What is an echo? How is it produced?

Explanation

An echo is the phenomenon of repetition of sound of a source by reflection from an obstacle. To distinguish an echo from the original sound, the obstacle must be situated at a suitable distance from the source of sound.
The sensation of sound lasts in our brain for 1/10 of a second. This property is called persistence of hearing. Therefore, to hear a distinct echo of a sound, the time taken by this sound to reach the listener after reflection should be 1/10 of a second. Let us take
`d =` minimum distance of the obstacle from the source(S), as shown in figure,
`v=` speed of sound in air,
`t =` total time taken by the sound to reach the listener (L) after reflection.
As total distance travelled by sound `=` speed of sound `xx` total time
`2d = v xx t" "...(i)`

Substituting `v= 344"m s"^(-1)` (speed of sound in air at `20^(@) C`) and
`t = (1)/(10)` s, we get from equation (i)
`2d = 344("ms"^(-1) ) xx (1//10)s`
or `d = 17.2" m"`
Hence, for hearing a distinct echo, the minimum distance of the obstacle from the source of sound should be 17.2 m. It is to be noted that this distance changes with changes temperature of air.