Explain why echoes cannot be heard in a small room.

(N/A) To hear an echo,the minimum distance between the source of sound and the reflecting surface (obstacle) must be approximately $17 \,m$. This is because the human brain retains sound for $0.1 \,s$ (persistence of hearing). Since the speed of sound in air is about $344 \,m/s$,the total distance traveled by sound in $0.1 \,s$ is $34.4 \,m$. Therefore,the distance to the obstacle must be at least $34.4 / 2 = 17.2 \,m$. In small rooms,this distance is not maintained,so the reflected sound reaches the ear before the $0.1 \,s$ interval,causing the original sound and the echo to overlap,making the echo inaudible.