Sonar - SpeedyLook encyclopedia

Active sonars

The first military application of the active sonar was ASDIC (acronym of has Lied S ubmarine D etection I nvestigation C ommittee).

An active sonar emits a sound impulse and listens to its echo on the obstacles which it meets. The bases of emission and reception of the signals are called antennas and are made of several Hydrophone S.

The antenna is:

is, directional and the operator mechanically directs it in a given direction where it emits and listens;
either fixed and omnidirectional, the emission is then or omnidirectional or in beam formed electronically in a given direction. With the reception, the direction of the echo is also determined by an electronic process (measurement of dephasings on each hydrophone).

The distance is obtained by the measurement of the time passed between the emission and the reception of the echo (the propagation velocity of the sound in sea water being equal to approximately 1500 m/s).

The frequency of emission of the sonar is selected according to its use. The high frequencies (several tens or hundreds of Khz) are quickly absorbed by the sea water (several hundred meters), but on the other hand allow the target detection small and can thus carry out true images. They are thus used for the hydrographic sounders, the sonars of fishing, for the search for mines, the detection of torpedes. The impulse can have a constant frequency, but also a frequency slightly variable and modulated for a finer detection of the echo.

The more one goes down in frequency, the more the distances from detection are tall, but one loses in smoothness and the antennas become very large and very doors. In practice, the active sonars very low frequency (ATBF) hardly go down in lower parts of 3 Khz. The ranges of detection do not exceed a few tens of kilometers (in the zones of convergence).

Because the other encountered difficulty is due to the fact that the sea water is not a homogeneous medium. On the one hand, the propagation is disturbed by the relief of the bottom, the marine animals and the plankton. In addition, the speed of sound varies according to the temperature (bathythermy) and from the pressure (and marginally of salinity). These parameters vary with the depth, but between 30 or 100 meters is generally a marked limit, called the Thermocline, and which divides warm water of the cold water surface of the bottom. The wave generated by a sonar on a side of the thermocline is reflected by this one. This phenomenon does not exist in not very deep coastal water, but the echoes on the bottom are then very awkward. Beyond this thermocline, the temperature becomes constant, but the waves are subjected to the influence of the pressure which “will rectify them” towards surface, thus creating zones of convergence (ZC). The sound waves thus do not follow of the rectilinear ways on the vertical plan, (and also at long distance on the horizontal level) and thus create according to the bathythermic conditions of sound the “remote regions”, used by the submarines and of the zones of annular detections (ZC) favorable to detection. For this reason, the buildings of underwater anti fight use at the same time sonars of hull and sonars towed, whose “fish” is immersed to a few hundred meters “to light” by in lower part the thermocline. These buildings use also towed passive sonars.

Indeed, the placement of an active sonar makes its carrier detectable and identifiable, since emitting a very characteristic sound. It for this reason is practically not used by the submarines, which implement several types of passive sonars.

The helicopters of anti-submarine fight also implement “soaked” active sonars said to intermediate frequency, which they winch and immerse in hovering.

The smallest sounders resemble a tight lamp-torch. The user points it in water, presses a button, and can then read a distance there. The sonar of fishing in is an alternative making it possible to see the fish benches on a screen. Certain sounders or sonars of fishing can have capacities close to those of the military sonars, with three-dimensional representations of the zone located under the boat.

Sonars and marine animals

Certain marine animals, like the Whale S with teeth and the Dolphin S, use systems of echolocation similar to the active sonars to locate their predatory and their preys. Currently, there exists a doubt about the harmlessness of the sonars with regard to these animals. They are in particular shown to lead them to lose their way, to even prevent them nourishing themselves or from reproducing. A recent article on the site of the BBC (see low) brings back a study of the newspaper Nature on the effects induced on Baleine S by military sonars, which would cause faintnesses of decompression (followed by a stranding).

