The technology of anechoic tiles was developed by the Kriegsmarine in the World War II. Codenamed Alberich, it was named after the invisible sorcerer from Germanic Mythology. The coating was made up of sheets approximately 1,000 millimetres (3 ft 3 in) square and 4 millimetres (0.16 in) thick, with rows of holes in two sizes, 4 millimetres (0.16 in) and 2 millimetres (0.079 in) in diameter. It was manufactured by IG Farben from a material known as Oppanol. The material was not homogeneous but contained air cavities; it was these cavities that degraded the reflection of ASDIC. The coating worked in the 10 to 18 kHz range, reducing ASDIC return by about 15%. 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
