Submarines could one day be cloaking themselves in cocoons of laser light in order to maintain their 'acoustic hygiene'. According to a feasibility study concluded earlier this year, an array of lasers could paint an envelope of light around a sub and use it to monitor the vessel's noise output. By knowing what noise the submarine was making, its commander could take appropriate cancellation measures, thus making detection by the enemy less likely.
The study was undertaken by Dr. Mike Podlesak from Australia's Defence Science and Technology Organisation (DSTO) during a year-long attachment to the Naval Research Laboratory in Washington DC. Basically, the idea is to provide a submarine with a self-diagnostic tool for measuring its own noise output and determining how that noise would appear to opponents. The lasers would continually scan a 'skin' of water a few meters out from the sub's hull and following the vessels' contours. This skin needs to be far enough out to avoid interference from the roiling water rushing over the hull, an area known as the turbulent boundary layer.
The lasers measure the motion of micro-particles in the water, using the laser doppler technique. These micro-particles resonate in sympathy with any sound waves moving through the water, giving an indication of what noise is leaking from the sub. This is known as the 'acoustic signature'.
The proposed laser system could also employ a procedure known as Near-field Acoustic Holography (NAH). This can predict how the acoustic signature would alter over varying distances and therefore determine what the submarine's opponents would be able to hear. By determining what sound frequencies the sub is radiating on, active broadcasts of slightly out-of-phase signals could be used to cancel some of the noise, effectively 'cloaking' the sub's acoustic signature.
If the theory bears fruit, there will be great interest from the world's most powerful navies. A submarine's most potent weapon is invisibility. Once detected, a sub's relatively slow speed and lack of defensive firepower make it a sitting duck, especially from air attack. The anti-submarine weapons of today mean that there is only a slim chance for an exposed vessel to escape. For this reason, silence is taken to fanatical levels of engineering. The USA's best ballistic missile submarines have been compared to 'black holes in the ocean' and have reputedly never been successfully tracked by opposing forces.