A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo of his sound ? Give a reason for your answer.

As we know,

$\text{Speed of sound (V)} = \dfrac{\text{Total distance travelled (2d)}}{\text {time interval (t)}}$

Total distance travelled by the sound in going and then coming back = 2d

Given,

d = 12 m

V = 340 ms^-1

Substituting the values in the formula above, we get,

$340 = \dfrac{2 \times 12}{t} \\[0.5em] t = \dfrac{2 \times 12}{340} \\[0.5em] t = 0.07 s \\[0.5em]$

Hence, time = 0.07 s.

An important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 s after the original sound dies off.

The time in this case is less than 0.1 s. Hence, the man will not hear the echo.