Sound waves get reflected when they hit the interface between two mediums. Imagine standing on one end of a long hallway of length L and emitting a sound wave. The reflected wave takes some time to return to the source, given by the total distance traveled and the speed of sound. Human ears can distinguish between two sources of sound if they are separated by a time gap greater than one-tenth of a second. The emitted and reflected waves are perceived as distinct sound waves if the time difference exceeds the distinction time. This phenomenon is called an echo. By equating the two sides and substituting the speed of sound in ambient air at twenty degrees Celsius, the minimum distance to hear an echo is obtained. Conversely, if the time difference between the emitted and reflected waves is known, the distance between the source and the reflector can be calculated. Ships use this technique to measure the distance of various objects on the ocean floor.