Sonar Technologies

Summary

Sonar technology embraces the use of sound waves to facilitate travel and use of waterways, primarily the ocean. Through the use and detection of sound waves, sonar enables ships to avoid underwater obstacles and discover the ocean's resources. Naval forces use sonar to locate, target, and destroy hostile forces. Sonar started as simply listening to sound in the ocean, but as technology improved, sonar became a complex system of emitters, receivers, and processors that enabled a much wider use in bodies of water of all sizes.

Definition and Basic Principles

The idea of using sound to locate submerged objects came from nature. Many marine animals, most notably whales and dolphins, generate sounds of various frequencies (the high-frequency dolphin “squeak” or the low-frequency whale “song”) that they use to communicate, navigate, and locate food. Humans soon copied this natural phenomenon. In its most common form, sonar is a means of acquiring data from bodies of water. Sonar, an acronym for SOund NAvigation and Ranging, takes advantage of the sound-amplifying qualities of water to distinguish objects hidden beneath the surface. Usually mounted on a ship or submarine, sonar systems convert electrical energy into sound energy, propagate the sound energy into the water in the form of a wave, measure the time interval between the transmission of the wave and its return to the ship and conversion back into electrical energy, and convert that measurement into processed data. The term sonar applies not only to the sound wave but also to the apparatus that generates it. Because water amplifies and transmits sound more than air, sonar is the primary means of detecting objects in areas where light and clear visibility are often in short supply. Depending on circumstances, water will transmit sounds hundreds of times farther than open air and at a greater speed. Modern sonar is an especially useful tool for exploring the nautical realm because it can search and record data on a wide area and can also be focused to provide detail on specific areas. Consequently, sonar has become the preferred tool in mapping the vast reaches of the ocean.

Background and History

In the early twentieth century, scientists developed the first acoustic systems to study the ocean floor. Using sound emitters, oceanographers could determine the depth of ocean waters and the contours of the bottom by measuring the time interval between the emitting of a sound wave and its return to the emitter. One of the first uses of acoustical devices was a means to detect icebergs, developed in the aftermath of the 1912 sinking of the RMS Titanic. Such systems proved their military value during World War I when German submarines wreaked havoc on British shipping. To counter the submarines, the British developed a device known as ASDIC that enabled them to locate submerged submarines before the German vessels could get within firing distance of their torpedoes. When the German submarine menace reappeared during World War II, sonar (the American term for ASDIC) again became a useful tool in stopping the destruction of merchant ships.

In the aftermath of World War II, the military use of sonar expanded. As nuclear power made submarines capable of faster speeds and diving to deeper depths, sonar became the only reliable means to target hostile threats. Advances in sonar also aided submarines in finding their enemy forces and directing weapons against them by providing electronic-targeting information instead of relying on visual inputs from binoculars or the periscope. Sonar also became effective at detecting the new generation of mines that rested on the bottom of the ocean as well as locating underwater obstacles and navigating choke points.

Postwar sonar advances also had civilian applications. Sonar proved a great asset to the fishing and oil industries, as it could efficiently detect resources, permitting more efficient use of those resources and avoiding the risks of sending deep-sea divers into dangerous circumstances.

How It Works

Types of Sonar. Sonar falls into the two categories of active and passive use. Active sonar uses a transducer to propagate a sound wave into the water, and a receiver, either on the emitting transducer or at another location, detects the echo of the sound wave bouncing off objects nearby and determines their depth and range by measuring the time interval between the initiation of the sound wave and its return. The data collected by the receiver must take various circumstances into context as water conditions affect the power of the sonar sound wave and the speed at which it moves through the water. Passive sonar relies on listening for sounds in the ocean rather than using a transducer-produced sound wave to generate data. Passive sonar is useful for determining conditions in the water without disturbing or altering the condition by introducing active sonar waves. Passive sonar also allows the user to remain silent and hidden, a useful element for the military, in which stealth is important.

