# Obtain the minimum distance of the obstacle from the source of sound to har distinct echoes ?

To hear an echo the time interval between original sound and the reflected one must be at least 0.1 second.i.e.

t=0.1 s

Speed of sound at 20

^{0}C is 344 m/s

Now we know that, speed = distance /time

so distance= speed * time =344 x 0.1s=34.4 m

Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be at least half of this distance,

i.e. 34.4/2=17.2 meter is the minimum distance in air required from sound reflecting surface to hear an echo.

Regards

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