Sound Waves – Class 9 Science Worksheet
NCERT-Style Practice Worksheet with Answers
Conceptual Questions | MCQs | Numericals | Assertion–Reason | Application-Based Questions
Read the complete chapter notes to understand the concepts, definitions, diagrams and formulas.
Read Class 9 Sound Waves Notes- Attempt all questions carefully.
- Choose the most appropriate option in the MCQ section.
- Show the formula and calculation steps in numerical questions.
- Use SI units wherever required.
- Check your answers using the answer key at the end.
Speed = Distance / Time
Speed of sound = Frequency × Wavelength
Wavelength = Speed / Frequency
Frequency = 1 / Time Period
Time Period = 1 / Frequency
Distance of reflecting surface = (Speed × Echo time) / 2
Section A – Multiple Choice Questions
- Sound is produced by:
- Sound requires which of the following to travel?
- Sound waves travelling through air are:
- The region of high pressure and density in a longitudinal wave is called:
- The SI unit of frequency is:
- The number of vibrations completed in one second is called:
- The time taken to complete one vibration is called:
- The distance between two consecutive compressions is:
- The approximate audible frequency range for a healthy human ear is:
- Sounds having frequencies below 20 Hz are called:
- Sounds having frequencies above 20,000 Hz are called:
- The persistence of sound due to repeated reflections in a large hall is called:
- An echo is heard due to:
- Which instrument uses ultrasonic waves to measure the depth of the sea?
- The loudness of sound mainly depends on its:
Section B – Fill in the Blanks
- Sound is produced by __________ objects.
- Sound cannot travel through a __________.
- The SI unit of frequency is __________.
- The time taken for one complete vibration is called the __________.
- The distance between two successive compressions is called __________.
- A region of low pressure and low density in a longitudinal wave is called __________.
- The audible range of a healthy human ear is approximately __________ to __________ Hz.
- Sound waves with frequencies above 20,000 Hz are called __________ waves.
- The repetition of sound due to reflection is called an __________.
- SONAR stands for Sound Navigation and __________.
Section C – State True or False
- Sound can travel through a vacuum.
- Sound is produced by vibrating bodies.
- Sound travels faster in solids than in gases in general.
- Frequency is measured in hertz.
- The pitch of sound depends mainly on frequency.
- Ultrasonic waves have frequencies below 20 Hz.
- An echo is caused by the reflection of sound.
- SONAR uses ultrasonic waves.
Section D – Match the Following
| Column A | Column B |
|---|---|
| 34. Frequency | a) Reflection of sound |
| 35. Echo | b) Hertz |
| 36. Wavelength | c) One complete vibration time |
| 37. Time period | d) Distance between successive compressions |
| 38. SONAR | e) Measuring underwater distance |
Section E – Very Short Answer Questions
- What is sound?
- How is sound produced?
- Why can sound not travel through a vacuum?
- Define frequency.
- Define amplitude.
- What is wavelength?
- What is meant by one vibration?
- What is an echo?
- What are infrasonic sounds?
- What are ultrasonic sounds?
Section F – Short Answer Questions
- Explain how sound is produced by a vibrating object. Give one example.
- Differentiate between compression and rarefaction.
- Explain why sound is called a mechanical wave.
- Differentiate between frequency and time period.
- How are pitch and loudness related to the characteristics of a sound wave?
- Why is sound heard more clearly through a solid object than through air in some situations?
- What is reverberation? Mention one way to reduce it in an auditorium.
- State two applications of ultrasonic waves.
Section G – Long Answer Questions
- Explain the propagation of sound through air with the help of compressions and rarefactions.
- Describe the characteristics of a sound wave: amplitude, frequency, time period and wavelength.
- Explain the reflection of sound. State the conditions required to hear a distinct echo.
- Explain the working principle of SONAR and describe how it is used to measure the depth of the sea.
Section H – Numerical Problems
Show the formula, substitution and final answer with the correct unit.
- A sound wave travels a distance of 680 m in 2 seconds. Calculate its speed.
- A tuning fork produces 256 vibrations in one second. Find its frequency.
- The frequency of a sound wave is 200 Hz. Calculate its time period.
- A sound wave travels at 340 m/s and has a frequency of 170 Hz. Calculate its wavelength.
- An echo is heard 1 second after a sound is produced. If the speed of sound is 340 m/s, calculate the distance of the reflecting surface.
- A sound wave has a frequency of 50 Hz. Calculate its time period.
- The speed of sound in air is 330 m/s and its frequency is 550 Hz. Calculate its wavelength.
- A person hears an echo 1.8 seconds after shouting near a cliff. If the speed of sound is 340 m/s, calculate the distance between the person and the cliff.
