SONAR stands for Sound Navigation and Ranging. It is the technique that uses sound propagation to navigate. There are two types namely active and passive sonar. Passive sonar is essentially listening the sounds made by vessels. Active sonar is emitting pulses of sound and listening for echoes. These is the equipment used to generate and receive the sound. The study of under water sound is known as underwater acoustics or hydro acoustics.
Sound travels more slowly in fresh water than in sea water. The speed of sound is determined by the water’s bulk modulus and mass density. The bulk modulus can be affected by temperature, dissolved impurities and pressure. The density of effect is small. Active sonar may harm the marine animals. Some animals (dolphins and bats) have used sound for communication and object detection for millions of years. Humans initially recorded sound in the water by a tube inserted into the water to detect vessels by placing an ear on the tube.