Acoustics/Sonar

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

History

The French physicist Paul Langevin, working with a Russian émigré electrical engineer, Constantin Chilowski, invented the first active sonar-type device for detecting submarines in 1915. Although piezoelectric transducers later superseded the electrostatic transducers they used, their work influenced the future of sonar designs. In 1916, under the British Board of Inventions and Research, Canadian physicist Robert Boyle took on the project, which subsequently passed to the Anti- (or Allied) Submarine Detection Investigation Committee, producing a prototype for testing in mid-1917, hence the British acronym ASDIC.

By 1918, both the U.S. and Britain had built active systems. The UK tested what they still called ASDIC on HMS Antrim in 1920, and started production of units in 1922. The 6th Destroyer Flotilla had ASDIC-equipped vessels in 1923. An anti-submarine school, HMS Osprey, and a training flotilla of four vessels were established on Portland in 1924.

The U.S. Sonar QB set arrived in 1931. By the outbreak of World War II, the Royal Navy had five sets for different surface ship classes, and others for submarines. The greatest advantage came when it was linked to the Squid anti-submarine weapon.

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 beamforming.

The pulse may be at constant frequency or a chirp of changing frequency. For 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 energy were emitted. In practice, the chirp signal is sent over a longer time interval; therefore the instantaneous emitted power will be reduced, which simplifies the design of the transmitter. 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.

Active sonar is also used to measure distance through water between two sonar transponders. A transponder is a device that can transmit and receive signals but when it receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transponder transmits an interrogation signal and measures the time between this transmission and the receipt of the other transponder's reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two transponders. This technique, when used with multiple transponders, can calculate the relative positions of static and moving objects in water.

Analysis of active sonar data

Active sonar data is obtained by measuring detected sound for a short period of time after the issuing of a ping; this time period is selected so as to ensure that the ping's reflection will be detected. The distance to the seabed (or other acoustically reflective object) can be calculated from the elapsed time between the ping and the detection of its reflection. Other properties can also be detected from the shape of the ping's reflection:

• When collecting data on the seabed, some of the reflected sound will typically reflect off the air-water interface, and then reflect off the seabed a second time. The size of this second echo provides information about the acoustic hardness of the seabed.
• The roughness of a seabed affects the variance in reflection time. For a smooth seabed, all of the reflected sound will take much the same path, resulting in a sharp spike in the data. For a rougher seabed, sound will be reflected back over a larger area of seabed, and some sound may bounce between seabed features before reflecting to the surface. A less sharp spike in the data therefore indicates a rougher seabed.

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 Web site (see below) reports findings published in the journal Nature to the effect that military sonar may be inducing some whales to experience decompression sickness (and resultant beachings).

High-powered sonar transmitters may indirectly harm marine animals, although scientific evidence suggests that a confluence of factors must first be present. In the Bahamas in 2000, a trial by the United States Navy of a 230 decibel transmitter in the frequency range 3 – 7 kHz 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 January 2002. However, at low powers, sonar can protect marine mammals against collisions with ships.

A kind of sonar called mid-frequency sonar has been correlated with mass cetacean strandings throughout the world’s oceans, and has therefore been singled out by environmentalists as causing the death of marine mammals. International press coverage of these events can be found at this active sonar news clipping Web site. A lawsuit was filed in Santa Monica, California on 19 October 2005 contending that the U.S. Navy has conducted sonar exercises in violation of several environmental laws, including the National Environmental Policy Act, the Marine Mammal Protection Act, and the Endangered Species Act.

Passive sonar

Passive sonar listens without transmitting. It is usually employed in military settings, although a few are used in science applications.

Speed of sound

Sonar operation is affected by sound speed. Sound speed is slower in fresh water than in sea water. In all water sound velocity is affected by density (or the mass per unit of volume). Density is affected by temperature, dissolved molecules (usually salinity), and pressure. The speed of sound (in feet per second) is approximately equal to 4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet) + salinity (in parts-per-thousand)). This is an empirically derived approximation equation that is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 metres there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, for a sound originating on one side of the thermocline tends to be bent, or refracted, off the thermocline. The thermocline may be present in shallower coastal waters, however, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation. Increased pressure increases the density of the water and raises the sound velocity. Increases in sound velocity cause the sound waves to refract away from the area of higher velocity. The mathematical model of refraction is called Snell's law.

