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For a distinct echo to be heard, the reflecting surface must be at a minimum distance of (taking speed of sound = 346 m/s):

A3.46 m from the source of the sound
B10 m from the source of the sound
C17.3 m from the source of the sound
D34.6 m from the source of the sound
Answer & Solution
Correct answer: C. 17.3 m from the source of the sound
The ear needs at least 0.1 sec between original and reflected sound. At 346 m/s, sound covers 34.6 m in 0.1 sec. Wave travels there AND back, so the reflector must be at least 34.6/2 = 17.3 m away.
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