NCERT Solutions Class 9 Science Chapter 11 - Sound

Sound – Important Questions with Answers

1. What is sound?
Answer: Sound is a form of energy that produces the sensation of hearing in our ears.

2. How is sound produced?
Answer: Sound is produced by vibrating objects.

3. What type of wave is sound in air?
Answer: Longitudinal wave.

4. Through which medium can sound travel?
Answer: Sound can travel through solids, liquids, and gases, but not through a vacuum.

5. What is the speed of sound in air at room temperature?
Answer: Approximately 343 m/s.

6. Which medium allows sound to travel the fastest?
Answer: Solids.

7. What is the SI unit of frequency?
Answer: Hertz (Hz)

8. Define amplitude.
Answer: Amplitude is the maximum displacement of particles from their mean position in a wave.

9. Define frequency.
Answer: Frequency is the number of vibrations per second.

10. Define time period.
Answer: Time period is the time taken to complete one full vibration or oscillation.

11. What is the relation between frequency and time period?
Answer:
Frequency (f)=1Time period (T)Frequency (f)=Time period (T)1​

12. What is the wavelength of a sound wave?
Answer: The distance between two consecutive compressions or rarefactions.

13. State the formula for the speed of sound.
Answer:
$v=\lambda \times f$
Where:
v = speed,
λ = wavelength,
f = frequency

14. What is ultrasonic sound?
Answer: Sound with frequency more than 20,000 Hz.

15. What is infrasonic sound?
Answer: Sound with frequency less than 20 Hz.

16. What is the audible range of human hearing?
Answer: 20 Hz to 20,000 Hz.

17. Name one animal that can hear ultrasonic sound.
Answer: Bat

18. What is the function of the eardrum?
Answer: The eardrum vibrates when sound waves strike it and sends signals to the brain via the inner ear.

19. What is echo?
Answer: Echo is the reflection of sound that is heard after the original sound.

20. What is the minimum distance required to hear an echo?
Answer: 17.2 metres (at room temperature, assuming sound speed as 343 m/s).

21. What is SONAR?
Answer: SONAR (Sound Navigation and Ranging) is a device used to find the depth of a sea or locate underwater objects using sound waves.

22. What is reverberation?
Answer: The persistence of sound due to repeated reflections after the source has stopped producing sound.

23. How can reverberation be reduced in halls?
Answer: By using sound-absorbing materials like curtains, carpets, and false ceilings.

24. What is pitch of sound?
Answer: Pitch is the sensation (highness or lowness) of a sound determined by its frequency.

25. How does sound help in communication?
Answer: Sound allows us to speak, listen, and understand, thus making verbal communication possible.