Sonar

Sonar is a special way to find things using sound. It stands for sound navigation and ranging. This tool helps us see underwater, like how we use our eyes above water. Sonar can find ships, measure distances, and talk to objects under the water.

There are two main types of sonar. One type, called passive sonar, listens for sounds that ships make. The other type, called active sonar, sends out sound pulses and listens for the echoes that come back. This tells us where things are and how far away they are.

People have used sonar for a long time. The first known use was in 1490 by Leonardo da Vinci, who used a tube to hear ships in the water. Sonar became important during World War I because submarines were hiding ships. Today, sonar helps ships, robots, and scientists who study the air and weather.

History

Animals like dolphins, bats, and some shrews have used sound to talk and find objects for millions of years. Humans first tried using sound underwater in 1490 when Leonardo da Vinci used a tube to listen for ships.

In the late 1800s, bells were placed near lighthouses or lightships to warn of dangers. After the sinking of the Titanic in 1912, people began using sound to find things underwater, just like bats use sound to fly. The first patent for a device to find things underwater was filed by Lewis Fry Richardson after the Titanic sank. Another scientist named Alexander Behm got a patent for a similar device in 1913.

During World War I, there was a big need to find submarines, so scientists worked hard on using sound. The British made early listening devices called hydrophones. A scientist named Paul Langevin worked on active sound devices to detect submarines in 1915.

ASDIC

In 1916, a Canadian scientist named Robert William Boyle started working on active sound detection. They used special crystals to create the first practical underwater sound detection device. To keep it secret, they called it “ASDIC” instead of using the word “sound.” By 1918, Britain and France had built prototypes, and Britain started using them on ships in the 1920s.

SONAR

In the 1930s, American engineers developed their own underwater sound detection technology. They started using the term SONAR to describe their systems.

US Navy Underwater Sound Laboratory

In 1917, the US Navy began using new technology to detect underwater signals. They used vacuum tubes to improve detection. During World War II, they kept developing better sonar systems to find submarines, mines, and torpedoes.

Materials and designs in the US and Japan

In the early 1940s, the US improved their sonar technology. They found better materials to use in their devices. Japan also used special materials in their sonar devices during World War II.

Later developments in transducers

After World War II, scientists kept looking for better materials for sonar. They tried using metals and other materials, but some didn’t work as well as others. New materials like Terfenol-D were discovered, which helped make better sonar devices.

Active sonar

Active sonar uses a special device that sends out a sound pulse, called a "ping." It then listens for the sound to bounce back. This helps measure distances and find objects underwater. The sound pulse is made using equipment that turns electrical signals into sound waves. These waves travel through the water and bounce off objects, returning to the sonar device.

By timing how long it takes for the sound to return, the distance to the object can be found. Microphones can also help learn the direction of the object. This technology is used in many ways, such as finding fish or helping submarines navigate. Some small sonars are easy to use and work like waterproof flashlights, showing distances to things underwater.

Passive sonar

Passive sonar listens for sounds without sending out any of its own. It is often used in the military but can also help scientists study fish and other sea life. By listening to sounds made by ships or other objects in the water, passive sonar can help identify what is nearby.

Passive sonar uses special tools to pick up sounds and can tell where sounds come from. For example, ships from the United States often make a specific sound that helps identify them. Even sounds like tools being dropped can be detected. Computers and trained people work together to sort out these sounds and learn more about what is making them. To get clearer results, some sonar tools are pulled behind ships to stay away from noisy parts of the vessel.

Performance factors

Sonar works differently based on where it is used and the tools being used, whether it is making sounds or listening for them. Things like water temperature, depth, and saltiness can change how sound moves. For example, sound moves slower in fresh water than in sea water.

Sound can bend when it moves through layers of water with different temperatures, which can make it harder to find objects. In deep water, sound can travel far with little loss if the conditions are good. In shallow water, sound often bounces between the surface and the bottom, which can cause losses. Other things like waves, ships, and tiny particles in the water can create noise that makes it harder to hear the sounds we want.

Military applications

Modern naval warfare uses sonar from ships, airplanes, and fixed locations. In World War II, ships used sonar, but submarines avoided it to stay hidden. Today, technology allows better ways to find and track enemies.

Active sonar is used to find submarines, but it can also show the user's position. So, it is used only for short times. Submarines use active sonar only when needed. Passive sonar is quiet and can detect targets from far away by listening to their noise, such as engine sounds. This helps identify what the target is and where it is going.

