The main device used to detect and locate objects like submarines and torpedoes under the sea water is called ‘sonar’. The word ‘sonar’ is an acronym for ‘sound navigation’ and ranging. This instrument is particularly useful in sea warfare. However, in peace time also, this is used in ships and submarines. It can locate objects at distances ranging from 100 metres up to 10 kms.
Sonar mainly consists of two parts: the transmitter and the receiver. Both these parts are submerged in the sea water. The transmitter with the help of a transducer produces sound waves of high frequencies (5,000 to 300,000 hertz). These waves are called ultrasonic waves and cannot be heard by human ears. The transmitter transmits these waves in pulses in all the directions. Whenever these pulses strike some object inside the sea water, they get reflected. These reflected waves are received by the receiver. The time taken for the waves to reach the object and come back to the receiver is measured. Half of this time when multiplied by the speed of sound in sea water gives the distance of the object. This instrument contains a display device also, which accurately shows the distance and position of the object.
Sonar, however, is not a foolproof system. The sounds produced by some aquatic animals sometimes interfere with its transmissions and can be misleading about the position of the objects. The enemy submarines can be detected by this equipment and thus can be destroyed. These days there are various kinds of sonars being used for studying the objects inside water. Sonars also help in large scale fishing by locating those spots where large groups of fishes are present. Nowadays blue-green laser pulses are also being used for underwater detection of submarines and torpedoes. Laser based devices also work on the echo sounding principle just as a sonar works.