Question

A boy stands 85 m in front of a high wall and blows a whistle. Calculate the time interval when he hears an echo. $\left( \text{Speed of Sound}=340m/s \right)$

Answer 1

Hint: An echo is heard when a sound from a source bounces or reflects back from an obstruction and returns back to the source. The sound travels back and forth in this process so that the total distance is twice the distance between the source and obstruction.

Complete Step-by-Step solution:
So in the problem, the distance between the boy and the high wall is 85 m. So in order to hear an echo from the sound produced by the boy, the sound has to go and hit the wall and should reflect back and reach the boy.
So the speed at which the sound waves travel in this case is given as $340m/s$. The total distance travelled by the sound wave is twice the distance between the source and the wall.
$\text{Distance}=2\times 85m=170m$
So the time taken to hear an echo is,
$\text{Time}\left( \text{T} \right)=\dfrac{\text{Distance}}{\text{Velocity}}$
Substituting the values of distance and velocity in the equation we get,
$T=\dfrac{170m}{340m/s}$
$\therefore \text{ Time(T)}=0.5\text{ seconds}$
So a time of $0.5\text{ seconds}$ is needed to hear an echo.

Note: In physics, an echo is defined as the reflection of sound which arrives at the listener with a delay after the original sound was made.
Echo is used in deep-sea explorations in order to find the depth of the ocean or sea or to analyze the sediments on the sea bed.
The minimum distance needed between the source and the obstruction in order to produce an echo is 17 m.
Echo can be used to determine the velocity of sound in a medium.