38. State one use of reflection of sound. 39. What is an echo ? 40. What minimum distance is required between the source of sound and the reflecting surface to hear an echo ? Give reason. 41. List four substances which are good absorbers of sound. 42. List the measures that you will take when designing a sound-proof room.
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Answer:
38.The reflection of sound is used in making instruments like megaphone, sound board, trumpet etc.
39.In audio signal processing and acoustics, an echo is a reflection of sound that arrives at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the listener
40.Since sound has to travel an equal distance in going up to the reflecting surface and in coming back from the reflecting surface, therefore it must travel nearly 33/2 = 16.5m either way. Thus, to hear the echo clearly in the air, the reflecting surface should be at a minimum distance of 16.5m from the source of sound.
41.When sound falls on the sofa, fluffs and other light substances such as clothes, papers, thermocol, coating of plaster of paris, carpets, curtains, furniture, wood etc., they absorb the sound to a good extent. These are called good absorbers of sound.
42.The roof of the enclosure must be covered by plaster of Paris after putting in the sheets of thermocol.
The walls of the enclosure should be covered by wooden strips.
The floor must be laid down by thick carpets.
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Answer:
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