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Tactics 101: Anti-Submarine Warfare (ASW) - Part 1


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TACTICS 101: ANTI-SUBMARINE WARFARE (ASW): PART 1 - THE FUNDAMENTALS

In Part 1 of this discussion, we'll address the fundamentals of naval oceanography, i.e. the nature of the subsurface environment and how it impacts on anti-submarine warfare (ASW).

A. FEAR GOD, DREADNOUGHT, AND THE SUBMARINE!

The watchword of subsurface warfare, and by extension, anti-submarine warfare (ASW), is stealth. Submarines are absolutely dependent on stealth for both mission effectiveness and self-defense.

The introduction of submarines brought about a new and revolutionary dimension in naval warfare, one that was measurable in fathoms. Compared to the heavily armored ironclads, dreadnoughts, battleships, and battlecruisers that ruled the surface, directly confronting each other in hails of gunfire, a submerged submarine instantly became invisible, allowing it closely approach the most powerful surface ship. It could then deliver a torpedo, a weapon which could avoid the armor and strike below the waterline, where a ship was most vulnerable, thus allowing the smallest submarine to sink the largest ship.

This unique combination of stealth and offensive punch has therefore been the defining characteristic of the submarine from the very beginning.

British Admiral Sir John Fisher, the father of the the first big gun battleship, HMS Dreadnought, had the prescience to know that the submarine would seriously constrain surface ship operations, so much so that during WWI, the Grand Fleet did not operate in the southern part of the North Sea because of the threat posed by U-boats. Fisher had hoped to develop a “blue water” force projection capability that would combine the surface strike firepower of the battlecruiser with the submarine, making both the open ocean and the narrows untenable for the enemy. He envisioned that that the future of naval warfare lay in the ability to move quickly and strike at long range, as well as to go below the surface. Unfortunately for the Allies, Fisher's vision proved to be ahead of its time.

The goal of modern ASW has remained unchanged from the time that the effectiveness of submarine warfare was first demonstrated during WWI. That goal is to deny an adversary submarine the ability to exercise its considerable influence on military objectives throughout an area of interest.

ASW no longer involves destroyers and corvettes racing around, pounding the ocean with active sonar and dumping seemingly endless numbers of depth charges on the heads of hapless and terrified submariners. (Though, admittedly, popping the movie Das Boot into the DVD player is still a favorite pastime).

das-boot.jpg
Image credit: Das Boot, 1981, Columbia Pictures et al.

Rather, in the modern era, passive sonar is the instrument of choice (on both sides of the equation) when it comes to trying to detect and locate an enemy, and active sonar is usually relegated to the final attack and other special circumstances. Therefore, since passive sonar is so critically important, both surface ships and submarines take every effort to reduce their acoustic signatures.

Despite their stealth, submarines are inherently quite vulnerable, and they are easiest to detect when they poke their masts (or their hulls) through the surface. In an encounter between a surface ship and a submerged submarine, however, the advantage most definitely shifts to the submarine, as it tends to have a quieter acoustic signature.

This advantage diminishes significantly, however, when the surface ship can act in concert with other friendly assets, especially aircraft. In fact, aircraft are perhaps the most effective way of countering submarines.

More on that later, but firstly, let's examine the environment in which submarines operate.

B. NAVAL OCEANOGRAPHY

1. The basics of SONAR

When the first sonar (sound navigation and ranging) systems were developed (the ASDIC system) between the world wars, it was believed that submarines would henceforth be stripped of their stealth. As was made obvious during the Battle of the Atlantic in World War II, this was hardly the case.

Sonar exploits underwater sound propagation to navigate, communicate, or as a means of acoustic location. There are two kinds of sonar: active and passive.

Active Sonar

Active sonar requires a transmitter to create a sound pulse (the famous "ping") and a receiver, to "listen" for the reflection (the echo) of that sound pulse.

"Give me a ping, Vasili. One ping only, please."

