SONAR is the acronym for sound navigation and ranging. It is a remote sensing device that is used for detecting and locating sound waves underwater. It has many other uses too. Naval uses this mechanism for the detection of submarines, and underwater navigation. Civilian uses include water depth measurement, locating fish schools and mapping the ocean floor. It is also used for locating objects such as pipelines or shipwrecks lying on the ocean floor, for measuring water current profiles and determining characteristics of ocean bottom sediments.
Sonar is just an alteration of echo. Transmitted wave and reflected wave combination give a picture of the distance and position of an object. Whales and specialized machines use this mechanism to locate distant objects and figure out their structure and movement.
There are two types of SONAR; active (echo-location) and passive. An active sonar sends out a signal in a narrow beam that propagates at a speed of about 1500m/s into water. If there is an object in the path of the beam it reflects a part of the sound energy back to the sonar receiver which detects the echo. By measuring the time taken by the echo to return, the distance of the target can be determined.
Passive sonar simply listens for sound given off by objects such as a ships propeller. It can determine the direction in which an object is located but not its distance. But it has one advantage; it does not give off any sound that another sonar unit might detect. Sonar also helps in identifying a target because the noise a ship gives off may reveal what type of ship it is. Submarines generally use passive sonar for locating enemy ships.
The range of sonar depends on their frequency. Low frequencies SONAR work remarkably. Many aquatic species like whales and dolphins are using these techniques for their survival. LFA sonar being used by military generally as it can travel thousands of miles. Only 4 broadcasting point is enough to cover 80% of total water land on earth.
Acoustic Transducer is the major development of the sonar system to project acoustic projectors. It generally use piezoelectric crystals (Quartz, tourmaline) , magnetostrictive crystals(iron,nickel) , electrostrictive crystal(barium titanate) . These crystal changes its shape when subjected to electric or magnetic fields and convert electric energy to acoustic energy. Suitably mounted in an oil-filled housing, they produce beams of acoustic energy over a wide range of frequency.