to produce sound. These vibrations disturb the surrounding medium (like air, water, or solids), creating sound waves that travel to our ears.

3. How does a sound-making object differ from one that is silent?

A sound-making object vibrates, creating disturbances in the surrounding medium that propagate as sound waves. A silent object does not vibrate or produces vibrations too weak or at frequencies outside the human hearing range (20 Hz to 20,000 Hz).

4. Name the part which vibrates to produce sound in the following:

(a) Drums: The stretched membrane (drumhead) vibrates when struck.
(b) Sitar: The strings vibrate when plucked.
(c) Flute: The air column inside the flute vibrates when air is blown across the opening.

5. What brings the sound of a ringing telephone bell to our ears?

The sound of a ringing telephone bell reaches our ears through vibrations in the air. When the bell vibrates, it creates sound waves that travel through the air to our ears, where they are detected as sound.

6. What is the length of vocal cords in a man?

The vocal cords in an adult man are typically about 20 mm long. This length contributes to the deeper pitch of male voices compared to females.

7. Out of a man and a woman:

(a) Who has shorter vocal cords? A woman has shorter vocal cords, usually about 15 mm long.
(b) Who produces sound of higher pitch? A woman produces a higher-pitched sound because shorter vocal cords vibrate at a higher frequency.

8. Give any four sources of sound in a marketplace.

Four sources of sound in a marketplace include:

Shouting vendors calling out to attract customers.
Horns from vehicles like rickshaws or scooters.
Music from shops or street performers.
Chatter and footsteps of crowds.

9. Name the sound-producing organ in humans.

The sound-producing organ in humans is the larynx, also known as the voice box.

10. Which part of our body vibrates when we speak?

The vocal cords in the larynx vibrate when we speak, producing sound as air from the lungs passes through them.

11. What does the working of a toy telephone tell us about sound?

A toy telephone, often made with cups and a string, demonstrates that sound travels through solids. When one person speaks into a cup, the vibrations travel along the taut string to the other cup, where they are converted back into sound, showing that sound requires a medium to travel.

12. Name one solid, one liquid, and one gas through which sound can travel.

Solid: Wood
Liquid: Water
Gas: Air

13. Which of the following cannot transmit sound? Water, Vacuum, Aluminium, Oxygen gas

Sound cannot travel through a vacuum because it lacks particles to carry the vibrations. Water, aluminium, and oxygen gas can all transmit sound.

14. Is the speed of sound more in water or in steel?

The speed of sound is higher in steel (about 5,000 m/s) than in water (about 1,500 m/s) because solids are denser and more elastic, allowing faster wave propagation.

15. Where would sound travel faster: in wood or in water?

Sound travels faster in wood (around 3,000–4,000 m/s depending on the type) than in water (1,500 m/s) due to the higher density and elasticity of solids.

16. In which medium does sound travel faster: air or iron?

Sound travels faster in iron (approximately 5,000 m/s) than in air (340 m/s) because iron is a solid with tightly packed particles.

17. In which medium does sound travel fastest: air, water, or steel?

Sound travels fastest in steel (around 5,000 m/s), followed by water (1,500 m/s), and slowest in air (340 m/s).

18. Out of solids, liquids, and gases:

(a) In which medium does sound travel slowest? Gases (e.g., air, 340 m/s).
(b) In which medium does sound travel fastest? Solids (e.g., steel, 5,000 m/s).

19. What is the speed of sound in air?

The speed of sound in air is approximately 340 m/s at room temperature (20°C).

20. Which of the following is the speed of sound in water and which in steel?

(a) 5,000 m/s: Steel
(b) 1,500 m/s: Water
(c) 340 m/s: Air

21. Name the organs of hearing in our body.

The organs of hearing in our body are the ears.

22. Name that part of the ear which vibrates when outside sound falls on it.

The eardrum (tympanic membrane) vibrates when sound waves strike it.

23. Name the three tiny bones present in the middle part of the ear.

The three tiny bones in the middle ear are the hammer (malleus), anvil (incus), and stirrup (stapes).

24. What is the function of the three tiny bones in the ear?

The three tiny bones amplify and transmit sound vibrations from the eardrum to the inner ear. They act as a lever system, increasing the force of the vibrations to ensure efficient transfer to the cochlea.

25. Name the nerve which carries electrical impulses from the cochlea of the ear to the brain.

The auditory nerve carries electrical impulses from the cochlea to the brain.

26. What is the name of the passage in the outer ear which carries sound waves to the eardrum?

The ear canal (external auditory meatus) carries sound waves to the eardrum.

27. Name the quantity whose unit is hertz.

The quantity measured in hertz is frequency.

