Underwater ears

Jason Holmes, a mechanical engineering graduate student at

and guest researcher at

, has devised a low-cost, highly sensitive hydrophone array that is perking up interest in both homeland security and ocean research circles.

Holmes' underwater hydrophone array is designed to be towed by a small, autonomous submarine and can monitor for ocean-going threats to America's waterways or for sound for ocean acoustics studies.

He presented research on his underwater listening device in Vancouver on May 20 at the semi-annual meeting of the Acoustical Society of America.

Holmes' prototype system comprises six underwater microphones, or hydrophones, spaced inside a 30-foot plastic tube filled with mineral oil. The array tube is filled with mineral oil to create neutral buoyancy, allowing the array to float behind the underwater towing vehicle.

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

Listening arrays typically used by the military and ocean scientists are towed behind ships and are very long, the shortest being around 1,500 feet long, and are several inches in diameter.

By contrast, Holmes’s prototype system is just 30 feet in length and 1.1 inches in diameter and can easily be towed through the water by a 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 sound is refracted by the bottom. But the US Navy saw the device as a potential security tool, one that is less expensive than the multi-million dollar listening arrays currently in use. Parts for Holmes' array cost a mere $4,000 and are available as off-the-shelf technology.

Holmes is now working with the military to further develop the array for underwater intelligence gathering. Holmes says his next project will comprise four underwater hydrophone arrays towed by a fleet of unpiloted subs that could travel up to 4 kilometers per trip.

Holmes and his faculty advisor William Carey, a professor of aerospace and mechanical engineering in BU's College of Engineering, say they envision a fleet of entirely autonomous listening subs will prowl the seas, returning to underwater recharging stations to upload their data and refresh their batteries.

"A lot of people were skeptical this would even work," Carey says. "But the way Jason has designed this array, this will change the way ocean measurements are made."