Franco Lolan

Do US subs have anechoic tiles? If not, why not?

rickusn

yes

Franco Lolan

cool. what class of subs? do you have any pictures? (I searched but couldn't find any) are the tiles recent additions?

rickusn

Not recent. All AFAIK. No pictures although you might find some on one of the submarine or naval sites. But if there is no damage, wear or age giving them away I dont believe you should be able to notice them unless up-close.

rickusn

Also hull-coatings are now used either in place of or in conjunction with tiles.

Their is a picture on page 778 in the 1990 Edition of "COMBAT FLEETS of the WORLD" of an LA class sub puportedly showing a tile pattern but I would never have known if the caption hadnt alerted me.

rickusn

Sample Chapter
From Chapter Six: Silence Makes Perfect
As the Cold War progressed and the U.S. Navy lost its lead in underwater speed and diving depth, more attention was paid to quieting its submarines. Torpedoes could out-dive and out-run the best submarine, but only if it could be detected. The one-of-a-kind submarines Tullibee and Lipscomb experimented with turbo-electric drive to eliminate reduction gears. Although indeed the ships were quieter, they lacked the speed of their gear-driven sisters and the experiment was dropped.
Quietness gives a submarine twin advantages: it is harder to detect,while its own sensors become more sensitive as self-induced noise diminishes. The idea of silencing a floating 45,000 horsepower propulsion plant is daunting, yet the Seawolf is claimed to make less noise at speed than its predecessor at dock. "In fact, the Seawolf at its tactical speed will be as quiet as the SSN 688 alongside the pier," said Adm. Bruce DeMars, director of naval nuclear propulsion. At one point, he indicated the Seawolf would inject into the ocean less acoustic energy than the few watts necessary to burn a light bulb.

Such profound quieting comes not from a single technique. While a principal cause of noise is machinery, as we saw in the last chapter, hydrodynamics too plays a part. In this chapter, we shall look at the control of non-hydrodynamic signatures.


ANECHOICS PUT SOUND ****S ON ACTIVE SONARS
The classic method of locating submarines is active sonar. Devised prior to WWII in Britain and the United States, active sonar transmits a sharp sound into the water (the ping) and waits for an echo. The larger and closer the reflecting object, the stronger the return signal. This factor is called "target strength."
German submarine research during WWII developed a coating, which absorbed sonar pings,and, thus, reduced target strength. It was known by the code-name Alberich. "Alberich was a 4-mm thick layer of rubber, which ****led echo reflections in the range of 10 kHz -18 kHz up to approximately 15%, but not at all diving depths," wrote German naval historian Eberhard Rössler. The first tests began in 1940, but the synthetic rubber coating "did not stick very efficiently to the hull." The first operational use came in 1944.

The Alberich material was the first anechoic coating for submarines, and used a technique still in use today. The material was not homogeneous, but instead contained voids and air cavities. These cavities degraded the reflection of the sonar ping. Alberich was considered a curiosity until the mid-1970s, when Russian submarines began appearing with tiles coating the hull. Western analysts at the time pointed with humor at the frequency with which the tiles fell off. The tiles have stopped falling off, and the analysts have stopped laughing because the tiles are one of the reasons Soviets submarines became so difficult to detect.

The United States began applying similar coatings to its submarines in 1988, calling them "special hull treatments." They provide soft, almost pillow-like footing for a sailor walking on deck.

The tiles and coatings are called anechoic because they absorb rather than reflect sound. Writing in the July 1983 issue of The Submarine Review, a well-informed writer, using the nom de plume Phoenix, said, "The very thick anechoic coating on the hulls of most of the Soviet submarines (of several inches) not only greatly reduces active sound ranging off such a coated hull but also cuts down markedly the terminal acquisition range of a torpedo's active sonar."

The British navy was quick to adopt anechoic titles, along with the Swedish and Norwegian undersea forces. One British source indicated the tiles reduce by a factor of four a submarine's target strength when struck by active sonar. A similar reduction might be expected in a torpedo's final acquisition range.

Anechoic tiles perform a second acoustic function by damping internal noise from the submarine. Although not perfect, they serve as a soundproof coating, cutting radiated noise.

Phoenix wrote, "This very effective torpedo countermeasure by itself, may force U.S. submarines into closer firing ranges to ensure more precise locating of Soviet submarines, or it might force U.S. submarines to go active before firing- thus, disclosing an attack."

