Question

A boy stands $85\,m$ in front of a high wall and then blows a whistle. Calculate the time interval when he hears an echo. (Speed of sound $ = 340\,m{s^{ - 1}}$)
(A) $0.1\,s$
(B) $0.2\,s$
(C) $0.5\,s$
(D) $1\,s$

Answer 1

Hint:Here we have to apply the concept of distance, velocity and time to get the answer.Velocity is defined as the displacement divided by time and speed is defined as the distance divided by time.So,we are going to apply these formulas in order to reach the correct solution.

Complete step by step answer:
Echo is a sound projection that appears to the listener after the actual sound with a pause. The delay is directly proportional to the distance from the source and the listener of the reflective surface.
Given,
Speed of sound $S = 340\,m{s^{ - 1}}$
Distance between the wall and the boy$ = 85\,m$
Since, echo is produced,
So, total distance $d = 2 \times 85 = 170\,m$
Let the time interval be $t$.
We know that-
Distance = speed $\times$ time
Time$ = \dfrac{{{\text{distance}}}}{{{\text{speed}}}}$
$
t = \dfrac{d}{S} \\
t = \dfrac{{170}}{{340}} \\
t = 0.5\,s \\
$
The time interval when he hears an echo is $0.5\,s$.
Hence, option C is correct.
Additional information:
An echo is a signal which is replicated and it reflects back the sound waves. As a rubber ball bounces off the ground, sound waves will bounce off flat, hard surfaces in the same way. The echo sounds the same as the original sound, even though the direction of sound varies.An echo is less strong than the initial sound due to the fact that it is created when a sound wave passes through the air and then touches another surface.


Note:Here we have to pay attention to the distance of sound waves. As an echo is produced, we have to take the distance twice of the original distance otherwise our answer will be wrong.In a perfect gas the speed of sound depends solely on its temperature and composition. In ordinary air, the velocity has a poor reliance on frequency and pressure, marginally deviating from ideal behaviour. So, the speed of sound may somewhat vary in different mediums.