The minimum distance from a reflecting surface needed to hear a CLEAR echo (with speed of sound ≈ 340 m/s) is about:
A3.4 m
B34 m
C100 m
D17 m
Answer & Solution
Correct answer: D. 17 m
For two sounds to be heard separately, the time gap between them must be at least 0.1 s. Sound travels 340 m/s × 0.1 s = 34 m in this time — but this is the TO-AND-FRO distance. So the reflector must be at least 34/2 = 17 m away. Closer than this and the original sound and its echo blur together.
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