In the 1980s, as Western navies scrambled to follow the Russian lead, anechoic technology was top secret. At the first Underseas Defense Technology conference in London in 1988, a specimen of an anechoic tile was seized by security agents. At the conference in 1994, a variety of companies were offering to tailor-make anechoics for just about any client. The technology is so simple, apparently it was deemed impossible to thwart its proliferation.

For example, the anechoic treatment planned for the second Collins-class submarine for the Australian navy will be a locally developed and produced product. David Oldfield with the Defense Science and Technology Organization was the man responsible for developing it.

"First I went back and redeveloped the old Alberich material, based on historical records," he said, holding an example in his hand. "It used big and little voids in synthetic rubber. I used an elastic polymer sandwich, and created voids all the same size." As for Alberich's adhesion problem, "We solved that," he said.

So have others. In 1994, a British-French joint venture offered to analyze a submarine, design an anechoic treatment, and apply the material. Steven Burton, director of Metod, a joint venture of W&J Tod, Ltd. and Metravib of France, said, "We offer a turnkey solution. We design and apply it to submarines."

It is actually a misnomer to call these materials anechoic, because they do more than absorb the sharp pings of sonars. "You may need multilayer materials," said Burton. "Anechoic to absorb, damping and decoupling materials to reflect, and Rho C materials to transmit sound such as your sonar dome."

Metod can apply materials to the hull either by affixing tiles, casting in place or spraying. Using tiles as a factor of 1.0, Burton said casting will cost 80%-90% of tiles, and experience a 150% application rate; the spray technique would cost about 85% of tiling, and go on 250% faster.

A typical treatment would consist of either cast or spray anechoics on the control surfaces and sail, anechoic tiles for the hull, and decoupling materials around the engineering spaces. Key to a successful treatment is surface preparation. Metod used shot-blasting, but is exploring ultrahigh-pressure water sprays, with pressures up to 36,000 psi. The surface is then primed and spray-coated or tiled.

As for the critical voids, which make anechoics work, "we can add voids by the spoonful while mixing a batch to spray," said Burton. Each pass produces a 2 mm layer. The Metod system works at the 30 kHz frequency of active torpedo homing sonars, the 4 kHz-6kHz range of active ASW sonars, and even below the 1 kHz used by modern low-frequency active sonars

Two different techniques are used. The dispersion of small voids (i.e. bubbles) in the material works in the frequency range of 1 kHz up to 500 kHz. For frequencies below 1 kHz, a technique using either larger voids or rigid pieces within the polymer coating is used. "For instance, a honeycomb structure filled with an elastomer can provide a large attenuation in this range of frequency."

Other companies were showing their skills at the 1994 conference as well. Avon Rubber had their sample snatched away in 1988, but were offering "advanced signature management technology" six years later. "The Signature Management Group has access to some of the most sophisticated test equipment. Using computer modeling techniques, thorough theoretical and practical design skills, and expertise in elastomer technology, successful signature management solutions are devised for individual requirements," said a company brochure.

In case adhesion was still a problem, even the British Ministry of Defense was willing to lend a hand. A brochure from the Defense Research Agency offered its facilities and advice for problems of adhesive formulation and evaluation, durability assessment and surface preparation techniques. "Special in-house skills and experience have been developed in the use of adhesives underwater and in wet environments generally for the bonding of a wide range of materials from steel, aluminum and copper alloys to GRP rubbers," the brochure said.

Not to be outdone, the consulting engineering group Yard Ltd., a subsidiary of the SEMA Group, offered to design "anechoic coatings" and "decoupling or damping layers." The top secrets of yesterday had become, in six years, more grist for defense commerce.


TILES USE DAMPING LAYERS TO CONVERT SOUND TO HEAT
The current generation of acoustic treatment uses layers of material to specialize in both anechoic and damping roles. The latest Russian attack submarines of the Akula and Sierra classes use such a layered system, as do British navy submarines.
The inner tiles allow engineers to tailor sound defeating qualities. For example, if a piece of rotating machinery at a certain site produces a specific frequency, the inner tile can be designed to damp that specific sound, and turn the acoustic energy into heat. Such damping would reduce the capability of a hostile passive sonar to pick up the noise. If the passive sonar was a torpedo homing device, the acquisition range would be cut.

