The human ear cannot distinguish between two sources of sound if they happen to reach within a specific time interval, typically 0.1 seconds apart. More than this, and they are perceived as separate sources.
Imagine the sound is reflected back to the ears. Assuming that the source is very close to the human, the difference between hearing the two sounds—the emitted sound and the reflected sound—may be more than the minimum time for perceiving distinct sounds. If this is the case, then the phenomenon is called echo.
As it turns out, the minimum distance to hear an echo is around 16.5 meters. As such, long hallways or distant cliffs are required to hear an echo.
The calculation can be reversed to calculate the distance between the emitter and the reflector. If the time difference between the emitted and reflected sound waves can be measured precisely, the distance between the emitter and the reflector can also be calculated. In the animal world, bats use this technique to locate prey.