What is the minimum distance from a reflecting surface needed to hear a distinct echo in air (speed of sound about 340 m/s)?

Answer: B. 17 m

Answer: B) 17 m
Explanation: The sensation of sound persists in the brain for about 0.1 s, so the echo must return after at least 0.1 s. In 0.1 s sound travels 340 x 0.1 = 34 m, which covers the distance to the wall and back. So the minimum distance is 34 / 2 = 17 m.

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