When you make waves in the center of a sink you can see the waves reflect or bounce off the sides of the sink and return to the starting point. Just like this sound waves can also be reflected. The reflection of your voice is called an echo. Echoes can be heard from almost any large surface such as a large wall, building, or cliff. The surface must also be relatively flat, perpendicular to the source of the sound, and far enough to notice the time difference, but not so far away that the echo is too weak.19
The further you are physically from the surface the more time that is needed for the sound wave to reach it and bounce back to your ear. This increases the time between when a sound is made and when you will hear its echo. By knowing the speed of sound and measuring the time it takes to hear the echo, you can calculate the distance of the object. The equation for this is: Distance = ½ X (Speed of Sound) X (Time until you hear the echo). The ½ is there because the sound travels the distance twice, both there and back.20 For example, the sound waves travel outward, bounce or reflect off a flat surface and return producing an echo. So the sound wave travels to the flat surface and back again.