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2016-02-17T19:23:50+05:30

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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
1 5 1
2016-02-17T19:24:48+05:30
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 

2D = 34.4

D = 17.2

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



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