Sonar is a machine that uses underwater sound waves to find other objects in the sea. A sonar can work by sending out sound and listening for echoes (active sonar), like a radar, or by listening for sound made by the object it is trying to find (passive sonar).
The basic parts of an active sonar are here:
The transmitter creates the sound waves.
By doing this, the sonar can find out where the object is.
Active sonar was first used in 1917 by Paul Langevin. The word SONAR was created during the Second World War as an acronym for Sound Navigation and Ranging. This acronym replaced the British acronym ASDIC. The word is now thought by many people as a regular word, no longer as an acronym.