Hydroacoustics(Underwater acoustics) is the study of sound in water and the branch of acoustics that deals with the radiation, propagation and reception of acoustic waves underwater to solve acoustic problems related to the detection of underwater targets and information transmission processes.
Hydroacoustics is the only form of energy known to man that can travel long distances in the ocean. Other physical media such as visible light, electromagnetic waves, lasers, etc., are quickly attenuated when propagated in seawater and therefore cannot travel long distances. The term sonar originated during the Second World War and is made up of three words: sound, navigation and ranging. Today, sonar is defined as 'a method and device that uses underwater sound waves to determine the presence, location and type of objects in the ocean.
The development of sonar has been around for over 500 years since the discovery of the sound tube by the Italian Leonardo da Vinci in 1490. What da Vinci described:'If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.' which is in fact the prototype of modern passive sonar.
In 1912, the Titanic, the world's largest passenger liner at the time, crashed into an iceberg and sank on its maiden voyage. The tragic shipwreck has prompted companies to develop devices that can warn of icebergs and other dangers to navigation.In 1914, the first practical echosounder was developed and patented in the USA by Fessenden (Reginald Aubrey) of the Boston Underwater Signal Company, it‘s an electronic device that sends out low-frequency sound signal and then switches to a listening state to receive an echo signal. Using this device can detect icebergs 3 kilometers away, but it is still impossible to accurately determine the location of the iceberg.
In 1918, Paul Longivan, a renowned French scientist, developed a piezoelectric transducer to produce ultrasound and applied the then newly available vacuum tube amplification technology to detect long-range targets in the water, receiving the echoes of submarines for the first time and pioneering modern hydroacoustics.
Sonar technology is an important means of acquiring, utilising and processing marine information. The invention of modern sonar predates radar, but the public's understanding of sonar is far less than that of radar. This is because sonar is mainly used in the military for the detection of surface ships and submarines, and in modern times it has been extended to underwater warning and anti-frogmen, so sonar is cloaked in mystery.
In the civil sector, sonar technology is an important tool for understanding various parameters of the sea surface, water column and seafloor, including sound velocity profiles, temperature and salt depth distribution, currents, internal waves, mesoscale eddies, seafloor geomorphology/topography, etc.Sonar equipment is also used in subsea scientific observation networks, subsea oil and gas field exploration, maritime distress rescue, underwater archaeology and marine disaster warning. In the field of national security, sonar equipment is installed on a variety of platforms, including surface ships, submarines, helicopters, unmanned/manned submersibles, shore stations, torpedoes, mines, etc. It plays a role in information gathering, long-range warning, target location and identification, navigation, and proximity/long-range weapon guidance.