SONAR is a technique that makes use of the property of sound. SONAR stands for SOund NAvigation and Ranging. This works on the principle of echo of transmitted sound waves from objects. For instance, if you hit a wall in front with a tennis ball, the ball will bounce back to you. But if the wall is removed, the ball will not come back to you.
Thus, even with eyes closed, you have a way of knowing whether there is an object or a rebounding surface in front. SONAR works in the same fashion.
Use of sound waves to detect objects is based on the above simple example. The advantage of using sonar wave over electromagnetic waves is that electromagnetic waves lose energy fast in the ocean water because water can conduct electricity. In contrast sonar waves can travel farther in water.
There can be two types of SONAR set-ups. One is Passive and the other Active.
In Passive SONAR, one detects sound waves that are present around. Leonardo da Vinci did it as early as 1490 AD. He dipped a pipe in water and placed his ear next to the end which was out of water. He used this to detect the waves generated by ships. Today, the techniques are far more sophisticated. SONAR became a topic of very serious studies during the World War II as detection of movements of ships and submarines assumed significance.
Now Active SONAR is very important. It has two major components:
One has to ensure that the signal is sent as a narrow beam. If not, then the reflections will be coming from many directions and will be confusing. Theoretically, the distance travelled by the wave is twice the distance between the transmitter or detector and the target to be detected. So, if velocity of the sound in water is v, then distance of the object is
d = 1/2 x v x t
where t is the time-lapse between transmission and detection of sonic signal.
The wave may be reflected from surface or bottom of the sea, ships, whales or other animals, submarines and other objects. The whole thing looks very simple but in practice, there are several other factors to be considered. For instance, the velocity of sound in a medium depends on the density and bulk modulus of the medium.