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21 Two astronauts on the surface of the moon cannot talk to each other like on the earth. Why?

The moon contains no atmosphere so the material medium is absent in the moon. For the propagation of the sound, the material medium is required i.e. the sound does not travel in vacuum. Hence in the moon there is no material medium through which the sound propagates. Hence, two astronauts on the surface of the moon cannot talk to each other like on the earth.

22  A sharp and intense sound is heard when a tuning fork struck against a rubber pad is held over a resonance tube and the length of the air column in the tube is adjusted for particular value. Explain why?

If a tuning fork is struck against a rubber pad and held over a resonance tube with random length, a feeble sound is produced. If the length of air column in the tube is adjusted for a particular value, the natural frequency of that air column when matches with the frequency of the tuning fork which gives resonance and the air column vibrates with its maximum amplitude. As a result, a sharp and intense sound is heard.

23. Why is there a hole in the musical instruments?

The musical instruments like stringed instruments are provided with a hole (hollow box) called sound box. when strings are set into vibration, forced vibrations are produced in the sound box. As there is large area, it sets a large volume of air into vibration. As a result, a loud sound of same frequency as that of the string is produced. Hence, there is a hole in the musical instruments.

24. Sometimes, the glasses of the windows of a house are found to be shattered when large sounding bodies like aeroplane pass near the house. Explain how?

The main reasons of the shattering of the windows of a house when large sounding bodies like aeroplane passes near the house are:

1. Due to the large sound coming from the aeroplane, the glass of windows are set into vibration. Large sound causes large amplitudes of forced vibration of the glass windows. In due course, its elastic limit may be exceeded, which then breaks the glass and causes shattering.

2. The frequency of the sound produced by the aeroplane may match with the natural frequency of the glass window resulting in resonance. During resonance, the maximum amplitude of forced vibration of the glass window may exceed its elastic limit which might lead to breakage and shattering.

25. A man is standing close to a closed room having glass windows and wants to call his friend. When he speaks, his friend does not hear him but when he knocks on the window, his friend hears him. Explain how?

If a man speaks to call his friend standing close to a closed room having glass windows then his friend does not hear him because most of the sound wave is reflected by the glass window and the energy of his sound is not enough to cause proper vibration in the glass. As a result, very less sound passed through the glass window which causes very less air to cause vibration which may or may not be perceived by his friend. There is less probability of perceiving by his friend and if it is not perceived then his friend does not hear him.

But, when he knocks on the window, the energy is directly fed into the glass, with no occurrence of reflection. So the window is set into sufficient vibration which causes the air of the closed room to vibrate and the sound can be easily perceived by his friend. So, his friend hears him.

26 Explain why people usually prefer knocking at the door instead of speaking to call someone inside a closed room?

If a man speaks to call someone (say his friend) inside a closed room then his friend does not hear him because most of the sound wave is reflected and the energy of his sound is not enough to cause vibration of the door. As a result, very less sound passed through the the door which causes very less air to cause vibration which may or may not be perceived by his friend. There is less probable of perceiving by his friend and if it is not perceived then his friend does not hear him. But, when he knocks at the door, the door is set into vibration which causes the air of the closed room to vibrate and the sound can be easily perceived by his friend. So, his friend hears him. Hence, people usually prefer knocking at the door instead of speaking to call someone inside a closed room.

27. Is a sound louder in a hall or in the open?

A sound is produced louder in a hall compared to the open place. In a hall, the repeated reflections of sound take place from the walls and ceilings that produce constructive interference with the sound. Hence, the loudness and intensity of the sound increases in the hall than in the open. But, there is no increase in the loudness and intensity of sound in the open as there is absence of the reflection phenomena of the sound. Hence, a sound is produce louder in a hall prefer to the open place.

28. Why do the stages of large auditoriums have curved backs?

The stages of large auditoriums have curved backs because when the speaker (host) stands near the focus of that curved surface then his voice travels parallel after reflection from that surface. As a result, his sound can be heard at the larger distances clearly. Hence, the stages of large auditoriums have curved backs.

29. Solids can propagate both longitudinal and transverse waves but liquid and gas can propagate only longitudinal wave. Why?

There is a tangential force develops which acts on the particles of medium when a transverse wave travels through that medium. Similarly, a normal force acts on particles of the medium when a longitudinal wave travels through that medium. Hence, the modulus of rigidity &bulk modulus of the medium are responsible for propagation of a transverse wave & longitudinal wave respectively. Solid is the medium that have both modulus of rigidity and bulk modulus. Hence, solids can propagate both longitudinal and transverse waves. But, gases & liquid have only bulk modulus so they can propagate only longitudinal wave.

