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When we stand in one corner of a big empty hall and shout the word ‘hello’, we will hear the word coming from the empty hall in the form of an echo a little while later. It appears as if the hall is repeating our ‘hello’. This happens because the sound of our ‘hello’ is reflected from the walls of the hall and this reflected sound forms the echo (which we hear as ‘hello’ coming from the empty hall). We can now say that 'The repetition of sound caused by the reflection of sound waves is called an echo'.

 When a person shouts in a big empty hall, we first hear his original sound. After a little while, we hear the reflected sound of his shout. This ‘reflected sound’ is an ‘echo’. Thus, an echo is simply a reflected sound. If we shout at a wall from 344 m away, the sound takes 1 second to reach the wall. The sound reflects from the wall, and takes 1 second to return. So, we hear the echo 2 seconds after we have shouted. We can now calculate the minimum distance from a sound-reflecting surface (like a wall), which is necessary to hear an echo.


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