Dolphins use ultrasonic calls with frequencies above 200-350 kHz for "echo location", and we invented "sonar" by using this principle. SONAR is a transliteration of the English abbreviation "sonar", and its full name in Chinese is: acoustic navigation and ranging. It is an electronic device that uses the propagation characteristics of sound waves under water to complete underwater detection and communication tasks through electroacoustic conversion and information processing. It has two kinds, active and passive, and belongs to the category of acoustic positioning. Sonar is an electronic device that uses underwater sound waves to detect, locate and communicate underwater targets. It is the most widely used and important device in hydroacoustics.