Sonar is important for many military tasks, including finding and tracking submarines, guiding weapons, and detecting underwater mines. It is also used to communicate underwater and to watch large areas of the ocean. Some sonars can detect swimmers or divers near ships or ports.

Civilian applications

Fishing is an important job that needs good tools to find fish. Because there are fewer fish now, fishermen use technology like sonar to help them. Sonar uses sound waves to find fish. This works well underwater because sound travels better there than in air. Today, most fishing boats use sonar to find fish, measure water depth, and see the ocean bottom.

There are different kinds of sonar tools. One type, called echo sounding, sends sound down to the water bottom and listens for the echo to find out how deep the water is. Another tool, called a net sounder, is attached to fishing nets and shows how close the net is to the bottom or surface. This helps fishermen know when to pull the net in to catch fish. Some small sonars are even used on underwater robots to help them see in murky water.

Scientific applications

Biomass estimation

Main article: Bioacoustics

Scientists use sonar to find and study fish and other sea creatures. They send out sound pulses and listen for echoes to learn about the size, location, and number of fish. This works best with fish that have special air-filled parts called swim bladders, like herring and cod. The data helps us understand more about sea life.

Wave measurement

Special sonar devices placed on the seafloor can measure the height and timing of ocean waves. This helps scientists learn about the conditions at different places in the water.

Water velocity measurement

There are short-range sonars designed to measure how fast water is moving in certain areas.

Bottom type assessment

Sonar can also help identify what the seafloor is made of, like mud, sand, or gravel. By analyzing the sound echoes, scientists can classify the type of material on the ocean floor.

Bathymetric mapping

Side-scan sonars can create maps of the seafloor by moving just above it. Some sonars cover large areas, while others give more detailed maps of smaller spots.

Hull-mounted multibeam echosounders on big ships can quickly gather detailed seafloor data. For example, the General Instrument "Seabeam" system uses special equipment to map the ocean bottom.

Sonar imaging

Sonar data can be used to create two and three-dimensional images of underwater objects and landscapes.

Sub-bottom profiling

Strong low-frequency sonar devices can show the layers just below the ocean floor. These tools help in underwater archaeology and other studies.

Gas leak detection from the seabed

Both passive and active sonar can detect gas bubbles leaking from the seafloor. This includes natural leaks of methane and carbon dioxide, as well as possible leaks from underground storage sites.

Synthetic aperture sonar

Some synthetic aperture sonars have been developed for use in searching and detecting underwater mines. More information can be found in synthetic aperture sonar.

Parametric sonar

Parametric sonar uses water’s properties to create specific sound frequencies. This method has benefits like wide frequency range and narrow beams but works with low efficiency. More details are in Parametric array.

Sonar in extraterrestrial contexts

Scientists have suggested using sonar on other worlds, like Titan, to study its hydrocarbon seas. However, they must be careful because conditions there are very different from Earth.

Ecological impact

Effect on marine mammals

Further information: Marine mammals and sonar

Using active sonar can sometimes cause groups of sea animals, like whales, to come ashore. Some whales, especially beaked whales, are very sensitive to this kind of sonar. Other sea animals, such as blue whales and dolphins, also move away from the sonar.

Some sea animals use sound to find food and stay safe. Studies show that certain sonar can interrupt this, which might affect their health.

A review in 2019 looked at cases where beaked whales came ashore after naval exercises using sonar. It found these whales are especially affected by this type of sonar. The review also noted that when sonar use was stopped, there were fewer cases of whales coming ashore, suggesting that limiting sonar use could help protect these animals.

Effect on fish

Low frequency sonar can slightly change how some fish hear for a short time.

Frequencies and resolutions

Sonar uses different sound frequencies to work. Lower frequencies can travel farther, but higher frequencies give clearer details and can be made in smaller devices.

During World War II, American sonars used frequencies around 20–30 kHz to stay small but had a range of about 2,500 yards. Later sonars used lower frequencies, like 10 kHz, to reach up to 5,000 yards. Some sonars even used 3 kHz to reach up to 20,000 yards, though they needed to be very large.

Japanese sonars from the same time used a mix of frequencies. One type worked at 9 kHz, while another used 17.5 kHz and could reach up to 6 km. Later models used frequencies between 13 and 20 kHz.

Higher frequency sonars, such as 600 kHz, can see very small details but only over short distances, like 75–150 meters. Lower frequencies, like 30 kHz, can see farther but with less detail, up to 6,000 meters. Water conditions can affect how well sonar works, especially in shallow coastal areas where higher frequencies are often needed.