Question Bank — Sound Waves: Characteristics and Applications

QUESTION BANK

Chapter 10: Sound Waves: Characteristics and Applications
(CBSE Class 9 – NCERT)

Section A — MCQs (1 mark each)

1. Sound is produced when an object
(a) moves with uniform velocity
(b) vibrates
(c) is heated
(d) is placed in vacuum

2. Which of the following is a correct statement about sound propagation?
(a) Sound can travel through vacuum.
(b) Sound travels fastest in gases.
(c) Sound needs a material medium to propagate.
(d) Sound travels as a transverse wave.

3. Assertion (A): Astronauts on a spacewalk cannot hear each other directly.
Reason (R): Sound requires a medium to travel and there is near-vacuum in outer space.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) A is false but R is true.

4. Assertion (A): In a sound wave, particles of the medium travel from the source to the listener.
Reason (R): Sound is a longitudinal mechanical wave in which particles only oscillate about their mean positions.
Choose the correct option:
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) A is false but R is true.

5. The region of higher density in a sound wave is called
(a) rarefaction
(b) compression
(c) crest only
(d) trough only

6. The SI unit of frequency is
(a) metre
(b) second
(c) hertz
(d) joule

7. If the frequency of a sound wave is doubled while speed remains constant, its wavelength will
(a) double
(b) become half
(c) remain the same
(d) become four times

8. Human audible range is approximately
(a) 20 Hz to 20 kHz
(b) 20 kHz to 200 kHz
(c) below 20 Hz
(d) above 20 kHz

9. An echo is heard due to
(a) refraction of sound
(b) absorption of sound
(c) reflection of sound
(d) diffraction of sound

10. Which of the following animals uses echolocation?
(a) Elephant
(b) Bat
(c) Snake
(d) Sparrow

Section B — Very Short Answer (2 marks each)

11. Define vibration. How is sound produced by a vibrating object?
12. What is a medium? Give one example each of solid, liquid and gaseous medium through which sound can travel.
13. Differentiate between compression and rarefaction in a sound wave.
14. What are mechanical waves? Why is sound called a mechanical wave?
15. Define wavelength and time period of a sound wave.
16. What is the audible range of human hearing? Name the waves having frequency (i) below 20 Hz and (ii) above 20 kHz.

Section C — Short Answer (3 marks each)

17. With the help of a labelled diagram, show how sound propagates through air as a series of compressions and rarefactions.
18. Explain why sound travels faster in solids than in liquids and gases. Give one example from daily life to support your answer.
19. Distinguish between infrasonic and ultrasonic waves. State one application of each.
20. What is reverberation? How is it reduced in large halls and auditoriums?
21. A tuning fork produces a sound of frequency 256 Hz. Calculate its time period. If the speed of sound in air is 340 m s⁻¹, find the wavelength of the sound wave.

Section D — Long Answer (5 marks each)

22. (a) Draw a neat labelled diagram of a longitudinal wave and mark compression, rarefaction, wavelength and amplitude on it.
(b) Explain why sound cannot travel through vacuum. Describe the bell-jar experiment to show that sound needs a medium.
(c) State any two differences between longitudinal and transverse waves.

23. (a) Define the following terms related to a sound wave: wavelength, frequency, amplitude and speed.
(b) Derive the relation between speed, wavelength and frequency of a sound wave.
(c) A sound wave travels a distance of 340 m in 1 s in air. If its frequency is 340 Hz, calculate its wavelength.

24. (a) What is an echo? Derive the minimum distance required to hear an echo clearly (speed of sound = 340 m s⁻¹).
(b) A person standing at a distance of 85 m from a wall claps his hands and hears an echo after 0.5 s. Verify whether the observation is consistent with the speed of sound. Show all calculations.
(c) State two conditions necessary for a distinct echo to be heard.

Section E — Case/Source-Based Questions (4 marks each)

25. Case Study: Echolocation in Bats
Bats fly in complete darkness and hunt insects using sound. They emit short bursts of ultrasonic waves. These waves strike insects or obstacles and return as echoes. The bat analyses the time taken and nature of the reflected waves to determine the position, size and even movement of the prey.

Answer the following:

(i) What type of waves do bats use for echolocation?
(ii) Why can humans not hear these waves?
(iii) Name the phenomenon used by bats to locate prey.
(iv) State one more animal that uses a similar method for navigation.

26. Case Study: Sonar
A naval ship uses sonar to detect underwater objects. It sends ultrasonic waves into seawater. The waves reflect from a submarine and return to the ship after 0.90 s. The speed of sound in seawater is 1530 m s⁻¹.

Answer the following:

(i) Expand the acronym SONAR.
(ii) Why are ultrasonic waves preferred over audible sound for underwater detection?
(iii) Calculate the distance of the submarine from the ship. Show the working.
(iv) Name the type of wave (longitudinal/transverse) used in sonar and justify.