Sonar

Sonar (sound navigation and ranging) is a technique that uses sound propagation under water to navigate or to detect other watercraft. There are two kinds of sonar, active and passive.

Active sonar

Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections of the pulse. To measure the distance to an object, one measures the time from emission of a pulse to reception. To measure the bearing, one uses several hydrophones, and measures the relative arrival time to each in a process called beam-forming.

The first active sonar technology was originally called ASDIC after the "Allied Submarine Detection Investigation Committee".

The pulse may be at constant frequency or a chirp of changing frequency. If a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total power were emitted. In general, long-distance active sonars use lower frequencies. The lowest have a bass "BAH-WONG" sound.

The most useful small sonar looks roughly like a waterproof flashlight. One points the head into the water, presses a button, and reads a distance. Another variant is a "fishfinder" that shows a small display with shoals of fish. Some civilian sonars approach active military sonars in capability, with quite exotic three-dimensional displays of the area near the boat. However, these sonars are not designed for stealth.

When active sonar is used to measure the distance to the bottom, it is known as echo sounding.

Sonar and marine animals

Some marine animals, such as whales and dolphins, use echolocation systems similar to active sonar to locate predators and prey. It is feared that sonar transmitters could confuse these animals and cause them to lose their way, perhaps preventing them from feeding and mating. A recent article on the BBC website (see below) reports findings published in the Journal Nature to the effect that military sonar may be inducing the whales to experience decompression sickness (and resultant beachings).

High-powered sonar transmitters can kill marine animals. In the Bahamas in 2000, a trial by the US Navy of a 230 decibel transmitter in the frequency range 3000-7000 hertz resulted in the beaching of sixteen whales, seven of which were found dead. The Navy accepted blame in a report published in the Boston Globe on 1/1/2002. However, at low powers, sonar can protect marine mammals against collisions with ships.

Passive Sonar

Passive sonars listen without transmitting. They are usually military (although a few are scientific).

Sonar in freshwater lakes is different in operation from sonar at sea. In salt water sonar operation is affected by temperature. Ocean temperature varies with depth, but at between 30 and 100 metres there is often an marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. Regarding sonar, a sound originating from one side of the thermocline tends to be reflected off the thermocline, unless it is very noisy. The thermocline is not present in shallower coastal waters. Pressure also affect sound propagation as convergence zones (CZ).

Sound waves that are radiated down into the ocean bend back up to the surface in great arcs due to the effect of pressure on sound. Under the right conditions these waves will then reflect off the surface and repeat another arc. Each arc is called a CZ annulus. CZs are found every 33 nm, forming a annular pattern of concentric circles around the sound source. Sounds that can be detected for only a few miles in a direct line can therefore also be detected hundreds of miles away. The signal is naturally attenuated but modern sonar suites are very sensitive.

Identifying sound sources

Military sonar has a wide variety of techniques for identifying a detected sound. For example, U.S. vehicles usually operate 60Hz alternating current power systems. If transformers are mismounted (without proper vibration insulation from the hull), or flooded, the 60Hz sound from the windings and generators can be emitted from the submarine or ship, helping to identify its nationality. In contrast, most European submarines have 50Hz power systems. Intermittent noises (such as a wrench being dropped) may also be listened for.

Passive sonar systems usually have large sonic databases. A computer system frequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or the type of weapon released), and even particular ships. Most navies continuously update the information in these databases.

Noise

Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle. For this reason, many submarines operate nuclear reactors that can be cooled without pumps, using silent convection, or fuel cells or batteries, which can also run silently. Vehicles' propellers are also designed and precisely machined to emit minimal noise. High speed propellers often create tiny bubbles in the water, and this cavitation has a distinct sound.

The sonar hydrophones may be towed behind the ship or submarine in order to reduce the effect of noise generated by them. Towed units also combat the thermocline, as the unit may be towed above or below the thermocline.

For many years, the United States operated a large set of passive sonar arrays at various points in the world's oceans. As permanently mounted arrays in the deep ocean, they were very quiet.

In war-time, emission of an active pulse is so compromising for a submarine's stealth that it is considered a very severe breach of tactics.