 

With active sonar, the source of the sound is the sonar system itself, the transducer (which converts electrical energy into acoustic energy). You also need a receiver to catch the signal (echo) reflected back from the target. That signal is degraded with increasing distance to the target in exponential fashion, so that four times the power of the active sonar produces only about twice the range.

A modern hull mounted active sonar can produce hundreds of thousands of watts of sound energy, reaching up to around 250 decibels (dB). Compare that to the sound level of a jet engine at 30 meters distance, which is just 150 dB!

Passive Sonar

With passive sonar, the sound source is the target itself. A transducer that can only receive acoustic energy is called a hydrophone.

Passive sources fall into two main categories: broadband and narrowband.

Broadband sources produce acoustic energy over a wide range of frequencies. When it comes to ships and submarines, typical broadband sources are noises that emanate from the propeller or shaft, flow noise, and some kinds of propulsion systems (e.g. steam boilers).

Narrowband sources, on the other hand, produce acoustic energy within a small band or class of frequencies. Typical ship and submarine sources are the various pieces of machinery found in every ship. For example, pumps, motors, electrical generation equipment and propulsion systems. When specifying narrowband sources, it is important to also specify the frequency at which it occurs.

Each type of sonar is dependent on a different characteristic of the sonar target that in turn influence sonar performance. For active sonar, that characteristic is the target's sound reflection characteristics, or "target strength". For passive sonar, the target's own radiated noise is critical.

Frequency

Lower frequency (longer wavelength) sounds - generally in the 1-5 kHz range - tend to propagate the furthest distance, and require a large sonar dome to maintain gain (directionality). Medium frequency sonars fall generally into the 5-15 kHz band, and high frequency in the 20-30 kHz range.

High frequency (shorter wavelength) sonar can be broadly associated with short, direct path ranges in the surface layer (again, more to follow). Medium frequency sonars can be used to detect targets below the surface layer, and can sometimes exploit the bottom bounce technique. Low frequency sonars, meanwhile, have the potential to achieve convergence zone (CZ) detections. More to come on the terms direct path, surface layer, and bottom bounce.

It is also very important to note, however, that in recent years, the distinctions between frequency classifications and capabilities (as well as the boundaries of the frequencies being achieved) have blurred somewhat as technology has pushed the boundaries of what sonar can do. It is also important to realize that the ocean environment (including such variables as temperature, salinity, depth, and even geographic location) plays a huge role in affecting how sonar behaves, and notably, what it can and cannot do.

2. The Characteristics of Sound in the Ocean Environment

While light is highly absorbed by water, sound waves are not. Sound waves can thus be effectively used to probe the ocean's depths, communicate underwater, and most importantly for the purposes of this discussion, locate submerged objects.

It is notable that although sound travels vastly farther through water than does light, it is lost in reverse order, i.e. highest frequency sounds are absorbed first, and lowest frequency sounds last. This frequency loss is also one reason why it can take quite awhile to determine the nature of, or develop, a sonar contact. It requires analysis of many sounds of many different frequencies, coming from different sources on the target, to determine its identity.

The speed that sound travels underwater varies from about 4,750 feet per second (fps) to about 5,150 fps. Its speed increases with:

1. Temperature, at a rate of about 4.3 fps per degree Fahrenheit.
2. Salinity, at a rate of about 4.3 fps per thousandth part increase in salinity.
3. Depth, at a rate of about 1 fps per 60 feet of depth.

The speed of sound underwater (in fps) can be determined as follows: 4388 + (11.25 × temperature (in degrees Fahrenheit)) + (0.0182 × depth (in feet)) + salinity (in parts per thousand)

(A special instrument, called a sound velocimeter, can be used to measure the sound velocity profile of a certain area of the ocean).

The average salinity of the world's oceans is 35 parts per thousand (ppt), that is to say, the same as 35 grams of salt dissolved in each kilogram of water. Salinity in the oceans varies from about 32-37 ppt, except in the polar regions and near the shore (where fresh water from ice and the flow from rivers and streams act to dilute the salinity), where it may be less than 30 ppt.

