1. The problem statement, all variables and given/known data: At a prayer meeting the disciples sing JAI RAM .The sound amplified by a loudspeaker comes back after reflection from a building at a distance of 80 m from the meeting. What maximum time interval can be kept between one JAI-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 m s-1. 2. Relevant equations: s/t =v 3. The attempt at a solution: I did a blind guess and worked out the problem as follows: Time taken to go to the wall = Time taken to come back = 80/320 s ⇒ Time difference = 2 x 80/320 = 1/2s. Answer matches with the one given in the text. I just want to know significance of the line "echo does not disturb a listener"