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Q. What is the minimum distance (in metres) required to hear an echo?
Answer: 17
Notes: The human ear cannot distinguish echo from the original direct sound if the delay is less than 1/15 of a second. The velocity of sound in dry air is approximately 343 m/s at a temperature of 25°C. Therefore, the reflecting object must be more than 17.2m from the sound source for echo to be perceived by a person located at the source.

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