Question

What is the minimum distance from an obstacle in order to hear an echo? (minimum time = 0.1 s)

• A. 17 m
• B. 16.6 m
• C. 16 m
• D. 17.6 m

Answer 1

The correct option is B

16.6 m

consider an observer producing 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 for returning to the observer. Since the standard speed of sound in air is 332 m/s and to hear two sounds distinctly minimum time required is 0.1 second, accordingly,

Speed = distance/time

Distance = speed x time

2x = 332 m/s x 0.1 s

X = ( 332 x 0.1 ) / 2 m

X = 33.2 / 2 = 16.6 m