The outer layer is designed to act anechoically - to absorb active sonar signals - and can also be tailored to match the primary frequencies used by potentially hostile forces. Thickness is dependent on the specific frequencies of interest. This is often in the region of 30mm-50mm, but could run as thick as 100 mm.

Sources indicate the Russian tiles are approximately 2.8 X 3.0 feet and four-inches thick. Earlier Russian anechoic tiles were smaller in area and thinner. These sources indicate the tiles reduce the acoustic signature of an Akula or Sierra between 10 and 20 dB. This reduction, in the frequency range of an American AN/BQQ-5 sonar, causes a reduction in detection range of between 25% and 50%. The detection range shrinkage is not constant because it varies with the temperature and composition of the water in which the sound is traveling. British sources indicate the two-layer system, also in use with the British navy, is similarly tailored to counter Russian sonars and shows similar results.

The physical principles are well documented through research in a field called auxiliary mass damping. A paper presented at the 1988 London UDT conference by Ed Parker with Plessy Naval Systems Ltd., Templecombe, U.K., concerned how such tiles can be tuned to depress noise levels over a broad frequency range, including the lower frequencies where long-range passive sonars work.

The technique described in the paper closely matches the Russian system described by intelligence sources, including the two-tile approach.

Conventional mass damping turns vibration into heat. The vibrating mass is connected to a base plate with a spring and a viscous damper. "The maximum energy is dissipated when the relative damper displacement is a maximum and this occurs at the damped resonant frequency of the mass on the spring. The bandwidth over which energy can be usefully dissipated is controlled by the viscous damping coefficient," said Parker.

By replacing the spring and viscous damper with a polyurethane rubber compound exhibiting both spring and damper characteristics, Parker creates what he calls auxiliary mass damping tiles. By varying the properties of the polyurethane rubber material, the acoustic damping characteristics can be tailored for specific frequencies.

"To obtain required stiffness properties in the elastic rubber layer, force-free areas are produced by introducing air pockets. This is done by either blowing the layer to produce a foam or molding the layer with inclusions. Manufacturing experience has shown that the air fraction can be controlled sufficiently to produce the required tile resonant frequency," he said.

To produce the topmost or mass layer, Parker's paper noted, "...it should be dense and appear mass like' when attached to the elastic layer. Steel, lead and lead oxide-loaded rubbers have been used for this purpose but the choice of material very much depends upon the specific application. Bonding the two layers to the structure to be damped is easily achieved with either epoxy or contact adhesive."

He said the tiles can be tuned to give less noise attenuation over a broad band of frequencies, or higher attenuation rates at more specific frequencies. This would allow designers to stipulate attenuation rates around particularly noisy submarine machinery such as coolant pumps in nuclear boats and internal combustion engines in conventional subs.

A brochure by Dowty Woodville Polymer of Britain indicates the company could tailor anechoic materials to a customer's specific requirements. "From the information supplied and using our in-house range of laboratory equipment with its extensive polymer database, it is possible to model and predict the type of material necessary to achieve the acoustic and anechoic performance and quality control at all stage of production," the brochure says.

The capabilities of Dowty's Woodville facilities included "the ability to predict the performance of anechoic materials for changes in temperature, frequency and pressure; the ability to measure and certify the performance of materials; the ability to advise on materials and their locations to reduce self-radiated noise; the ability to cary out our original research into acoustic and anechoic materials for passive/active sonar arrays; [and] the ability to design and manufacture acoustic materials to meet specific customer requirements."



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Franco Lolan

thanks a lot Rick! 100 mm in come cases, crazy! I went on an LA class prob 6 yrs ago, I cant remember tiles. being a submariner, did you hear Akula's getting significantly quiter as a result of these tiles? or did you hear your own subs getting quiter after treatment? If you can't disclose that information, just say that you can't disclose.TY

Franco Lolan

any one else with comments or experience?

rickusn

Way after my time. LOL But apparently tiles and coatings do indeed make submarines quieter in addition to harder to detect actively.

Franco Lolan

may I ask what time/ sub was that?

rickusn

1977-81 USS HL Stimson SSBN 655 Gold

Franco Lolan

I checked out some pics at http://home.att.net/~little-jeff/

The Chap

Yes.

__________________
Where's the bloody gin? An army marches on its liver, not its ruddy stomach.

Franco Lolan

"Yes" to what? Please expound.

The Chap

Sorry Rubber tiles. (long story short)