Acoustic Phenomenon Short Answer Question | Acoustic Phenomenon Most Important Question for Board | Doppler Effect Question

 1. What is Doppler's effect in sound?

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2. What do you mean by Red Shift?

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3. Show that Doppler's effect in sound is asymmetric.

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4. Whistle of an approaching train is shriller. Why?

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5. Can there be cases that bodies are moving but no Doppler effect is observed?

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Is it necessary that Doppler Effect is observed when bodies are moving?

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6. The pitch of an approaching ambulance seems to be rising. Why?
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Whistle of an approaching train is shriller, why?

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7. The pitch of an engine whistle to a passenger in a moving train appears constant whereas to a stationary listener outside the train it appears to change.

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8. The pitch of a train engine approaching a cliff appears to be high to the passengers whereas when the train just emerges from the cliff, the pitch appears to be less, why?

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9. Will there be any Doppler's effect when the sound or observer is moving in the direction perpendicular to the direction of propagation of sound?

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10. A sounding body is moving in a circle. A person 'A' is standing outside the circle and another person B at the center of the circle. Do they receive the same pitch of sound? Justify your answer.

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11. What are the difference between Doppler's effect in sound an that in light?

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12. How is Doppler's principle used in radar?
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13. What would a person hear if he moves away from stationary source of sound with speed of sound?

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14. How can a bat catch its prey in dark night?
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Bats can ascertain distances, directions and size of obstacles without use of eyes. Explain.
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How are bats able to fly around even if they don't have eyes?
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How can bats fly around without colliding with objects that come in their way?

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15. Why cannot we hear echo in a small room?

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16. Define Reverberation.

ans: If the reflecting surface is less than about 17m, the reflected sound follows the original sound so closely that the listener cannot distinguish them as seperate sounds. It feels as if the original sound has been extended. Such phenomenon is called Reverberation.

17. Why are bells made of metal and not of wood?

ans: Bells are meant to produce specific sounds for long durations of time. This requires that they have low damping. Since wood produces very high clamping, it rapidly dampens the vibrations. On the other hand, metals have low damping. So, bells are made of metal and not of wood.

18. Why do we hear more clearly in curtained room than in a non-curtained room?

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19. A beating drum with a large surface area produces a loud sound as compared to the beating of drums with small surface area.

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20. Why does the sound produced by a tuning fork become louder when the stem of the fork is pressed on the top of a table?

ans: This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of sound depends on is the surface area of the area available for vibration. When only the tuning fork is used, it causes vibration of air of a very small area, which makes it appear feeble. But when its stem is pressed against a tabletop, the whole of the top vibrates, which causes the vibration of a large area of air. This makes it appear loud.

21. Why are temple bells made larger?

ans: This phenomenon is bases on the surface area of the vibrating substances. One of the factors on which the loudness of sound depends on is the surface area of the available for vibration. When large bells are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If small bells are used, small area of air vibrates and so sound will be feeble.

22. A loud speaker produces louder sound than an ear piece does. Explain.

ans: This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of sound depends on is the surface area of the area available for vibration. When loud speakers are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If small ear pieces are used, small area of air vibrates and so sound will be feeble.

23. Why is the sonometer box hollow from inside?

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24. Explain why guitar is provided with hollow boxes.
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Explain why string type musical instruments are provided with hollow boxes.

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25. Striking harder on a bell or a tuning fork produces a louder sound. Why?

ans: This phenomenon is based on the amplitude of the vibrating substances; one of the factors on which the loudness of sound depends on. When struck hard, the amplitude of vibration of either the bell or the tuning fork will be the highest. This causes very large amplitude vibrations of the air around and cause louder sound.

26. A violin note and sitar note may have the same frequency, yet we can distinguish between the two notes. How?

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27. Two of your friends speak with the same frequency. How are you able to distinguish voice of one from the other?

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28. How is it that once can recognize a friend from his voice without seeing him/her?

