## Class 9 - Physics

#### Sound

MCQ
Q&A
Notes
Question:
A boy stands at one end of a corridor,both the doors of which are closed.When he claps his hands together,the echo of the hand clap continues for a few seconds with decreasing loudness.Why ?

It is due to the phenomenon of multiple reflections from both the doors of the corridor that the echo of the hand clap continues for sometime.The decreasing loudness is due to the absorption of sound by the doors and the walls.

## Questions and Answers

Sound - Questions
1. Sounds we hear inform us of the state of world around us,especially of that part of it which our other senses,such as touch and sight,cannot reach.Illustrate this with appropriate examples.
2. Describe with the help of a diagram,how compressions and rarefactions are produced in air near a source of sound.
3. What is reverberation ? How can it be reduced ?
4. How is ultrasound used for cleaning ?
5. Write the name of the wave which propagates in terms of compressions and rarefactions.
6. How is sound propagated through a material medium ? What is wave motion ?
7. We see flash earlier than the thunder.Discuss.
8. Calculate the wavelength of a sound wave having a frequency 300 Hz and speed 330 m/s.
9. What are infrasonics ?
10. What is the minimum distance of the obstacle from the source of sound for hearing distinct echo ?
11. How are compressions and rarefactions produced near a source of sound ?
12. What are transverse waves ?
13. Distinguish between music and noise.
14. What is ultrasound ? Describe some of its practical applications ?
15. Discuss the structure of human ear from auditory aspect only.
16. Try to imagine what you would feel, what sort of person you would be,had you been born totally deaf.
17. Cite an experiment to show that sound needs a material medium for its propagation.
18. Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound.Mark the position of compression and rarefaction on this curve.Also define wavelengths and time period using this curve.
19. Can sound waves travel through vacuum ?
20. What is audible range ?
21. What is a sonic boom ?
22. What is sound barrier ?
23. How can you make a building sound proof ?
24. What is the range of frequencies of audio waves,infrasonics and ultrasonics ?
25. What are the characteristics of a sound ? Discuss each one of them briefly.
26. The frequency of a source of sound is 100 Hz.How many times does it vibrate in a minute ?
27. Explain the working and application of a sonar.
28. What propagates along with the waves ?
29. Can you produce sound without utilizing energy ?
30. What is sound ? Discuss the method of its production.
31. Describe an experiment to show that sound waves can be reflected.What are the laws of reflection of sound waves.
32. Hearing may be damaged by excessive noise.So our ears sometimes need protection from continuous loud sound. Hearing damage depends on the sound intensity level (decibel level)and the exposure time,and the exact combinations vary for different people.Normally,at 90 dB,it takes 8 hours or less for the damage to receptor nerves to occur.Further,it is found that if the sound level is increased by 5 dB,the safe exposure limit is cut to half.
(a) How long will it take for a sound of a very loud lawn mower (or a motorcycle) of 95dB to damage the hearing ?
(b) What is the nonstop exposure time for a 105 dB sound to damage the ear ?
(c) What should be done to protect the society from the danger of losing the divine gift of hearing power ?
33. A boat anchor is rocked by waves whose consecutive crests are 100 m apart.If the wave speed of the moving crests is 20 m/s,calculate the frequency at which the boat rocks.
34. The sonic boom of an aircraft has a time period of 0 00005 s.Calculate the frequency of the sound produced.
35. A bat emits ultrasonic sound of frequency 100 kHz in air.If this sound meets a water surface,what is the wavelength of (a) the reflected sound wave (b) the transmitted sound wave ?
Given speed of sound in air = 340 m/s and in water = 1486 m/s.
36. A wave of wavelength 0-60 cm is produced in air and it travels at a speed of 300 m/s.Will it be audible ?
37. A man standing at 51 m from a wall fires a gun.Calculate the time after which an echo is heard.The speed of sound is 340 m/s.
38. What are wavelength,frequency,time period and amplitude of a sound wave ?
39. What is sound and how is it produced ?
40. Explain how the human ear works.
41. By how much does the speed of sound increase with rise of temperature ?
42. There are no echoes produced in small living rooms.Explain why echoes are produced only in large galleries and halls.
43. What do you mean by reverberation and reverberation time ? How is reverberation controlled ?
44. What is an echo ? How is it formed ?
45. A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s.If the speed of the ultrasound through sea water is 1531 m/s,what is the distance of the seabed from the ship ?
46. What is loudness of sound ? What factors does it depend on ?
47. What is acoustical transite ?
48. How can you show that sound waves are longitudinal waves ?
49. What is sonic boom and when is it heard ?
50. On what factors does the speed of sound in a material depend ?
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