Hint: In order to solve this question, we are going to firstly define what an echo is and how it is produced. After that, we are going to list the two important applications of the concept. Echo is very useful in nature and many other human applications too. Some of them are listed in the solution.
Complete step-by-step solution: In audio signal processing and acoustics, an echo is defined as a reflection of sound that generally arrives at the listener with a delay of a few seconds after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the listener. There are two applications of the echo: Dolphins use echo in order to detect their enemies and the obstacles by emitting the ultrasonic waves and thereby hearing their echo. Echo is also used to image the human organs in medical sciences.
Note: Alternatively, we can also say that when a sound wave is produced in air, some of it is absorbed after hitting with a reflector while some of the sound wave is reflected back to the listener. This reflected sound is called echo. The applications of echo are: Bats use the concept of echo in order to find their ways as they cannot see with their eyes, so , they use echo by producing sounds and sensing their reflections to find the ways. Echo helps to estimate the distance of the hills and the mountains.