28. What is the relation between time-period and frequency of an oscillating body?

Frequency (f) is the inverse of the time period (T). The relationship is given by the formula: f = 1/T, where frequency is measured in hertz (Hz) and time period in seconds (s).

29. Name three characteristics which are used to describe oscillations (or vibrations).

The three characteristics are:

Frequency: Number of oscillations per second.
Amplitude: Maximum displacement from the central position.
Phase: The stage of the oscillation cycle.

30. What is the scientific name for the number of vibrations made per second?

The scientific name is frequency.

31. What name is given to the maximum displacement of a vibrating body from its central position?

The maximum displacement is called the amplitude.

32. If 125 oscillations are produced in 5 seconds, what is the frequency in hertz?

Frequency = Number of oscillations / Time = 125 / 5 = 25 Hz.

33. How does loudness depend on the amplitude of vibrations?

Loudness is proportional to the square of the amplitude. A larger amplitude results in a louder sound because it produces more intense pressure waves.

34. By how much will the loudness of a sound change when the amplitude of vibrations is:

(a) Doubled? Loudness increases by a factor of four (since loudness ∝ amplitude², 2² = 4).
(b) Halved? Loudness decreases to one-fourth (since (1/2)² = 1/4).

35. Name the unit used to measure the loudness of sound. Also write its symbol.

The unit is decibel, symbolized as dB.

36. What is the loudness of a normal conversation in decibels?

The loudness of a normal conversation is about 60 dB.

37. On what factor does the pitch of a sound depend?

The pitch of a sound depends on its frequency. Higher frequency results in a higher pitch.

38. How is pitch related to frequency?

Pitch is directly proportional to frequency. A higher frequency produces a higher-pitched sound, and a lower frequency produces a lower-pitched sound.

39. Name the characteristic of sound which enables us to distinguish between a man’s voice and a woman’s voice even without seeing them.

The characteristic is pitch, determined by the frequency of the sound. Women typically have higher-pitched voices due to shorter vocal cords.

40. Arrange the following sounds in the order of increasing frequencies (keeping the sound of lowest frequency first):

(i) Baby’s voice
(ii) Man’s voice
(iii) Woman’s voice

Order: Man’s voice, Woman’s voice, Baby’s voice (babies have the shortest vocal cords, producing the highest frequency).

41. Which produces sound of a higher pitch: a drum or a whistle?

A whistle produces a higher-pitched sound because it generates higher-frequency vibrations compared to a drum.

42. Name the characteristic of sound which depends on:

(a) Amplitude: Loudness
(b) Frequency: Pitch

43. Name the characteristic of sound which can distinguish between the ‘notes’ (musical sounds) played on a flute and a sitar (both having the same pitch and loudness).

The characteristic is timbre (or quality), which depends on the waveform and allows us to distinguish between different sound sources.

44. Write the full form of dB.

The full form of dB is decibel.

45. What is the name of very high-frequency sounds which cannot be heard by the human ear?

These are called ultrasonic sounds (frequencies above 20,000 Hz).

46. Why do we not hear the screams of a bat?

Bats produce ultrasonic sounds with frequencies above the human hearing range (20,000 Hz), so we cannot hear them.

47. Which of the following frequency of sound can be heard by a dog but not by a man?

(a) 50,000 Hz: Dogs can hear this ultrasonic frequency, but humans cannot.
(b) 15,000 Hz: Both dogs and humans can hear this frequency.

Answer: 50,000 Hz

48. Name the substance which vibrates in a flute to produce sound.

The air column inside the flute vibrates to produce sound.

49. State whether the following statements are true or false:

(a) Sound cannot travel in vacuum: True. Sound requires a medium like air, water, or solids to travel.
(b) The number of oscillations per second of a vibrating object is called its time-period: False. It is called frequency.
(c) If the amplitude of vibrations is large, sound is feeble: False. Larger amplitude produces louder sound.
(d) The lower the frequency of vibration, the higher is the pitch: False. Lower frequency results in lower pitch.
(e) If the amplitude of vibrations is doubled, the loudness of sound also gets doubled: False. Loudness increases four times (proportional to amplitude²).
(f) When the amplitude of vibrations is halved, the loudness of sound becomes one-fourth: True (since (1/2)² = 1/4).
(g) Unwanted or unpleasant sound is termed as music: False. It is termed as noise.
(h) Noise pollution may cause partial hearing impairment: True. Prolonged exposure to loud noise can damage hearing.

