At a prayer meeting,
Question:

At a prayer meeting, the disciples sing JAI-RAM JAI-RAM. The sound amplified by a loudspeaker comes back after reflection from a building at a distance of $80 \mathrm{~m}$ from the meeting. What maximum time interval can be kept between one JAl-RAM and the next JAI-RAM so that the echo does not disturb a listener sitting in the meeting. Speed of sound in air is $320 \mathrm{~m} \mathrm{~s}^{-1}$.

Solution:

Distance travelled by sound to come back $=80 \times 2=160 \mathrm{~m}$

Time interval = distance $/$ speed

$=160 / 320$

$=0.5 \mathrm{sec}$