Weather will also influence salinity, particularly where precipitation exceeds evaporation (such as in the rainy North Pacific) or where evaporation exceeds precipitation (such as in the Indian Ocean). An isolated body of water also tends to have higher salinity, such as in the Mediterranean, where excess evaporation works to increase salinity.

The polar seas (the Arctic and Antarctic) are the least saline (at 30-33 ppt), followed by the Indian Ocean (32-35 ppt), the Pacific (32-36 ppt), and lastly, the most saline ocean is the Atlantic (at 34-37 ppt).

Water pressure, which increases with depth, also has an effect on the propagation of sound. As stated, the speed of sound underwater increases with greater depth (i.e. increased pressure), which causes sound waves to bend (or refract) away from the area of higher sound speed. More on the effect of refraction in just a bit.

Let's look more closely at one of the most significant factors affecting underwater acoustics: temperature.

3. The Thermocline or the Layer

The world's oceans are stratified vertically with respect to temperature. Considered globally, seawater has a relatively large temperature range that depends upon location and time of year. Water temperature in the open ocean varies from a low of 28.4 degrees Fahrenheit (-2 degrees Celsius) to about 86 degrees F (30 degrees C), and can reach nearly 100 degrees F (37.8 degrees C) in shallow coastal waters around the Equator.

The thermal structure of the ocean is divided into three zones:

First is the surface layer or the mixed zone, where temperatures are almost uniform with depth. This is where waters are mixed together by wave action, solar driven circulation cells, tides, and so on. The depth of the surface layer varies with location and season. During winter months, it becomes more defined at all latitudes, and may extend to depths of 1,000 feet or more in mid latitudes during stormy weather. In the polar regions, water can become thoroughly mixed in winter, so that it is very nearly the same temperature from the surface to the bottom. Under normal conditions, the daily temperature of the surface layer varies as little as 5 degrees.

The bottom of the surface layer or mixed zone is marked by the next zone, the thermocline, where the temperature begins to decrease rapidly with depth.

The third zone is the deep layer, where temperature decreases very slowly with depth.

Ocean layers
ocean-layers.gif
Image credit: ES310, Introduction to Naval Weapons Engineering.

Here we are most concerned with the characteristics (i.e. acoustic properties) of the thermocline.

There may be a number of seasonal thermoclines present, which vary in depth and numberin accordance with the season (being most numerous and extending to the the deepest depths during the summertime), and also, standing or permanent thermoclines which appear year around and usually occupy deeper waters than do the seasonal kind. (Thermoclines may be identified on a bathythermograph trace, abbreviated BT or XBT, which is a graph that measures temperature as a function of depth, with temperature normally on the horizontal axis, and depth on the vertical axis).

Distinct density boundaries appear at the various thermoclines that change the water's acoustic properties. Within the surface layer, sound travels generally in straight lines (particularly at angles of less than 15 and more than 45 degrees relative to the horizontal from the source).

The "layer"
layer.gif
Image credit: ES310, Introduction to Naval Weapons Engineering.

However, sound tends to bend (i.e. refraction) as it passes through the thermoclines and tends to produce "shadow zones" above and below the angle of the sound. The boundaries of these shadow zones are referred to as limiting rays. As a result, much of the sound generated by surface sources is trapped in the mixed layer and can travel over substantial distances (sometimes called the "surface duct").

Surface duct
surface-duct.gif
Image credit: ES310, Introduction to Naval Weapons Engineering.

There are exceptions to this effect, a good example being in the Red Sea, where hot water seeping from thermal vents pool and accumulate at the bottom, and another example, in the polar seas, where extremely cold surface water is present. These produce reverse thermoclines with temperatures that increase with depth (rather than the usual behavior of decreasing with depth).

Unsurprisingly, submariners pay close attention to keeping an eye on where the thermoclines are located, and may pass across these boundaries periodically to listen for targets above and below the layer.