Sonars of high power can also kill the marine animals. With the the Bahamas in 2000, an experiment of the US Navy with 230 dB of a transmitter in the frequency band 3 000-7 000 hertz caused the stranding of sixteen whales, of which seven were found died. US Navy recognized its responsibility in this stranding in a report published in the Boston Globe , on January 1st 2002. However, it is it should be noted that low power, the sonars can make it possible the animals to avoid the boats.

Passive sonars

The principle of the passive sonar is to listen to sounds without emitting some. They are generally used in military or scientific applications.

The listening of the sounds is the principal activity of a submarine in patrol, it listens:

of the sounds emitted by the propellers of the boats and the hulls moving, even of the engines of the planes of maritime patrol or the helicopters in low altitude;
of the emissions sonars;
of its own noises to reduce them, remain discrete and preserve a good listening.

These sounds are detected, analyzed, followed by a whole series of hydrophones, are more or less specialized according to their frequency of collection, on the hull even of the submarine or a towed vertical wire aerial (a “flute”) of listening very low frequency (ETBF) (a few tens to a few hundreds of Hz). This equipment makes it possible to detect a presence at distances being able to exceed the hundred kilometers.

The posting of the majority of the passive sonars is traditionally appeared as a falls of two-dimensional water , with in X-coordinate the direction (the azimuth) of the sound and in ordinate time (or the frequency for a spectral analysis). Certain postings use colors. Most recent employ a representation inspired of that of the Radar S.

The passive sonar measures only one direction; the distance is obtained by the calculation and relative kinematics of the sound-effects man and the submarine. In the case of detection of emissions sonar, the distance can also be given by the time intervals between the echoes due to the successive reflections on the bottom and surface.

For listening, the submarine profits from a great advantage compared to its adversary of surface, it is not disturbed by the noise of the waves nor, at low speed, by its own noises, and it can exploit all the bizarreries of the propagation of the sound in the sea. It will always detect its adversary of surface much further this one in is able.

These bizarreries make all the interest of the military Océanographie which studies the water masses, their temperatures and their salinity, the currents and the swirls, to determine the favorable conditions with the underwater detection and conversely the zones favourable with the discretion of the submarines.

The planes of maritime patrol and the helicopters of underwater fight also use passive sonars in the form of released acoustic buoys and of which signals are transmitted by wave radio and received and analyzed by the aircraft, even relayed towards a center with ground.

Identifications of the sound-effects men

Of the specialized personnel is formed with listening (familiarly called “gold ears”); a great practice and a long drive enable them to recognize the type of ship, even to identify it individually, to calculate its speed and to detect its regime changes. The passive sonars use also very important sound databases and the calculator analyzes the frequency spectrum of the sound-effects man to identify it. The performance of the human ear remains however still higher in many cases than that of the calculator, and the two means are complementary.

Noises

The passive sonars generally are very limited because of the noises which are emitted in the building itself. For this reason, the buildings of surface, can use them only in the shape of towed vertical wire aerial (it is thus uncoupled from the hull).

By design, the submarines are quiet; the engines and auxiliaries are connected to their frame (" suspendus") by the means of studs (generally elastomeric), dissipating the vibrations (thus noise), in the form of heat. The submarines are the subject of a particular care intrinsically to avoid any vibration by the choice of very quiet equipment, by neat hydrodynamic forms, avoiding generating indiscreet disturbances in water and by discrete engines. The Hélice S are the subject of an attentive design, in order to limit the emissions of noises, in particular those related to the phenomenon of Cavitation (formation of small bubbles on the blades of the propellers during their fast rotation by weak immersion). An external acoustic coating is often laid out on the external hull to absorb the sounds (one speaks then about material of masking) and, or, to decrease the echo of an impulse of an unfavourable sonar (one speaks then about anechoic coating).

See too

anti-submarine Fight
Echolocation
SOSUS (Sound System Monitoring)

External bonds

site of [[SHOM] on the sounders]

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