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. The ocean must be at least 6000 feet (1850 meters) deep, or the sound waves will echo off the bottom instead of refracting back upwards. Under the right conditions these waves will then be focused near the surface and refracted back down and repeat another arc. Each arc is called a convergence zone. Where an arc intersects the surface a CZ annulus is formed. The diameter of the CZ depends on the temperature and salinity of the water. In the North Atlantic, for example, CZs are found approximately every 33 nautical miles (61 km), depending on the season, forming a 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. Typically the first, second and third CZ are fairly useful; further out than that the signal is too weak, and thermal conditions are too unstable, reducing the reliability of the signals. The signal is naturally attenuated by distance, but modern sonar systems are very sensitive.

Identifying sound sources

Military sonar has a wide variety of techniques for identifying a detected sound. For example, U.S. vessels usually operate 60 Hz alternating current power systems. If transformers are mounted without proper vibration insulation from the hull, or flooded, the 60 Hz 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 50 Hz power systems. Intermittent noises (such as a wrench being dropped) may also be detectable to sonar.

Passive sonar systems may have large sonic databases, however most classification is performed manually by the sonar operator. 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. Publications for classification of sounds are provided by and continually updated by the U.S. Office of Naval Intelligence.

Sonar in warfare

Modern naval warfare makes extensive use of sonar. The two types described before are both used, but from different platforms, i.e., types of water-borne vessels.

Active sonar is extremely useful, since it gives the exact position of an object. Active sonar works the same way as radar: a signal is emitted. The sound wave then travels in many directions from the emitting object. When it hits an object, the sound wave is then reflected in many other directions. Some of the energy will travel back to the emitting source. The echo will enable the sonar system or technician to calculate, with many factors such as the frequency, the energy of the received signal, the depth, the water temperature, etc., the position of the reflecting object. Using active sonar is somewhat hazardous however, since it does not allow the sonar to identify the target, and any vessel around the emitting sonar will detect the emission. Having heard the signal, it is easy to identify the type of sonar (usually with its frequency) and its position (with the sound wave's energy). Moreover, active sonar, similar to radar, allows the user to detect objects at a certain range but also enables other platforms to detect the active sonar at a far greater range.

Since active sonar does not allow an exact identification and is very noisy, this type of detection is used by fast platforms (planes, helicopters) and by noisy platforms (most surface ships) but rarely by submarines. When active sonar is used by surface ships or submarines, it is typically activated very briefly at intermittent periods, to reduce the risk of detection by an enemy's passive sonar. As such, active sonar is normally considered a backup to passive sonar. In aircraft, active sonar is used in the form of disposable sonobuoys that are dropped in the aircraft's patrol area or in the vicinity of possible enemy sonar contacts.

Passive sonar has fewer drawbacks. Most importantly, it is silent. Generally, it has a much greater range than active sonar, and allows an identification of the target. Since any motorized object makes some noise, it may be detected eventually. It simply depends on the amount of noise emitted and the amount of noise in the area, as well as the technology used. To simplify, passive sonar "sees" around the ship using it. On a submarine, the nose mounted passive sonar detects in directions of about 270°, centered on the ship's alignment, the hull-mounted array of about 160° on each side, and the towed array of a full 360°. The no-see areas are due to the ship's own interference. Once a signal is detected in a certain direction (which means that something makes sound in that direction, this is called broadband detection) it is possible to zoom in and analyze the signal received (narrowband analysis). This is generally done using a Fourier transform to show the different frequencies making up the sound. Since every engine makes a specific noise, it is easy to identify the object.

Another use of the passive sonar is to determine the target's trajectory. This process is called Target Motion Analysis (TMA), and the resultant "solution" is the target's range, course, and speed. TMA is done by marking from which direction the sound comes at different times, and comparing the motion with that of the operator's own ship. Changes in relative motion are analyzed using standard geometrical techniques along with some assumptions about limiting cases.

Passive sonar is stealthy and very useful. However, it requires high-tech components (band pass filters, receivers) and is costly. It is generally deployed on expensive ships in the form of arrays to enhance the detection. Surface ships use it to good effect; it is even better used by submarines, and it is also used by airplanes and helicopters, mostly to a "surprise effect", since submarines can hide under thermal layers. If a submarine captain believes he is alone, he may bring his boat closer to the surface and be easier to detect, or go deeper and faster, and thus make more sound.

In the United States Navy, a special badge known as the Integrated Undersea Surveillance System Badge is awarded to those who have been trained and qualified in sonar operation and warfare.

In World War II, the Americans used the term SONAR for their system. The British still called their system ASDIC. In 1948, with the formation of NATO, standardization of signals led to the dropping of ASDIC in favor of sonar.