Anechoic Tiles

In its quest for ever quieter submarines, the Navy began affixing sound absorbing tiles (anechoic tiles) to submarines in 1980.  The tiles are an inch or so thick and are made of different layers, materials, and void spaces designed to deaden specific frequencies of sound. (1) 

Interestingly, anechoic tiles were first used on German U-Boats at the end of WWII.

While effective, the tiles have a disturbing tendency to fall off during patrols.  Tile loss reduces the effectiveness of the silencing and increases flow noise due to the “holes” left on the submarine’s surface.  The rough edges of these holes likely create additional turbulence and noise just as any rough edge or projection on a hull would.  Addressing a photo of a ragged looking USS Mississippi on return from patrol, former submariner Bryan Clark stated,

Bryan Clark, a senior fellow at the Center for Strategic and Budgetary Assessments and a former Navy submariner, said the amount of acoustic coating missing on the Mississippi "could create enough flow noise to be a sound problem at even relatively slow speeds. Also, there is enough tile missing that it could reduce the coating's ability to absorb sonar energy and make the submarine easier to find with active sonar." (3)

The Navy has formed various study groups to address the problem and have claimed success multiple times but tile loss remains a problem as this photo of USS Virginia in 2018 shows.

USS Virginia - Note the patches of missing tiles.

One alternative approach to anechoic tiles is being developed by researchers at the University of Michigan.  They are developing a “superhydrophobic (water repellent)” paint-like coating presumably filled with bubble voids which reduces drag thereby increasing speed, fuel efficiency, and quietness. (2)  The challenge, as with anechoic tiles, is to make the coating durable.

Similar research at the Université Paris Diderot in France has examined the use of microscopic bubbles in thin coatings. (1)  The bubbles dissipate acoustic energy.

In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.

To make submarines invisible to the sound frequencies used in sonar, larger bubbles are needed. Still, the researchers predicted that a 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible. (1)

While the experimental results are encouraging, the challenge, again, is to produce an easily applied, durable coating.  This is technology that is worth keeping track of in the future.

____________________________________

(1)Live Science website, “Thin 'Bubble' Coatings Could Hide Submarines from Sonar”, Charles Q. Choi, 4-Feb-2015,

https://www.livescience.com/49689-stealth-coatings-hide-submarines.html

(2)Popular Mechanics website, “Navy Testing Superhydrophobic Hull Coatings For Submarines”, Kyle Mizokami, Jul 5, 2018,

https://www.popularmechanics.com/military/research/a22063315/navy-testing-superhydrophobic-hull-coatings-for-submarines/

(3)Military.com website, “Navy Subs Still Show Issue with Stealth Coating”, The Honolulu Star-Advertiser By William Cole, 2018,

https://www.military.com/daily-news/2017/03/06/navy-subs-still-show-issue-stealth-coating.html

Bagikan Artikel ini