Submarines enjoy an additional beneficial effect, in that sound generated below the surface duct may often only be detected in the mixed layer within a 45 degree cone from the source. Surface vessels, and sonobouys dropped by aircraft, get around this obstacle by using variable depth sonars (VDS) and hydrophones. The detection gear is suspended below the layer to detect sounds under it.

4. Convergence Zones (CZ)

Convergence zones (CZ) are another effect caused by the refraction of underwater sound and are chiefly a result of changes in sound speed velocity as depth increases.

Sound waves traveling down into the depths (past the critical depth, where velocity matches that at the source) tends to bend or refract back toward the surface due to the increased pressure (and hence, increased sound velocity). Under the right conditions, the refraction creates parabolically arcing paths through the water, and when it reaches the surface, in a donut shaped area called an annulus, the sound is reflected back down again. Each focus at the surface is called a convergence zone (CZ). The process repeats itself until the sound waves have been attenuated or obstructed.

Convergence zone
convergence-zone.gif
Image: ES310, Introduction to Naval Weapons Engineering.

CZ require a minimum depth of about 400 meters in order to form, at which depth they have about a 50 percent chance of developing. The likelihood of CZ forming tends to increase with water depth, so a depth of 600 meters yields an 80 percent probability of CZ, with near certainty in even deeper water. In shallow water, the sound waves echo off the bottom rather than refract upwards. Notably, underwater topography, such as sea mounts and mid oceanic ridges, tend to disrupt CZ paths, as do ocean currents that have significant differences in temperature (e.g. the Gulf Stream).

CZ typically develop at about 30-33 nautical mile (nm) intervals, with the width of the annulus increasing with distance. The first CZ at 30-33 nm typically has a surface annulus some 3-5 nm wide; the second, at 60-66 nm, is about 6 nm wide; and the third, at 90-99 nm, has a width of about 9 nm. Fourth and fifth CZ are possible but rare. Notably, CZ ranges in the Mediterranean are reduced due to layers of warm water near the bottom.

Annulus
cz-annulus.gif
Image credit: ES310, Introduction to Naval Weapons Engineering.

CZ do not form perfect concentric cylinders, of course, but rather a series of curved 3-D shapes, so it is quite possible that a submarine within CZ range but below the CZ path could remain undetected. The same is true for the areas between the annuli, where shadow zones will develop.

Both active sonar and passive sonar may make use of CZ. This is especially true of low frequency sound, the kind generated by a high powered active sonar such as that found aboard the American Spruance (DD-963) class and the Soviet (Russian) Udaloy (Project 1155) class ASW destroyers.

In the same way that radar warning receivers (RWRs) and electronic surveillance measures (ESM) gear can detect actively emitting radars well beyond their own effective range, actively emitting sonar can also be detected well beyond the range at which it can effectively receive an echo. As a rough rule of thumb, active sonar can be heard at three times the distance at which it can detect targets.

5. Direct Path and Bottom Bounce

Sounds in the ocean are rarely confined to following a single path, and each of those potential paths must be considered when using acoustic devices to try and detect or locate ocean objects. In addition to the CZ phenomenon, sound may travel by direct path or bottom bounce.

Direct Path

The direct path is, just like the term suggests, the simplest acoustic propagation path. It occurs in the surface layer, and acts much like radar, being essentially a straight line path between the sonar source and the target, with no reflection and only a single change in direction due to refraction. It is the principle upon which fathometers are based.

The maximum achievable range obtained by exploiting the direct path is tied directly to the point at which the surface duct limiting ray is reflected from the surface. For most hull mounted medium frequency sonars, maximum direct path range is typically less than 10 nautical miles (nm), and most often, much shorter than that. (Range is of course largely dependent upon the power and frequency of the sonar in question).

Bottom Bounce

Bottom bounce is an acoustic propagation path that involves reflecting sound off the bottom, so that it "bounces" up to the surface, and then is reflected downward again. It is chiefly an effect of medium (MF) and high frequency (HF) sonars, since low frequency sound tends to be absorbed into the bottom.