50. Fill in the following blanks with suitable words:

(a) Sounds are produced by vibrating objects.
(b) The human voice box is called larynx.
(c) Sound cannot travel in vacuum.
(d) A set of three tiny bones in the middle part of the ear passes on sound vibrations from the eardrum to the liquid in the cochlea.
(e) The unit of frequency is hertz.
(f) The time taken by an object to complete one oscillation is called time period.
(g) The shrillness of a sound is determined by the frequency of vibration.
(h) Unpleasant sound is called noise.

Short Answer Type Questions

51. (a) Name the part of the human body responsible for producing sound.

The vocal cords in the larynx produce sound in humans.

(b) How do vocal cords produce sound?

When air from the lungs passes through the vocal cords, they vibrate, creating sound waves. The tension and length of the cords determine the pitch of the sound.

52. Explain how the human voice box works to produce sound.

The larynx (voice box) contains vocal cords that vibrate when air is forced through them. Muscles adjust the tension and gap between the cords, altering the pitch and volume of the sound produced.

53. Out of a lion and a tiger, which animal produces a sound of:

(a) Higher pitch? A lion, as it typically produces higher-frequency roars compared to a tiger.
(b) Lower pitch? A tiger, as its roars are generally lower in frequency.

54. Why are the voices of men, women, and children different?

The differences arise due to variations in vocal cord length and thickness. Men have longer, thicker vocal cords (lower frequency, deeper voice), women have shorter, thinner cords (higher frequency, higher pitch), and children have the shortest cords (highest pitch).

55. Why is it more convenient to put the ear to the track to hear an approaching train?

Sound travels faster through solids like steel (railway tracks) than through air. Placing an ear on the track allows you to detect the vibrations of an approaching train sooner than hearing the sound through the air.

56. During the rainy season, why is the flash of lightning seen first but the sound of thunder heard later?

Light travels much faster (300,000,000 m/s) than sound (340 m/s). Thus, the flash of lightning reaches our eyes almost instantly, while the sound of thunder takes longer to reach our ears.

57. Explain why the flash of lightning is seen first but the sound of thunder is heard later.

This is due to the vast difference in the speeds of light and sound. Light travels at 300,000,000 m/s, reaching us almost instantly, while sound travels at 340 m/s, causing a delay in hearing the thunder.

58. Name the object (or part) which vibrates to produce sound in the following musical instruments:

(a) Sitar: Strings
(b) Dholak: Membrane
(c) Flute: Air column
(d) Cymbals: Metal plates
(e) Veena: Strings
(f) Tabla: Membrane

59. Name one musical instrument each in which the sound is produced:

(a) By vibrating a stretched string: Sitar
(b) By vibrating air enclosed in a tube: Flute
(c) By vibrating a stretched membrane: Tabla
(d) By vibrating metal plates: Cymbals

60. Give two examples of each of the following:

(a) Stringed musical instruments: Sitar, Guitar
(b) Wind musical instruments: Flute, Trumpet
(c) Membrane musical instruments: Tabla, Dholak
(d) Plate type musical instruments: Cymbals, Gong

61. Which of the sounds having the following frequencies can be heard by human beings and which cannot?

(a) 6 Hz: Cannot be heard (below 20 Hz, infrasonic).
(b) 5,000 Hz: Can be heard (within 20 Hz–20,000 Hz).
(c) 10,000 Hz: Can be heard.
(d) 35,000 Hz: Cannot be heard (above 20,000 Hz, ultrasonic).
(e) 18 kHz (18,000 Hz): Can be heard by young humans, though some adults may not hear it.

62. Answer the following:

(a) What is the upper limit of frequency of human hearing? 20,000 Hz
(b) What is the lower limit of frequency of human hearing? 20 Hz
(c) Name one animal which can produce ultrasonic sounds. Bat
(d) Name two animals which can hear ultrasonic sounds. Dogs, Dolphins

63. (a) What is a vibration (or an oscillation)? Define 'amplitude' of vibration of an object.

A vibration is a repeated back-and-forth motion of an object. The amplitude is the maximum distance the object moves from its central (equilibrium) position during vibration.

(b) What is the frequency of a vibrating body whose time-period is 0.05 second?

Frequency = 1 / Time period = 1 / 0.05 = 20 Hz.

64. (a) State two methods of producing sound.

Vibrating strings (e.g., in a guitar).
Vibrating air columns (e.g., in a flute).

(b) How does sound from a sound-producing body travel through air to reach our ears?

The vibrating body creates pressure waves in the air, which travel as longitudinal waves, compressing and rarefying the air. These waves reach the ear, causing the eardrum to vibrate, which is interpreted as sound.

65. (a) Why can a sound not be heard on the moon?

There is no atmosphere on the moon, so there is no medium (like air) for sound waves to travel through.