Transmission and Reception. The method of acquiring information from a sonar system depends on the type of sonar employed. Passive sonar relies entirely on reception systems without an active transmission component. The first passive system was using a hydrophone, essentially a microphone placed into the water. First introduced during World War I, hydrophones were a cheap and basic means of detecting hostile submarines. The hydrophone operator simply listened to sounds emanating through the water, trying to sort artificial man-made noise from the ambient sea sounds. The main handicap was the motion of the ship, as water moving past the hydrophone and propeller noise muffled the sound of distant objects. Modern passive sonars work on the same principle of reception-based detection but use advanced receptor arrays positioned at various locations to counter the noise generated by the ship. The early hydrophones used carbonized paper as a baffle to transform the sound wave energy into electrical signals heard in the operator's headset, but modern systems employ fiber-optic receptors that decode the incoming sound. Active systems have a standard receptor and an active emitter to generate sound in the water. The first systems used quartz crystal oscillators to generate sound, but later systems used more powerful and adaptive piezoelectric materials, such as lead magnesium niobate. The more powerful the electrical charge, the farther the detection range of the sonar system, but transmitting sound itself is insufficient for object detection.

Environmental Challenges. Although modern power plants on ships and submarines, especially nuclear ones, generate vast amounts of power for their sonar systems, the environment in which they operate limits the capability of the sonar to transmit and receive sound. The temperature of the water affects sound propagation, as does the salinity level, with sonar capability seriously degrading in freshwater compared with saltwater. The amount of marine life also affects sonar effectiveness. Large amounts of algae or other microscopic life can, like a fog, cause different levels of water density. Marine animals that employ biosonar can also generate enough sound in a specific locality to drown out man-made sonar emissions. However, the biggest obstacles to effective sonar use are geography and temperature. Sonar works best in deep water, where reflections off the shore or bottom do not cause multiple echoes or reflections. In shallow water, there are multiple reflective surfaces, and sonars have trouble discerning between the multiple inputs to determine whether a sound is artificial or natural. As water depths increase, the declining level of sunlight also causes various strata or layers of water with different temperatures, known as thermal layers. Sonar-emitted sound tends to bounce off these layers and hide an object or become warped, which distorts the sonar beam and provides an inaccurate measurement of a target's location.

Data Processing. Acquiring data from sound propagation is one thing, but converting it into useful information is another. Unless the received data is converted into a useful form, the sonar is worthless. Early sonar sets used the human ear to discern useful information from the ambient ocean sounds, a process fraught with difficulties ranging from changing conditions to human error. The advancement of electronics and computers, however, provided a means to turn random noise into coherent sonar images. Advanced data processing permitted sonar operators to “scrub” radar returns of unwanted clutter as well as enhance weak or scattered returns to provide otherwise useless information. Computers also allowed sonar to take additional forms aimed at overcoming geographical and conditional limitations. Towed arrays, long cables with many attached hydrophones, provide multiple sonar sources and permit the operator to triangulate a sound source or concentrate on the particular hydrophones that are providing the clearest data. To overcome the limitations of thermal layers, a vessel might employ a variable depth sonar (VDS), a towed array with a small maneuvering craft, something like a small submarine, on the end of the array. By passing signals down the array, the operator can direct the maneuvering craft through the thermal layers, concealing nothing. The most modern sonar sets feature synthetic aperture sonar, which uses Doppler-shift differences in sonar return to convert the incoming data into a three-dimensional image.

Applications and Products

Military Use of Sonar. Although created for scientific purposes, sonar owes its development to the immediate military need to hunt German submarines during World War I. Starting with the basic sonar sets and hydrophones that equipped Allied subhunters in World War I, sonar continued its development as an antisubmarine weapon through the years after the conflict.

By World War II, sonar had become a key electronic aid to all major navies, as submarines continued to develop as a fighting force. By the 1950s, the arrival of faster and more capable nuclear-powered submarines made early detection even more vital. Sonar systems evolved to provide precise locations of potential submerged threats and data for new weapons to counter them. In addition to improved shipboard systems with long-range and greater data-processing power, sonar took on new forms. The US Navy developed the Sound Surveillance System (SOSUS), massive hydrophone arrays planted on the seafloor, to monitor geographical choke points for passing Soviet submarines during the Cold War. Complementing the shipboard sonars, airborne systems began to use sonar. Helicopters equipped with dunking sonar, a sonar transducer on the end of a cable that a hovering helicopter could drop into the water, greatly expanded a ship's sonar-coverage area. Helicopters and fixed-wing aircraft could also drop sonobuoys, buoys that can detect underwater sounds and transmit them via radio, within a general area identified by another sonar system. As nuclear-powered submarines could remain submerged for months at a time, sonar became the primary means of navigating without external visual cues. Sonar also provided targeting information for submarine weapons, such as torpedoes and cruise missiles.

