An echo is the repetition of the original sound because of reflection by some surface.
At 22°C, the velocity of sound in air is 344 m/s.
Our brain retains a sound for 0.1 seconds Thus, for us to be able to hear a distinct echo, the sound should take more than 0.1 seconds after starting from the source to get reflected and. come back to us.
We know that,
Distance = speed x time
= 344 m/s x 0.1 s
= 34.4 m
Thus, to be able to hear a distinct echo, the reflecting surface should be at a minimum distance of half of the above, i.e. 17.2 m.
As the velocity of sound depends on the temperature of air, this distance depends on the temperature.
