Sound from a vibrating object travels in every medium and spreads in all directions. If there is an explosion at any place, it is heard equally in all directions surrounding the explosion.
Sound may be directed or reflected: Sound can be directed and thereby more effectively received along a particular direction. This can be done by means of a megaphone. A megaphone is a large, funnel-shaped device used to reflect and channelize sound energies along a chosen direction. Stages in large auditoriums have curved backs so that sound can be better projected and heard. This is due to reflection of sound from a hard surface. This property of reflection of sound is similar to the reflection of light. Here the hard surface acts like a reflecting mirror, and the angle of incidence is equal to the angle of reflection. The laws of reflection for sound and light are similar.
Echo: Like light, sound is also reflected from the surface on which it falls. The sound travels to a reflecting surface, is reflected, then travels back to the ear. The sound now hears is called an echo of the original sound. Echo is generally produced by any large surface such as the walls of a building, the sides of a mountain, a cliff, tunnels, deep well or the trees at the edge of a forest.
Sound travels 330 m in one second in air. It takes about one-tenth of a second for the human ear to separate one sound from another. The human ear can hear two sounds separately and distinctly only if there is interval of 0.1s between the two. This is called persistence of hearing. Total distance traveled by sound in one-tenth of a second
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=330 m/s x 1/10 s = 33 m
Therefore to hear your echo it is necessary for you to stand at least (33/2)m=16.5m away from the reflecting surface. Sound from your voice will travel 16.5 m to and from the reflecting surface, a total of 33m. At the velocity of 330 m/s, this sound travels for one-tenth of a second. The echo will e clean and distinct because it will reach you one-tenth of a second after you hear your voice for the first time.