Modern sonar systems remain critical in military operations in the underwater sphere. Evolving sonar technology must detect quieter, technologically advanced submarines and effectively navigate or track marine objects. Sonar technology aims to improve data processing efficiency and expand detection sensitivity to inform decision-making in submarines and anti-submarine warfare operations. Improvements in technology are particularly important in areas that present natural challenges for traditional sonar techniques, like shallow littoral water—5 to 10 meters (16 to 33 feet) below the low tide level. Active sonar in these areas is often reflected and refracted, but accuracy may be improved with more sensors.

Civilian Use of Sonar. Sonar plays a significant role in a variety of industries. The fishing industry makes widespread use of sonar to locate schools of fish in the vast stretches of the ocean. Locating fish by sonar allows the fishing boat to sweep a specific area of water, as opposed to pre-sonar fishing, where operators had to drop their nets in spots likely to contain fish but which might not. Targeted fishing saves time and fuel by making the process more efficient. Using sonar in fishing has even filtered down to recreational sportsmen—small, portable “fish finders” now abound. The other major industry to employ sonar is resource exploration. Sonar permits the mapping of the ocean floor and the exploration of likely locations of valuable materials. Oil exploration, in particular, employs sonar. Before exploratory drilling, sonar is used to determine the geological formations on the ocean floor likely to contain pockets of oil.

The nature of high-frequency sound waves has contributed to other industrial applications, even out of water. Unlike radar waves, sound waves are not radioactive and latently lethal. Although there is a risk of hearing loss due to lengthy exposure to high-frequency sound, sound waves generated into the air, known as ultrasonic waves, do not pose the health risks of radiation emitters such as radar. Consequently, sound waves are used in industrial processes such as materials inspection, where they are bounced off a product to determine its structural soundness and uniformity. The most recognizable use of commercial sound waves, however, is the prenatal ultrasound. As radiation might harm an unborn child, physicians use sound waves to generate, through a synthetic aperture system, a three-dimensional view of the child for diagnostic and preventive-care measures.

Careers and Course Work

Because of their complex mechanical and electronic nature, sonars require various skills to construct, operate, and maintain them. As a sound-based means of detecting hidden objects, a course of study should emphasize a general knowledge of acoustics and a specific knowledge of hydroacoustics. Because of its particular operational environment, how to design and construct sonar systems requires a recognition and understanding of the aquatic environment. Sonar design requires extensive knowledge of materials engineering to determine the proper components of the system and marine engineering to allow the integration of the radar system into an ocean environment and ensure its successful operation. The clear reception and presentation of the radar signal is also vital, and as this is primarily a function of modern electronics, anyone interested in sonar operations needs to have competency in electronics, computer science, or signal processing. Anyone with these skills should find ready employment in any number of industrial or scientific concerns, especially as the use of nonaquatic sound waves in industry increases.

Social Context and Future Prospects

The use of sonar will only expand as the need for more and more resources from the ocean increases. As only a fraction of the ocean floor has been accurately mapped and sonar is the most effective means of doing so, sonar will be an important tool in future economic development. As with any use of technology in an unspoiled environment, using sonar will face scrutiny. Environmentalists claim that sonar use can confuse or injure aquatic life, especially dolphins and whales that use biosonar for navigation and hunting. Environmental groups have tried for years, with some success, to limit the use of low-frequency naval sonars because of their harm to ocean life. In 2015, the US Navy agreed to limit its use of sonar during training missions in the seas near Hawaii and California to protect marine life. At the same time, oceanographic research relies heavily on sonar to study and understand aquatic life forms, and finding a balance between artificial and natural sonar systems will have to emerge as ocean exploration continues.

In a trend toward multi-statics, the US Navy aims to create a network of ships, submarines, aircraft, and sonobuoys to share information and provide a more comprehensive prevalence ability. Additionally, using active and passive sonar techniques and creating unmanned solutions for anti-submarine warfare are prioritized in the twenty-first century. Research using artificial intelligence to automate tasks and increase accuracy by lowering sonar operators' cognitive load also offers promise.

In the civilian sector, sonar is increasingly used to find and map geological structures that may contain oil and gas, lowering the risks of such searches. Once a drilling spot is identified, sonar helps monitor drilling and infrastructure installation to limit oil spills and environmental impact.

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