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 ms$^{-1}$.[Pg. No. 201]

For an echo, sound has to travel to the reflecting surface and back, so the total distance travelled by sound is 2d.

Given, $v = 343 \text{ m s}^{-1}, t = 0.2 \text{ s}$

Distance = $v \times t$

$$2d = 343 \times 0.2$$

$$= 68.6 \text{ m}$$

$$d = \frac{68.6}{2} = 34.3 \text{ m}$$

Therefore, the reflecting surface should be placed at a minimum distance of 34.3 m from the source of sound.