(b) How do astronauts talk to one another on the surface of the moon and why?

Astronauts use radio waves to communicate, as these can travel through a vacuum. Sound cannot travel on the moon due to the lack of air.

66. (a) What is meant by the (a) 'pitch' of sound, and (b) 'quality' of sound?

Pitch: The perceived highness or lowness of a sound, determined by its frequency.
Quality (Timbre): The characteristic that distinguishes sounds of the same pitch and loudness from different sources, due to the shape of the sound wave.

(b) What is ultrasound? State two uses of ultrasound.

Ultrasound is sound with frequencies above 20,000 Hz, inaudible to humans. Two uses:

Medical imaging (e.g., to monitor a fetus).
Cleaning delicate objects (e.g., jewelry).

67. (a) What differences will you hear in a sound if there is an increase in:

(i) Amplitude: The sound becomes louder.
(ii) Frequency: The sound becomes higher-pitched.

(b) Calculate the time period of a pendulum which is vibrating with a frequency of 10 hertz.

Time period = 1 / Frequency = 1 / 10 = 0.1 seconds.

68. (a) How can you show that a sounding tabla is vibrating?

Sprinkle fine powder (e.g., talcum) on the tabla’s membrane. When struck, the powder will move or jump, indicating vibration.

(b) On what factor does the loudness of a sound depend?

Loudness depends on the amplitude of the vibrations.

69. Why can we hear an approaching train by putting our ear to the railway line when the sound cannot be heard through the air?

Sound travels much faster through solids like steel (about 5,000 m/s) than through air (340 m/s). The vibrations from the train reach the ear via the track before the sound through the air.

70. Why can sound not travel through a vacuum (or through outer space)?

Sound requires a medium (solid, liquid, or gas) to travel, as it relies on particle vibrations. A vacuum has no particles, so sound cannot propagate.

71. (a) What type of pollution is caused by the working of a mixer and grinder in the kitchen?

Noise pollution

(b) Why should we not put a pin or pencil in our ears?

Inserting objects can damage the eardrum or other delicate ear structures, leading to hearing loss or infection.

72. Name any two common musical instruments and identify their vibrating parts.

Flute: Vibrating part is the air column.
Tabla: Vibrating part is the membrane.

73. What is the difference between noise and music? Can music become noise sometimes?

Music is pleasant, organized sound with regular patterns, while noise is unpleasant, irregular sound. Music can become noise if played too loudly or discordantly, such as loud music at a party disturbing neighbors.

74. Draw a labelled diagram of the larynx and explain its functions.

Diagram Description: The larynx includes the vocal cords, glottis, and cartilage. Air from the lungs passes through the glottis, causing the vocal cords to vibrate and produce sound. Muscles adjust the tension to control pitch.

Functions:

Produces sound through vocal cord vibrations.
Directs air and food to the trachea and esophagus, respectively, via the epiglottis.

75. (a) Give two causes of noise pollution from homes.

Loud music from stereos or TVs.
Operating noisy appliances like mixers or vacuum cleaners.

(b) What are the usual causes of partial hearing loss suffered by a person?

Prolonged exposure to loud noise.
Ear infections, aging, or physical damage to the ear.

Long Answer Type Questions

76. How can you show that sound cannot travel through a vacuum? Draw a labelled diagram of the apparatus used.

Experiment: Use a bell jar experiment. Place an electric bell inside a glass bell jar connected to a vacuum pump. Ring the bell while air is present, and the sound is audible. Gradually remove air using the pump, and the sound fades, becoming inaudible in a vacuum.

Diagram Description: A bell jar with an electric bell inside, connected to a vacuum pump. Labels include: bell, jar, vacuum pump, and air outlet.

Explanation: Sound requires a medium to travel. In a vacuum, there are no air particles to vibrate, so the sound cannot reach the observer.

77. (a) What is meant by the ‘time-period’ of a vibrating object? State its unit.

The time period is the time taken for one complete oscillation. Its unit is seconds (s).

(b) Define ‘frequency’ of a vibrating object. Name the unit in which frequency is measured.

Frequency is the number of oscillations per second. Its unit is hertz (Hz).

(c) A pendulum oscillates 40 times in 4 seconds. Calculate its:

(i) Time period: Time period = Total time / Number of oscillations = 4 / 40 = 0.1 s
(ii) Frequency: Frequency = Number of oscillations / Time = 40 / 4 = 10 Hz
Can we hear the sound produced by the oscillations of this pendulum? No, the frequency (10 Hz) is below the human hearing range (20 Hz–20,000 Hz).

