Single correct option
An echo is heard when the reflecting surface is at least how far from the source? (Speed of sound in air = 340 m/s and the ear retains a sound for 0.1 s.)
- A. 34 m
- B. 17 m
- C. 8.5 m
- D. 68 m
Show answer and solution
Answer: (B) 17 m
The sound must travel to the wall and back, a total of 340 x 0.1 = 34 m, so the wall must be at least 17 m away. The trap is to quote 34 m, which is the total path and not the distance to the reflector. This is why echoes are not heard in small rooms.
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