Question

An experiment is being set up that requires echoes to arrive at least 0.2 s after the emission of sound. What minimum distance should a reflecting surface be placed at? Assume the speed of sound to be 343 m/s.

Answer 1

Sound must travel to the reflecting surface and back in 0.2 s, so the total distance travelled is $$343 \times 0.2 = 68.6,\mathrm{m}$$.

The reflecting surface must be at half this round-trip distance: $$34.3,\mathrm{m}$$.