78. Draw a neat and labelled diagram of the human ear. Explain its working.

Diagram Description: The ear consists of the outer ear (pinna, ear canal), middle ear (eardrum, hammer, anvil, stirrup), and inner ear (cochlea, auditory nerve).

Working:

Sound waves enter the ear canal via the pinna and strike the eardrum, causing it to vibrate.
The vibrations are amplified by the hammer, anvil, and stirrup in the middle ear.
The stirrup transfers vibrations to the cochlea, where fluid movement stimulates hair cells to generate electrical signals.
The auditory nerve carries these signals to the brain, which interprets them as sound.

79. (a) What is noise? Give two examples of sounds which are considered noise.

Noise is unwanted or unpleasant sound. Examples:

Honking of vehicles.
Loud construction machinery.

(b) What is a musical sound? Give two examples of musical sounds.

Musical sound is pleasant and organized. Examples:

Notes from a piano.
Melody from a flute.

80. (a) What is meant by noise pollution? Mention some of the sources of noise pollution in your surroundings.

Noise pollution is excessive, disruptive sound that harms human health or the environment. Sources:

Traffic noise from vehicles.
Loudspeakers at events.
Construction activities.

(b) Explain how noise pollution is harmful to human beings.

Noise pollution can cause hearing loss, stress, sleep disturbances, and reduced concentration.

(c) State the various measures which can be taken to control noise pollution.

Use soundproofing in buildings.
Limit the use of loudspeakers.
Plant trees to absorb sound.

(d) What can be done along the roads to reduce noise pollution caused by traffic?

Install noise barriers.
Plant trees along roads.
Enforce traffic rules to reduce honking.

Multiple Choice Questions (MCQs)

81. Voice of which of the following is likely to have the minimum frequency?

82. Sound can travel through:

(d) Solids, liquids, and gases

83. Which of the following vibrates when a musical note is produced by the cymbals in an orchestra?

84. A musical instrument is producing a continuous note. This note cannot be heard by a person having a normal hearing range. This note must then be passing through:

85. Which of the following sound frequencies can be heard by a woman having a normal hearing range?

86. When we change a feeble sound to a loud sound, we increase its:

(b) Amplitude

87. Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the strings suitably. By doing so he is adjusting:

88. A key of a mechanical piano is first struck gently and then struck again but much harder this time. In the second case:

(a) Sound will be louder but pitch will not be different

89. One of the following can hear infrasound. This one is:

90. The speed of highly penetrating ultrasonic waves is:

(d) Same as those of audible sound waves (speed depends on the medium, not frequency).

91. The ultrasound waves can penetrate into matter to a large extent because they have:

(b) Very high frequency

92. Which of these sound waves can be used to measure the depth of the sea?

93. (Assumed similar to 92) Which frequency is suitable for sonar?

94. Which of the following are used to study the growth of a fetus inside the mother’s womb?

(d) Sound waves (ultrasound).

95. We can distinguish between the musical sounds produced by different singers on the basis of the characteristic of sound called:

(b) Timbre

96. The maximum speed of vibrations which produce audible sound will be in:

97. (Assumed) Which medium allows sound to travel fastest?

98. (Assumed) What determines the pitch of a sound?

99. The speed of sound in a liquid medium is approximately:

(d) 1,500 m/s (e.g., water).

100. Which of the following modes is utilized in the production of sound by humans?

101. Identify X, Y, and Z in the context of sound types.

X: Ultrasonic sound (>20,000 Hz)
Y: Infrasonic sound (<20 Hz)
Z: Audible sound (20 Hz–20,000 Hz)

102. Which house would you suggest to your parents to reduce noise pollution?

The house three lanes away from the busy road, as it will experience less traffic noise.

103. (a) Calculate the time period if the frequency is 500 Hz.

Time period = 1 / Frequency = 1 / 500 = 0.002 s

(b) What is the frequency if the time period is 0.002 s?

Frequency = 1 / Time period = 1 / 0.002 = 500 Hz

(c) Can we hear this sound?

Yes, 500 Hz is within the human hearing range.

104. Name the three tiny bones in the middle ear and their roles.

Hammer (Malleus): Transfers vibrations from the eardrum.
Anvil (Incus): Amplifies vibrations.
Stirrup (Stapes): Transmits vibrations to the cochlea.

105. Explain why, if we strike a steel tumbler with a spoon, the sound produced is different from that produced by a wooden stick striking a tumbler.

The sound differs due to the material properties of the striking object. Steel is denser and more elastic, producing higher-frequency vibrations and a sharper, metallic sound. Wood is less dense, producing lower-frequency vibrations and a duller sound.

Friday, August 1, 2025