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Question
When a sound is made from a distance of 18 m, in front of a tall building, it is found to get repeated again. Which phenomenon is associated with this observation? Give a brief explanation of this phenomenon?
Solution
The associated phenomenon is known as Echo. We get echoes because of the reflection of sound. The reflected sound reaches the observer after a time 2d/v, where d is the distance of the reflecting wall from the source. For the human ear. this time must be at least 0.1 seconds to ensure that the reflected sound does not get mixed up with the direct sound. Since the velocity of sound in air is nearly 330 ms−1, we must have
2d/330 > 0.1
or d > 16.5 m
Since the building, in this case, is at a distance of 18 m, we can hear the reflected sound clearly. We thus get a clear echo in this case.
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