Applications of Echo:
1. Measurement of a distance: Distance of a distant hill or of any extended reflector can be determined with the help of echo. In this case, a sharp inarticulate sound, like the sound of a gunshot is produced and simultaneously a stopwatch is started. The stopwatch is stopped just by hearing the echo from the distant reflector.
2. Measurement of the depth of a sea: The depth of a sea can be measured with the help of an echo. A source S containing some explosive material is kept at a depth of h from one end of a ship and a hydrophone. H is an underwater microphone that is kept immersed at the same depth h from the other end. A loud sound is produced due to the explosion at S. Sound can reach the hydrophone along two paths –
i. Straight line path SH
ii. Path SOH after reflection from the bottom O of the sea.
The sound that reaches along SH is called original sound and the sound that reaches along SOH is called reflected sound. The hydrophone electrically records the time intervals in which sound travels along the direct path SH, as well as along the reflected path SOH.
3. Measurement of the altitude of an airplane: The altitude of a flying airplane can also be measured with the help of an echo. Let us consider an airplane flying horizontally at a height of h along the path AB with a velocity of v as shown in fig:
When the plane is at A, a loud sound is produced and the echo is heard after time t when the plane is at B. So, sound reflects from the ground at point O and reaches the airplane along the path AOB, where AO = OB.