Anechoic tile

NOTOC 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: To absorb the sound waves of active sonar, reducing and distorting the return signal, thereby reducing its effective range. To attenuate the sounds emitted from the vessel, typically its engines, to reduce the range at which it can be detected by passive sonar. The technology of anechoic tiles was developed by the Kriegsmarine during the Second World War, codenamed Alberich after the invisible guardian dwarf of the Rhinegold treasure from Richard Wagner's Der Ring des Nibelungen music dramas. The coating consisted of sheets approximately square and thick, with rows of holes in two sizes, and in diameter. Manufactured by IG Farben as a specially formed synthetic rubber tile and made using a stabilized, non-polar, high molecular weight polyisobutylene homopolymer with low-temperature elasticity; the rubber material itself was known by its trademark Oppanol. The material was not homogeneous, but contained air cavities; these cavities resulted in a degraded 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 , a Type IIB. , 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.