Bottom bounce normally occurs in areas where the bottom is fairly smooth and the water depth is slightly greater than critical depth (see our earlier discussion on that term), typically in depths greater than 2,000 meters but less than 5,000 meters. It is useful in shallow seas, such as the Mediterranean, or along the deeper portions of the Continental Shelf.

The angles at which the sound enter the water during bottom bounce are generally too steep to produce the sort of refraction that results in CZ (that is, provided the depth is less than 5,000 m), and are typically between 15 and 45 degrees from the horizontal. Maximum effective range for the bottom bounce technique is less than 20 nautical miles.

The chief advantage of the bottom bounce technique is that it can fill the gap between relatively short direct path range and long distance CZ detections, particularly where conditions limit CZ propagation. It isn't as popular in the modern era, largely due to higher than expected absorption rates across much of the ocean floor.

Bottom bounce
bottom-bounce.gif
Image credit: ES310, Introduction to Naval Weapons Engineering.

C. TACTICAL OCEANOGRAPHY IN ANTI-SUBMARINE WARFARE

What does all of this mean in anti-submarine warfare (ASW)?

Simply put, it means that ocean conditions have a huge influence on how ASW sensors and weapons behave. Because of this influence, it is critically important to be aware of the conditions in which your submarine (or surface ship) is operating.

1. Ocean Depth and Topography

As discussed, water depth, bottom contour, and bottom composition will affect sound propagation, and in turn, sonar performance.

If the sound source is deep and the conditions are right, sound propagation may occur in what is referred to as the "deep sound channel" or DSC (previously called the SOFAR channel, or Sound Fixing And Ranging Channel). Sound is "trapped" in the DSC, with no losses at its boundaries, in turn providing extremely low loss to the receiver. (This is similar in principle to the transmission of light in a fiber optic cable). Low frequency sounds can travel for literally thousands of miles in the DSC.

The famous US Navy SOSUS (Sound Surveillance System) - the chain of bottom laid hydrophones devices strung across the north Atlantic - exploited the deep sound channel phenomenon to listen for Soviet submarines.

The DSC tends to occur deeper near the Equator, and shallower at the Poles. You can even achieve similar propagation in the surface duct under ideal conditions, but there are always some reflection loss at the surface.

The deep sound channel, here at around 750 meters north of Hawaii
SOFAR-channel.png
Image credit: World Ocean Atlas, US National Oceanographic Data Center.

Sound propagation is also affected by absorption in the water itself as well as at the surface and bottom. Sonars tend to perform best where the ocean bottom is hard and flat, and worst where it is muddy (or rocky, in which case, you get more scattering than absorption). This absorption is frequency dependent, as we discussed earlier, with highest frequency sounds being absorbed first, and lowest frequency sounds last. This is why sonars required to operate over long ranges tend to utilise low frequencies to minimise absorption effects.

In shallow water, sound propagation can suffer considerable loss. Certain sonars, for example, such as low frequency types, tend to perform poorly in shallow waters due to reverberation (scattering) and the harmonic noise produced by wave action.

Since underwater topography also influences sound propagation, submariners try to remain aware of features such as ocean ridges, sea mounts, guyotes (which are flat topped features that look like underwater mesas), and islands. The composition of the ocean bottom can likewise influence the performance of bottom bounce sonar or affect the placement of mines. Another obvious, and ominous, reason to remain aware of undersea topography is the hazard they can pose to subsurface navigation. Witness what happened to the USS San Francisco (SSN-711) in April 2005:

ssn-711-sanfrancisco.jpg
Photo credit: US Navy.

2. Ocean Temperature and Salinity

Another consideration is the freshwater outflow from rivers and the outflow from marginal seas, which can significantly affect sound propagation characteristics.

