Sound as a way to see underwater
Light and radio waves do not travel far in seawater, but sound does. Sonar, short for sound navigation and ranging, uses this fact. In its active form, a device sends out a pulse of sound and listens for echoes, then converts the delay into distance, since sound moves through seawater at roughly 1,500 meters per second. In its passive form, it only listens, for example for the noise of a ship’s propeller. The idea of echo ranging developed unevenly over the twentieth century, driven first by disasters and wars and later by commerce and science.
Early ideas after the Titanic
Leonardo da Vinci is sometimes credited with noting that a tube placed in water could detect distant ships, but practical work began in the early 1900s. After the loss of the Titanic in 1912, there was strong interest in detecting icebergs. Reginald Fessenden, a Canadian-born engineer working in the United States, developed an electromagnetic oscillator that could send and receive underwater sound. In 1914, tests off Newfoundland showed that it could detect an iceberg at a distance of a couple of miles, although the device could not reliably tell objects apart or give a precise direction.
Around the same time, German scientist Alexander Behm patented an echo sounder in 1913 to measure water depth, reportedly motivated by the Titanic disaster as well. Echo sounding, as opposed to detection of objects, became one of sonar’s most lasting uses.
The First World War and submarines
The First World War’s submarine threat turned sonar into a military priority. In France, the physicist Paul Langevin worked with the Russian engineer Constantin Chilowsky and developed an ultrasonic transmitter using quartz crystals, which are piezoelectric, meaning they change shape when voltage is applied and generate voltage when squeezed. This gave stronger and more directional sound than earlier devices. British and American researchers developed similar equipment, and Britain’s version was called ASDIC. The name sonar was adopted in the United States in the early 1940s.
Results during the war were limited, and by 1918 the techniques were still being improved. Between the wars, detection ranges and reliability increased, and in the Second World War, sonar became a standard tool for escort ships hunting submarines, alongside depth charges. Submarines countered with new tactics and, later, quieter designs. The cat-and-mouse contest over naval submarine warfare shaped sonar research for decades, and the history of submarine design is closely tied to the history of sonar.
Echo sounders at sea
Away from warfare, sonar spread in the 1920s and 1930s as echo sounders on commercial ships and research vessels. Before this, depth was measured by lowering a weighted line, which was slow and sparse. The German Meteor expedition of 1925 to 1927 used echo sounders to take thousands of depth measurements across the South Atlantic, revealing underwater ridges and basins in a way that earlier surveys could not. Echo sounders let captains see the seabed under their ship continuously, which improved safety in unfamiliar waters, much as a coastal lighthouse had warned of danger from the surface.
Fishing and seabed mapping
Fishing boats began using echo sounders in the 1930s to detect schools of fish, since fish with swim bladders reflect sound differently from water. After the Second World War these became widely available, and later designs displayed images on screens, then on color displays. Critics note that better detection also contributes to overfishing when stocks are not well managed.
In the 1950s and 1960s, scientists such as Marie Tharp and Bruce Heezen used soundings to produce maps of the ocean floor that helped establish the global mid-ocean ridge system, an important input to the theory of plate tectonics. Side-scan sonar, developed in the 1950s and 1960s, produces image-like pictures of the seafloor by sweeping sound to the sides of a towed device. Multibeam echo sounders, which became widespread from the 1970s and 1980s, send many narrow beams at once and map a wide swath of bottom in one pass. These tools are used to chart sea routes, find shipwrecks, survey routes for submarine cables and support offshore construction. Accurate positioning from GPS receivers made the resulting charts far more useful.
Navigation and safety
On modern ships, sonar devices measure depth, detect hazards ahead, and sometimes measure speed over the bottom with Doppler sonar. Divers, rescue teams and archaeologists use handheld or boat-mounted units to search for objects underwater. Researchers also use sonar to study marine life, but the sound can affect some animals, notably whales and dolphins, and concern about this has led to limits on powerful sonar in some regions.
The same echo principle later appeared in other technologies, including diagnostic ultrasound used by doctors. In each case, a pulse is sent into a medium, an echo returns, and a computer converts timing into a picture. Sonar remains central to anything that moves or works beneath the surface, from fishing vessels to research ships and underwater robots.