- An echo is heard 2.4 seconds after a sound is produced. Calculate the distance of the reflecting surface from the source. Take the speed of sound as 340 m/s.
- A sound wave travels at 360 m/s and has a frequency of 450 Hz. Calculate its wavelength.
- A sound wave has a wavelength of 1.5 m and travels at 330 m/s. Calculate its frequency.
- A student hears thunder 5 seconds after seeing lightning. If the speed of sound is 340 m/s, estimate the distance of the lightning strike.
- A SONAR device sends an ultrasonic signal towards the seabed and receives its echo after 0.8 seconds. If the speed of sound in seawater is 1500 m/s, calculate the depth of the sea.
Section I – Assertion and Reason
Choose the correct option:
- A. Both Assertion and Reason are true, and Reason correctly explains Assertion.
- B. Both Assertion and Reason are true, but Reason does not correctly explain Assertion.
- C. Assertion is true, but Reason is false.
- D. Assertion is false, but Reason is true.
-
Assertion: Sound cannot travel through a vacuum.
Reason: Sound requires a material medium for propagation. -
Assertion: A sound with a higher frequency generally has a higher pitch.
Reason: Pitch depends on the frequency of vibration. -
Assertion: An echo is produced due to reflection of sound.
Reason: Sound waves can bounce back after striking a hard surface. -
Assertion: Ultrasonic waves can be used in medical imaging.
Reason: Ultrasonic waves have frequencies above the human audible range. -
Assertion: Loudness increases when the amplitude of a sound wave increases.
Reason: Greater amplitude generally means greater sound intensity.
Section J – Competency-Based and Application Questions
- A student strikes a metal plate and observes that it vibrates. When the plate is touched gently, the sound stops. Explain why the sound stops.
- A person standing in front of a large building shouts and hears the same sound again after a short interval. Identify the phenomenon and explain how it occurs.
- Bats use high-frequency sounds to locate obstacles in darkness. Which type of sound waves do they use? Explain how these waves help them.
- A ship sends an ultrasonic signal towards the seabed and receives the reflected signal. Explain why the time taken by the signal must be divided by two when calculating the depth.
- Two sound waves have the same frequency but different amplitudes. Compare their pitch and loudness.
Answer Key and Solutions
Use these answers to evaluate your understanding. Numerical solutions include the calculation steps.
Section A – MCQs
1. b) Vibrating objects
2. b) Material medium
3. b) Longitudinal waves
4. b) Compression
5. c) Hertz
6. b) Frequency
7. c) Time period
8. b) Wavelength
9. b) 20 Hz–20,000 Hz
10. b) Infrasonic
11. c) Ultrasonic
12. b) Reverberation
13. b) Reflection of sound
14. b) SONAR
15. b) Amplitude
Section B – Fill in the Blanks
16. Vibrating
17. Vacuum
18. Hertz (Hz)
19. Time period
20. Wavelength
21. Rarefaction
22. 20 and 20,000
23. Ultrasonic
24. Echo
25. Ranging
Section C – True or False
26. False
27. True
28. True
29. True
30. True
31. False
32. True
33. True
Section D – Match the Following
34. b) Hertz
35. a) Reflection of sound
36. d) Distance between successive compressions
37. c) One complete vibration time
38. e) Measuring underwater distance
Section E – Very Short Answers
39. Sound is a form of energy that produces the sensation of hearing.
40. Sound is produced when an object vibrates.
41. Sound needs particles of a material medium to transfer vibrations. A vacuum has no particles to carry the sound wave.
42. Frequency is the number of complete vibrations or oscillations per second.
43. Amplitude is the maximum displacement of a vibrating particle from its mean position.
44. Wavelength is the distance between two consecutive compressions or two consecutive rarefactions in a longitudinal wave.
45. One vibration is one complete to-and-fro motion of a vibrating object about its mean position.
46. An echo is the repetition of sound heard after its reflection from a distant surface.
47. Infrasonic sounds have frequencies below 20 Hz.
48. Ultrasonic sounds have frequencies above 20,000 Hz.
Section F – Short Answers
49. When an object vibrates, it causes nearby particles of the medium to vibrate. These vibrations travel through the medium as sound waves. For example, a vibrating tuning fork produces sound.
50. Compression is a region where particles are closer together and pressure and density are higher. Rarefaction is a region where particles are farther apart and pressure and density are lower.
51. Sound is called a mechanical wave because it requires a material medium to travel. It cannot propagate through a vacuum.