This is because, as we learned earlier in this discussion, both temperature and salinity will affect the speed of sound underwater. In turn, the behavior or performance of active and passive sonar is affected.

The temperature and salinity levels of the ocean vary widely between geographical regions. In the Arctic, low water temperatures can make for nearly uniform conditions from the surface to the ocean floor, which ordinarily might make for consistent sonar performance. However, the presence of ice also means obstacles to sound propagation (increased potential for scattering and reverberation) and high amounts of ambient noise (due to the constant grinding of the ice pack).

Varying salinity levels can also affect sonar performance. For example, the Mediterranean salinity "tongue", the name given to the warm outflow of water passing through the Strait of Gibraltar into the Atlantic, forms a mass of water that stretches nearly all the way to Bermuda and possesses acoustical properties that are distinct from other ocean regions.

Mediterranean-salinity-tongue.gif
Image credit: Dynamics of the Mediterranean Salinity Tongue, James C. Stephens and David P. Marshall, in Journal of Physical Oceanography, Vol 29, Issue 27, July 1999.

3. Ambient Noise

Even without ships and submarines present, the ocean is far from a quiet place. The ocean's own background noise, or ambient noise, varies widely with source, location, and frequency. Turbulence and geologic or seismic activity (such as underwater volcanoes) are the primary contributors to low frequency ambient noise. Ports and harbors will have increased localized noise levels due to industrial activity and merchant traffic. Distant ship traffic is perhaps the dominant noise source in the range of medium frequencies. Surface noise, such as wind and wave action, or rainfall on the surface, is the primary source of high frequency ambient noise.

Biological activity in the ocean also contributes to ambient noise and can influence your sonar. (Look up the fascinating story of the snapping shrimp). Tropical waters year round, and temperate and polar waters in the summer time, are especially prone. Marine mammals, especially toothed whales and dolphins, naturally use a method of echo location (or biosonar) to communicate and find food. These sounds add to the cumulative background noise of the ocean. (There is considerable controversy, as well, surrounding the use of high powered military sonars in the world's oceans and the negative effect it is believed to have on these same marine mammals).

The most obvious contribution to ambient noise is what is happening on the ocean's surface due to weather. Higher winds drive bigger waves, and consequently cause a greater amount of noise. Consistent wind speed is measured by the Beaufort Wind Scale, ranging from 0 (calm, wind speed less than 1 knot) up to 12 (hurricane force, wind speed over 64 kt). There is a direct relationship, then, between the steady wind speed and the sea state (the height, period and character of the waves). The World Meteorological Organization (WMO) sea state codes set out a scale for assessing the condition of the ocean surface, ranging from 0 (calm, glassy seas) up to 9 (phenomenal waves, over 14 meters high).

The greater the size of those crashing waves, of course, the greater the ambient noise contribution. Weather tends to degrade sonar performance, as increasing sea states increase ambient noise in the surface duct. This can deafen hull mounted surface ship sonars, and likewise, mask the presence of surface ships from submarine sonars. (The frequency of the noise from sea state tends to be greater than 300 Hz).

4. Self-Noise

Self-noise is the noise that your own surface ship or submarine produces, and which may be detected by your own sonar (or, worse, by the enemy's sonar system) and which contributes the overall degradation of sonar performance. Self-generated noise has therefore always been a problem for sonar platforms. Essentially, the faster a ship or submarine travels through the water, the more noise it creates and the less it hears. In both instances, it risks destruction by slower moving, stealthy opponents. The combination of a ship or submarine's self-noise, both narrowband and broadband, form its "acoustic signature" and can be exploited by an enemy's passive sonar to classify the vessel.

The noise from a ship or submarine's machinery, propellers, and even the sound of the hull passing through the water (flow noise), all add to self-noise.

Ships and submarines have plenty of potentially noisy machinery, including their engines, reduction gears, generators, hydraulics, pumps and turbines. In addition to their own noise, machinery produces vibrations in the hull that are in turn passed into the surrounding water. Machinery noise is generally independent of speed, as it is masked at high speed by flow noise, and forms the major component of self-noise at low speed. Even electronics and electrical devices can produce noise (thermal noise) if inadequately shielded.

