Question

why do echo need to heard after 0.1 second and at a distance of 17m in air

Answer 1

The minimum distance required for an echo to be produced is 16.6 m. Consider an observer producing a sound at a distance x from a rigid surface. The person hears the first sound as soon as it is produced and next when it travels through a total distance of 2x i.e. (x+x) for approaching the surface and and for returning to the observer.Since the standard speed of sound internationally accepted is 332 m/s and to hear the 2 sounds distinctly i.e., for formation of echo, the minimum speed required is 0.1 second. Accordingly, Speed(S)=Distance(d)/Time(t) or, distance = speed*time or, 2x = 332 m/s * 0.1 s s and s gets cancelled.. or, x = 332 *0.1 m /2 or, x = 33.2 m /2 or, x = 16.6 m (Answer)

Answer 2

The minimum distance required for an echo to be heard is 16.6m, but, 17m is used as a safe distance and to uncomplicate numerical solving!