Sonar, << SOH nahr, >> is a system that uses sound energy to locate objects; measure their distance, direction, and speed; and even produce pictures of them. The word sonar comes from _so_und _n_avigation _a_nd _r_anging.
Sound travels through water better than radio waves or light, and so it is useful for exploring and monitoring the world’s oceans. The word sonar is most commonly associated with devices for finding submarines. However, sonar systems are also used for many other purposes, including imaging (creating pictures of) the seafloor, spotting schools of fish, and tracking whales. Sonar systems are also found in nature. For example, dolphins, porpoises, some bats, and some whales use a form of sonar, known as echolocation, to find food and to avoid obstacles.
How sonar works.
There are two types of sonar: (1) active, and (2) passive. Active sonar systems purposely radiate sound into the water. Such a system includes an underwater device called a projector that converts electrical signals into sound waves. The system sends out short sounds, called pings, that allow for more accurate location of objects than sounds of longer duration. Each ping travels from the projector through the water until it strikes an object. Objects reflect and scatter the sound in various directions. The sound reflected by an object back to the sonar is called an echo. An underwater device known as a hydrophone converts the echo from sound to electrical signals. In modern sonar systems, a computer analyzes the echoes and processes them to obtain information about the object. Such information includes distance, speed, size, and direction.
The distance to an object of interest, called a target, is known as its range. Because sound travels at roughly 1 mile (1.6 kilometers) per second through water, a ping that returns after 2 seconds has traveled 2 miles—1 mile to the target and 1 mile back. Thus, the target’s range is 1 mile. The speed of the target can be determined by measuring the frequency of the echo and comparing it to that of the signal that was sent. The frequency of a sound is the number of sound waves that pass a given point each second. An object moving away from the projector will return a sound of lower frequency, and an approaching object will cause a shift to a higher frequency. This phenomenon is called the Doppler effect, and the frequency change is known as a Doppler shift. The faster the target is moving, the greater this shift.
Passive sonar systems do not employ a projector, but instead receive sounds radiated by other sources. These systems usually combine the sound information from several hydrophones to determine the direction of a sound. Active sonar is better for determining target range. But submarine crews prefer using passive sonar, because by not putting any sound in the water, they do not reveal their own position.
Other uses.
Sonar has many other applications besides its use in the ocean. For example, physicians can use a form of sonar called ultrasound, which operates at frequencies above the range of human hearing, to diagnose heart disease or to produce images of a fetus (unborn baby).
History.
Many scientists consider the first known reference to the idea of sonar to be an observation on the properties of sound made by the great Italian artist and scientist Leonardo da Vinci. In 1490, he wrote, “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.” Naval scientists in France, the United Kingdom, and the United States experimented with underwater sonar during World War I (1914-1918). During the 1920’s and 1930’s, scientists and engineers developed practical sonar systems that were used during World War II (1939-1945). Since then, engineers have continued to improve the sensitivity and accuracy of sonar systems. In addition, the use of sonar for scientific and commercial purposes has grown.
