Sound made in front of a tall building $18 \, m$ away is repeated. Name the phenomenon and briefly explain it.

(N/A) The phenomenon is known as Echo.
An echo is the repetition of sound caused by the reflection of sound waves from a distant surface.
For an echo to be heard,the reflected sound must reach the observer at least $0.1 \, s$ after the original sound,as the human ear has a persistence of hearing of $0.1 \, s$.
The total distance traveled by the sound is $2d$,where $d$ is the distance to the reflecting surface.
Given the speed of sound in air is approximately $340 \, m/s$,the minimum distance required is calculated as $d = (v \times t) / 2 = (340 \times 0.1) / 2 = 17 \, m$.
Since the building is at a distance of $18 \, m$,which is greater than $17 \, m$,the reflected sound reaches the observer after $0.1 \, s$,allowing the echo to be heard clearly.