Question

What is an echo???
List its two applications

Answer 1

an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room.

echo is used in mapping. sound bounces off an object and the shorter timelapse between the original sound and the echo, the smaller the distance. it is particularly useful in mapping the debth of the ocean and also finding lost objects at sea. it is also used in medical procedures to assess for blockages in the heart for example. 

ultra sonic echos use sound waves to determine a shape and size of an object not visible to us like sunken ships, baby in the womb etc...