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29. When a certain person talks over the phone with the microphone strapped with a handkerchief, his/her voice become indistinguishable, why?

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30. Which is the most fundamental property of a wave? Why?0

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31. Can sound waves be polarized?
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Is polarization possible for longitudinal waves? Why?

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32. Why does an empty vessel produce more sound than a filled one?
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An empty vessel sounds more than one when it is struck. Why?

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33. Under what conditions are beats formed?
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How are beats produced? What is Beat frequency?
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Define beats and beats frequency
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Explain with figure the meaning of beats and beats frequency.

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34. Can two sound waves of equal frequency produce beats? Explain.

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35. In order to hear distinct beats, why should the difference in the frequencies of the two sources be small( less than 10Hz)?

ans: If a person is to hear two events, the frequencies should be such that the time gap between them should be at least 0.1s. Since f = 1/T, the frequency should be at most 10Hz. This means it should be less than 10Hz.

36. An airplane mechanic notices that the sound from a twin engine aircraft varies in loudness when both engines are running. What could be causing this variation from loud to soft and vice versa?

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37. When same strings of two guitars are pluck, they give sound which sometimes gets loud and sometimes feeble, why?

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38. Why sis the roaring of a lion different from the sound of a mosquito?

ans: The main points of difference dare in the pitch and loudness. The sound of mosquito has a higher pitch (determined by frequency) than the roaring of a lion. In contrast, the lion roars loudly (with high amplitude) than a mosquito.

39. In a machine run saw, high pitch is heard initially and low pitch sound is heard after the saw touches the wood. Explain why?

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40. Two loudspeakers have been stalled in an open space to listen to a speech. When both the speakers are in operation, listener sitting at a particular place receives very feeble sound. What can be the reason of it?

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41. Why is a given sound louder in a hall(large closed room) than in the open?

ans: When a sound is produced in a hall, the waves are reflected repeatedly from the walls and ceiling. The reflected sounds produce constructive interference with the direct sound at different locations; as a result the intensity of sound increases. However, in open space, there are no such reflections and the intensity of sound will be low there.

42. What do you mean by ultrasonics and infrasonics?

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43. Explosions on other planets are not heard on earth. Why?

ans: There are mainly two factors involved. One is the distance. from inverse square law, the intensity of sound is inversely proportional to the square of distance, i.e. I (alpha) 1/r^2. Since the distances of the planets are very large from the earth, the intensity of sound on earth will be very less, so less that it would be very difficult to hear them.

The Other factor is absence of any medium between them and earth. Sound needs material medium to travel, the elastic and inertia property of the substances playing their roles. Since there aren't any such medium, sound will thus not travel up to earth.

44. If something goes wrong in a geostationary satellite and it explodes, can we hear the enormous sound of the explosion?

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45. As we move away from a sounding body, the sound we receive goes on decreasing. why?

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46. Why do you make your hand like a cup to call your distant friends?

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47. What do you mean by Threshold of Hearing?

ans: The intensity of the lowest audible sound that people can hear is called Threshold of Hearing. This is equal to the intensity of 10^-12W/m^2 for a pure frequency of 1000Hz (1 KHz). The intensity level of this intensity is 

48. If intensity of certain sound increases ten times, what would be the change in intensity level?

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49. If intensity of certain sound becomes twice(by the addition of one more source or otherwise), what would be the change in Intensity level?

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50. If intensity level of certain sound becomes 100dB from 50dB, what would be the change in intensity?

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51. Two ears help in determining the direction from which sound is coming. How?

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52. What would have happended if we have had only one ear?

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53. When a bell is hit har, the sound produced is louder, why?

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54. How can we distinguish between the music of a sitar from that of a harmonium without looking at the instruments when the sound is of same pitch and loudness?

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55. What is a noise pollution?

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56. Two waves are represented in usual notation as y1 = a1sinwt and y2 = a2coswt Their intensities are I1 and I2. What would be the ratio of their amplitudes when I1 = 2I2?

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