Flow noise is generated by the turbulence of water flowing over the hull, and is dependent upon speed, hull shape and, somewhat ironically, the placement of the sonar transducer.

Propeller noise (typically very low frequency) is dependent on rotation speed and the geometry of the propeller itself. In fact, cavitation noise is the loudest noise arising from normal ship operations. Cavitation occurs when partial vacuums or bubbles are formed by the motion of rapidly turning propellers (at high speeds), and when these bubbles collapse, they produce broadband noise that can be heard at a considerable distance (and also produce shock waves that can actually damage the propeller's blades ... remember the power of those snapping shrimp?).

Acoustic stealth can also be degraded by short, transient noises caused by such things as rudder movement, start up and stopping of machinery, a dropped wrench, or famously, the opening of the torpedo tube doors. Transient noises are very characteristic of submarines and can help classify them as such.

Unsurprisingly, navies have sought to reduce self-noise wherever possible in an effort to further enhance their own stealth. Vibration dampening, isolation, and shielding of noisy machinery and components, specially shaped or skewed propeller blade geometry, and pump jet propulsors, are examples of this effort. And, of course, crews exercise a serious noise discipline, particularly aboard submarines.

Considerable amounts of money and effort have been expended to find further ways to reduce the self-noise of submarines (and the surface ships that might hunt them), even since the days of WWII. One of the earliest efforts, one that continues today, was the addition of anechoic tiles or coatings to the hulls of submarines. The Kriegsmarine (German Navy) introduced the technology first in 1944, with a synthetic rubber coating for its U-boats called Alberich. Although the principle was (and is) sound, problems with the adhesive sometimes resulted in the coating coming off, flapping in the U-boat's own wake and causing even more noise.

Modern anechoic tiles or coatings serve two purposes: to absorb active sonar, reducing the strength of the return and thereby reducing its effective range (which also reduces the acquisition range of active homing torpedoes); and to reduce self-noise, thereby reducing the effective range of an enemy's passive sonar.

Clearly visible anechoic tiles on the bow of USS Key West (SSN 722)
ssn722-anechoic-tile.jpg
Photo credit: Rob Mackie, steelnavy.com, 7 October 2000.

A noise reduction system called "Prairie Masker" was developed by the US Navy for several classes of its warships (including the Spruance, Perry, Arleigh Burke, and Ticonderoga) to reduce or mask their self-noise. The Prairie system is fitted to the ship's propellers, while Masker is fitted to the external hull in the vicinity of the propulsion plant. Compressed air is forced through machined perforations to create air bubbles that form a barrier around the propellers and hull, thereby shielding their radiated noise, and interfering with the ability of enemy passive sonar (particularly submarine sonar) to conduct analysis of the sound to classify or identify its source. It is reported that the acoustic signature of Prairie Masker sounds like rain to a passive sonar.

Prairie Masker
Prairie-Masker.png
Image credit: Mariana Ruiz.

D. CONCLUSION

Hopefully this discussion has provided a decent foundation for an understanding of the anti-submarine warfare (ASW) environment, the "battlefield" in which submarines operate and their pursuers face off.

In Part 2 we'll move to the tools of ASW - the sensors and the weapons.

Source references:
Jane's Navy International, November/December 1995.
Naval Institute Guide to World Naval Weapons Systems, 1997-98.
Jane's Information Group: August 1999; June 2003; September 2005.
Journal of Electronic Defense, March 2001.
Proceedings, June 2002.
National Defense, January 2003.
World of Defence, UDT, Issue No.2, 2004.
Navy Times, August 2005.
Harpoon 3 Sonar Model, AGSI, 2007.
Proceedings, June 2007.
ES310, Introduction to Naval Weapons Engineering.
Ocean Talk, Naval Meteorology and Oceanography Command.
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