Question

Determine the minimum distance between the listener and reflector for an echo to be heard distinctively of a sound propagating with a speed v m/sec ? (v=340m/s)

Answer 1

As we know that the perception of a sound usually endures in memory for only 0.1 seconds
so, minimum time for echo = 0.1 sec
distance = d
sound have to travel, from listener to reflector then to listener
so total distance travelled = 2d
now, distance = speed× time = 340 ×0.1
⇒2d = 34
⇒d = 17 m
minimum distance for echo is 17 m

Answer 2

Okay lets get our facts straight...

Our ears have a time limit before they can switch between sounds 

That is 0.1 seconds....Now the speed of sound in air is about 344 $\frac{m}{s}$

So the Distance covered will Be

$s*d = 34.4 m$

But remember an echo has to get reflected so it must travel twice a given distance.

Hence 

[tex]2D = 34.4 D = 17.2[/tex]

Now we get that the minimum distance for echo is 17.2 m