SONAR was originally an acronomy for the phrase SOund Navigation and Ranging. It is essentially like radar, except that it uses sound pulses and is used underwater.
When I say that it is like radar, I mean that sonar is a system in which a bunch of sound waves are sent out from a ship. The sound waves bounce off of objects and then return to the ship where they are received by part of the sonar apparatus. When the waves come back, they do not all come back at once. Instead, things come back sooner if they hit a target near the ship and later if they hit one farther. In this way, the returning sound waves can sort of map out the shapes of objects that are within range of the sonar set.
SONAR is an acronym that stands for Sound Navigation and Ranging. It utilizes the fact that sound waves bounce off objects and can be received by the body sending them, to detect objects and also determine the distance they are at. The frequency of sound waves used here varies from those very low, called infra sonic to those with a very high frequency called ultrasonic. These usually help in very accurate detection. SONAR has been used for over 50 years by ships to navigate in the seas and save themselves from colliding with other ships and objects in the water that are not visible.
Sonar is a device that uses sound to locate position of objects submerged in deep waters. More sophisticated varieties of sonars can also generate pictures of these objects and determine speed of moving objects. The word sonar is formed by combination of two terms used in navigation - sound navigation and ranging.
As compared to radars sonars are better suited for many underwater applications as sound travels faster and more efficiently in water, whereas radio waves face greater distortion in water. Some sonar devices are designed to operate in air also. For example, some varieties of burglar alarms use airborne ultrasound waves to detect movement.
There are two types of sonars - active and passive sonars. Active sonar send out sound waves which travel through the water until they strike an object, and are reflected by them in various directions. Some of the reflected waves return to the sonar, where they strike a receiver, which converts the sound back into electrical signals. These signals are then analyse to produce the required information. Passive sonars use the sound given off by other objects. These type of sonars are not very effective in ascertaining the distances of the object under study.
It is interesting to note that dolphins and bats use natural sonar techniques called ecolocation to locate food, avoid obstacles and to communicate.