Sound made in front of a tall building 18 m away is repeated. Name the phenomenon and briefly explain it.

Ans. The 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 and v is its velocity. For the human ear, this time must be at least 0.1 second to ensure that the reflected sound does not get mixed up with the direct sound. Since the velocity of sound in air is nearly 340 m s^-1, we must have 2d/340 > 0.1 or d> 17 m. Since the building is at a distance of 18 m, we can hear the reflected sound in the form of an echo clearly.

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