July 1, 2005

Ears go underwater for national security

A low-cost, highly sensitive underwater listening device may soon aid in national security and ocean research.

Jason Holmes, a mechanical engineering graduate student at Boston University and guest researcher at Woods Hole Oceanographic Institute, has devised an array of underwater ears that is perking interest in homeland security and ocean research circles. Holmes' device, an underwater hydrophone array that a small, autonomous submarine can tow, can monitor for ocean-going threats to America's waterways or for sound for ocean acoustics studies.

The array combines sophisticated engineering with off-the-shelf hardware to create a relatively inexpensive but highly sensitive underwater listening device. The prototype comprises six underwater microphones, or hydrophones, spaced inside a 30-foot plastic tube filled with mineral oil. The array tube carries mineral oil inside it to create neutral buoyancy, allowing the array to float behind the underwater towing vehicle.

Signals from the hydrophones register to and store on mini-disc recorders aboard the unpiloted submarine Remus. Designed by Woods Hole Oceanographic Institution, Remus looks like a small torpedo and can navigate autonomously underwater around obstacles and through harbors using GPS sensors, sonar, and electronic maps.

Listening arrays typically used by the military and ocean scientists tow behind ships and are very long-the shortest being around 1,500 feet long-and several inches in diameter. At 30 feet in length and 1.1 inches in diameter, the extremely compact prototype can easily tow through the water behind a small, quiet, battery-powered craft. The compact size of the towing sub and array make it easy for one or two people to launch the system, compared to the fully crewed ships required for conventional hydrophone systems.

Holmes originally developed the array to help him study how sound waves travel through shallow water, where the bottom refracts sound. Until recently, most acoustic ocean studies have taken place in deep water, where the bottom has little effect on sound. Holmes constructed the hydrophone system to tackle the problem of how sound waves behave in shallow water, but the U.S. Navy saw the device as a potential security tool that is vastly less expensive than the multi-million dollar listening arrays currently in use. Parts for Holmes' array cost $4,000 and are available as off-the-shelf technology.