Question

What should be the minimum distance between the source and reflector in water so that the echo is heard distinctly? (The speed of sound in water = 1400 m $s^{-1}$).

Answer 1

The time period of persistence of ear for simple sound = 0.1 s.

Therefore, to hear the distinct echo of simple sound, the minimum time taken by the sound to reach the listener after reflection should be 0.1 s.

Let d be the distance between source and reflector.

The time taken by sound to reach the listener after reflection = t

The sound will travel the total distance of 2d.

$2d=vtd=\frac{vt}{2}$

Now, putting the values,

v = 1400 m $s^{-1}$)

t = 0.1 s

We get,

d = 70 m

Therefore minimum distance of reflector from the source should be 70 m.