Anechoic tile

Anechoic tiles are rubber or synthetic polymer tiles containing thousands of tiny voids, applied to the outer hulls of military ships and submarines, as well as anechoic chambers. Their function is twofold:

The technology of anechoic tiles was developed by the Kriegsmarine in the Second World War, codenamed Alberich after the invisible dwarf from Germanic Mythology. The coating was made up of sheets approximately 1 m (3 ft 3 in) square and 4 mm (0.16 in) thick, with rows of holes in two sizes, 4 mm (0.16 in) and 2 mm (0.079 in) in diameter. It was manufactured by IG Farben from a material known under its trade name of "Oppanol" (basically PIB, see also synthetic rubber). The material was not homogeneous but contained air cavities; it was these cavities that degraded the reflection of ASDIC. The coating reduced echoes by 15% in the 10 to 18 kHz range. This frequency range matched the operating range of the early ASDIC active sonar used by the Allies. The ASDIC types 123, 123A, 144 and 145 all operated in the 14 to 22 kHz range. However, this degradation in echo reflection was not uniform at all diving depths due to the voids being compressed by the water pressure. An additional benefit of the coating was it acted as a sound dampener, containing the U-boat’s own engine noises.

The coating had its first sea trials in 1940, on U-11, a Type IIB.U-67, a Type IX, was the first operational U-boat with this coating. After its first war patrol, it put in at Wilhelmshaven probably sometime in April 1941 where it was given the coating. The coating covered the conning tower and sides of the U-boat, but not to the deck. By 15 May 1941, U-67 was in Kiel performing tests in the Baltic Sea. During July, the coating was removed from all parts of the boat except the conning tower and bow. Further experiments and sound trials were made in the Little Belt but they presumably proved unsatisfactory, as all the coating was subsequently removed. Problems were encountered early-on, when it was found that the adhesive had insufficient strength to bond synthetic rubber with the pressure hull and casing. This resulted in the sheets loosening and creating turbulence in the water, making it easier for the submarine to be detected. Furthermore, the coating was found to have considerably decreased the speed of the boat.

...
Wikipedia