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

In a curtained room, the curtains absorb the sound waves reaching the walls of room that prevents the reflection of sound. But in non-curtained room, sound is reflected in many directions which causes the production of echoes or reverberations. So, the sound heard in non-curtained room is not clear due to production of echoes or reverberations but no echoes or reverberations are produced in curtained room. Hence, we hear more clearly in a curtained room than in a non-curtained room.

31. Two loudspeakers have been installed in an open space to listen to a speech. When the speakers are in operation, a person sitting at a place (not far) receives very feeble sound while another person sitting at another place might receive very loud sound. Explain why?

When two sound waves of same frequency traveling along same line in same direction, superimpose upon each other & the resultant wave shows alternate variation of intensity of sound. There are two regions where sound shows variation of intensity. One region where compression of one wave meets the compression of other, the amplitude of vibration increases & intensity of sound is maximum & this region is constructive interference. While other region where compression of one wave meets the rarefaction of the other, the net amplitude of vibration becomes zero and the intensity of sound is zero & this region is destructive interference.

Two loudspeakers have been installed in an open space to listen to a speech. When the speakers are in operation, a person sitting at a place (not far) receives very feeble sound while another person sitting at another place receives very loud sound. This is due to the phenomena of interference of sound. A person sitting at a place (not far) receives very feeble sound because of destructive interference of sound & another person sitting at another place receives very loud sound because of constructive interference of sound.

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

When two sound waves of nearly same frequency traveling along same line in same direction, superimpose upon each other & the resultant wave shows alternate rise & fall in intensity. This is known as phenomena of beats. If two sound waves of equal frequency are used, then it does not produce beats since the beat frequency obtained will be zero but interference of sound occurs. There are two regions where sound shows variation of intensity. One region where compression of one wave meets the compression of other, the amplitude of vibration increases & intensity of sound is maximum & this region is constructive interference. While other region where compression of one wave meets the rarefaction of the other, the net amplitude of vibration becomes zero and the intensity of sound is zero & this region is destructive interference. Hence, two sound waves of equal frequency can not produce beats.

33. The reverberation time is longer for an empty hall than for crowded hall why?

As there are a lots of sound absorbing materials in the crowded hall than the empty hall, more sounds are absorbed in the crowded hall and the sound cannot reflect more as in the empty hall due to being absorbed. Hence, the reverberation time is longer for an empty hall than for crowded hall.

34. How are bats able to fly around even if they don't have eyes?

Bats produce ultrasonic waves which detect if there is object or not. The waves of ultrasonic sound produced by bats are of the high frequency that travels toward the objects and when it is reached to it , it is reflected from them and bats analyze the reflected waves coming from the objects and they can visualize the shape, size, position and dimension of the object.

35. Why can't we hear echo in a small room?

When the sound wave is reflected from any object then it is called echo. To hear echo clearly, the interval between original sound and reflected sound must be at least 0.1 seconds, called the persistence of hearing. So, the minimum distance between the source and the listener must be 0.05 times the velocity of the sound in the air i.e. 16.5m. Hence, we can not hear echo in a small room.

36. You are given a watch, and asked to determine the length of a very large hall. Can you do that? Explain.

When the sound wave is reflected from any object then it is called echo. By counting the echo produced and noting the time in which the echo is produced, we can easily find the length of a hall. Let d be the length of a large hall, v be the velocity of the sound in air and t be the time at which the echo is produced.

37. Explain why does the flash of light reaches the earth before the sound coming from the same thunder?

As the velocity of the light is much more greater than that of the sound in the air, the lightening effect is first observed during thunder then later the sound of the same thunder is heard by the person. Hence, the flash of light reaches the earth before the sound coming from the same thunder.

38. Discuss the effect of wind on the velocity of sound propagation through the air.

EFFECTS OF WIND ON VELOCITY OF THE SOUND:

The velocity of the sound is altered by the effect of wind. Let the velocity of the sound be v and the velocity of the
wind be vw and the direction of the wind makes angle θ with the velocity of gas then the resultant velocity of the sound is given by:
conditions:
1. If θ = 00, then vR = v + vw.
2. If θ = 900, then vR= v.
3. If  θ = 1800, then vR = v - vw.

39. Two people sitting in two boats in a pond are talking to each other. Why can they hear other over long distances?

The air surrounding to the pond is highly humid. As the density of humid air is less than that of dry air so the velocity of sound in humid air is greater than the velocity of sound in dry air. The air close to the surface of the pond is greatly humid, sound waves travel faster in it and cover large distance. Hence, if two people sitting in two boats in a pond are talking to each other they cannot hear each other over long distances.

40. Sound can be heard more distinctly through carbon dioxide than through air. Why?

The intensity of sound produced as a result of vibration of particles of a medium varies directly to the density of the medium. As the density of carbon dioxide gas is greater than that of the air, the intensity of sound in carbon dioxide is greater than that in air. Hence, sound can be heard more distinctly through carbon dioxide than through air.

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