Explain how the speed of sound can be determined by the method of echo?

The echo method can be used to determine the speed of sound in air.

For this, sound is produced from a place at a known distance say, d at least 50 m from the reflecting surface. The time interval t in which the echo reaches the place from where the sound was produced, is noted by a stop watch having the least count 0.01 s . Then the speed is calculated by using the following relation —

V = $\dfrac{\text {Total distance travelled}}{\text{Time interval}}$

V = $\dfrac{\text {2d}}{\text {t}}$ ms^-1

The experiment is repeated several times and then the average value of speed of sound V is determined.