Question

what should be the minimum distance between the listener and the reflector to hear an echo of sound propagating with a speed v m/s?

Answer 1

perception of a sound usually endures in memory for only 0.1 second.
so minimum time for echo =0.1sec
distance =d
sound have to travel from listener to reflector then to listener.
so total distance =2d
we know distance =speed ×time
2d=340×0.1=34
d=34÷2=17
so minimum distance is 17m