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Question

Explain how the 'Echo' is produced. What should be the minimum distance between the listener and the reflector to hear an echo of sound propagating with a speed vms1?

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Solution

The repetition of sound caused by the reflection of sound waves. For example, if we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.
The sensation of sound persists in our brain for about 0.1 s. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1 s.
Given,
Sound is propagating with speed, v m/s
Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least,
D=speed×time
D=v×0.1
D=v10
Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is,
D2=v20

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