What is an echo? When can we distinctly hear the echo of a sharp sound? Why cannot we hear an echo in small halls?

(N/A) An echo is the phenomenon of the repetition of sound caused by the reflection of sound waves from an obstacle.
For hearing a distinct echo,the obstacle must be situated at a sufficient distance from the source.
Due to the persistence of hearing,the original sound and the reflected sound must reach the listener with a time interval of at least $0.1 \, s$ $(1/10 \, s)$.
If $d$ is the minimum distance of the reflecting surface from the source,the total distance covered by the sound is $2d$. Given the speed of sound $V \approx 340 \, m/s$,the time taken $t = 2d / V$.
Setting $t = 0.1 \, s$,we get $d = (V \times t) / 2 = (340 \times 0.1) / 2 = 17 \, m$.
Therefore,the minimum distance required to hear a distinct echo is $17 \, m$.
In small halls,the distance between the source and the reflecting walls is less than $17 \, m$,so the reflected sound reaches the listener before the persistence of hearing ends,causing the sound to overlap with the original sound rather than being heard as a distinct echo.