52. Frequency is the number of vibrations per second and is measured in hertz. Time period is the time taken for one complete vibration and is measured in seconds. They are reciprocals: T = 1/f.
53. Pitch depends mainly on frequency: higher frequency produces higher pitch. Loudness depends mainly on amplitude: greater amplitude generally produces a louder sound.
54. Sound generally travels faster in solids because their particles are closely packed and can transfer vibrations efficiently.
55. Reverberation is the persistence of sound due to repeated reflections. It can be reduced by using sound-absorbing materials such as curtains, carpets and acoustic panels.
56. Ultrasonic waves are used in medical imaging and in SONAR. They are also used to detect cracks in metal components and for cleaning delicate objects.
Section G – Long Answers
57. When an object vibrates, it moves nearby particles of air. During one part of the vibration, particles are pushed closer together, producing a compression. During another part, particles move farther apart, producing a rarefaction. These alternate regions travel through the medium and carry sound energy from one place to another.
58. Amplitude is the maximum displacement of a vibrating particle from its mean position. Frequency is the number of vibrations per second. Time period is the time taken for one complete vibration. Wavelength is the distance between two consecutive compressions or rarefactions. The relation between wave speed, frequency and wavelength is v = fλ.
59. Reflection of sound occurs when sound waves strike a surface and bounce back. A distinct echo can be heard when the reflected sound reaches the ear at least about 0.1 second after the original sound. The reflecting surface should be sufficiently large and hard, and the sound should be loud enough.
60. SONAR stands for Sound Navigation and Ranging. It sends ultrasonic waves through water. These waves reflect from the seabed or an underwater object and return to the receiver. The time taken for the round trip is measured. Depth or distance is calculated using distance = speed × time / 2.
Section H – Numerical Solutions
61. Given: Distance = 680 m; Time = 2 s
Speed = Distance / Time
= 680 / 2 = 340 m/s
62. Number of vibrations = 256 per second
Frequency = Number of vibrations per second
= 256 Hz
63. Given: Frequency = 200 Hz
Time period = 1 / Frequency
= 1 / 200 = 0.005 s
64. Given: Speed = 340 m/s; Frequency = 170 Hz
Wavelength = Speed / Frequency
= 340 / 170 = 2 m
65. Given: Echo time = 1 s; Speed = 340 m/s
Distance = (Speed × Echo time) / 2
= (340 × 1) / 2 = 170 m
66. Given: Frequency = 50 Hz
Time period = 1 / Frequency
= 1 / 50 = 0.02 s
67. Given: Speed = 330 m/s; Frequency = 550 Hz
Wavelength = Speed / Frequency
= 330 / 550 = 0.6 m
68. Given: Echo time = 1.8 s; Speed = 340 m/s
Distance = (Speed × Echo time) / 2
= (340 × 1.8) / 2 = 306 m
69. Given: Echo time = 2.4 s; Speed = 340 m/s
Distance = (Speed × Echo time) / 2
= (340 × 2.4) / 2 = 408 m
70. Given: Speed = 360 m/s; Frequency = 450 Hz
Wavelength = Speed / Frequency
= 360 / 450 = 0.8 m
71. Given: Wavelength = 1.5 m; Speed = 330 m/s
Frequency = Speed / Wavelength
= 330 / 1.5 = 220 Hz
72. Given: Time = 5 s; Speed = 340 m/s
Distance = Speed × Time
= 340 × 5 = 1700 m or 1.7 km
73. Given: Echo time = 0.8 s; Speed in seawater = 1500 m/s
Depth = (Speed × Echo time) / 2
= (1500 × 0.8) / 2 = 600 m
Section I – Assertion and Reason
74. A – Both statements are true, and the Reason correctly explains the Assertion.
75. A – Both statements are true, and the Reason correctly explains the Assertion.
76. A – Both statements are true, and the Reason correctly explains the Assertion.
77. B – Both statements are true, but the Reason does not directly explain the Assertion.
78. A – Both statements are true, and the Reason correctly explains the Assertion.
Section J – Competency-Based Answers
79. The metal plate produces sound while it vibrates. Touching it gently reduces or stops its vibrations, so it no longer produces the sound.
80. The phenomenon is an echo. Sound waves travel to the building, reflect from its surface and return to the listener after a short interval.
81. Bats use ultrasonic waves. These waves reflect from obstacles and return to the bat. By detecting the returning echoes, bats can locate objects and estimate their distance.
82. The signal travels from the ship to the seabed and then returns to the ship. The recorded time is for the complete journey, so the one-way time is half the total time. Therefore, depth = speed × time / 2.
83. Both waves have the same frequency, so they have the same pitch. The wave with the greater amplitude is generally louder.
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