Anechoic tile

Anechoic tiles are rubber or Sorbothane-like 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 sonar 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

History

The technology of anechoic tiles was developed by Germany in the Second World War. Code named "Alberich", it was a 4 mm thick coating of rubber for submarines that attentuated sound in the 10 kHz to 18 kHz range to 15% of its normal strength. This frequency range matched the operating range of the early ASDIC active sonar used by the allies. ASDICs operating range would have been correspondingly reduced from its optimal range of 2000 meters to somewhere around 300 meters. The rubber contained a series of small voids, which helped to break up sound waves. The problem was that the material performed differently at different depths, due the voids being compressed by water pressure. The first tests were conducted in 1940, but it was not operational until 1944.

After the war it was not used until the 1970s when the Soviet Union began coating its submarines in rubber tiles. These were initially prone to falling off, but as the technology matured it was apparent that the tiles were having a dramatic effect in reducing the submarines' acoustic signatures. Modern Russian tiles are about 100 mm thick, and apparently reduced the acoustic signature of Akula class submarines by between 10 and 20 decibels (10% to 1% of its original strength).

The modern materials consist of a number of layers and many different sized voids, each targeted at a specific sound frequency range at different depths. Different materials are sometimes used in different areas of the submarine to better absorb specific frequencies associated with machinery at that location inside the hull.

The US Navy began applying a similar coating to its submarines in 1988, and navies around the world quickly followed suit.

References

* [http://www.junobeach.org/e/4/can-tac-det-e.htm ASDIC information at Junobeach.org]
*Stan Zimmerman. "Submarine Technology for the 21st Century." Victoria: Trafford. ISBN 1-55212-330-8