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Taking the speed of sound as 344 m/s and persistence of hearing as 0.1 s, what is the minimum distance from a reflecting wall to hear a distinct echo?
A8.6 m
B34.4 m
C17.2 m
D68.8 m
Answer & Solution
Correct answer: C. 17.2 m
1. Note the data: $v = 344\,\text{m/s}$ and minimum time gap $t = 0.1\,\text{s}$.
2. The sound travels to the wall and back, a total distance $2d$, in time $t$.
3. So $2d = vt$, giving $d = \dfrac{vt}{2}$.
4. Substitute: $d = \dfrac{344 \times 0.1}{2} = \dfrac{34.4}{2}$.
5. Therefore $d = 17.2\,\text{m}$.
6. Trap C (34.4) forgets to divide by 2 for the return trip; trap A (8.6) halves the answer once too many.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 5 Acoustics "Acoustics", p.71_
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