Recent progress in towed hydrophone array research

Since World War I, various attempts have been made to minimize own ship noise by towing hydrophones at the end of long cables. Recently, because of advances in technology (such as improved cables and low-noise transistors) as well as improved understanding of the hydrodynamics of tow, most of the problems limiting this technique in the past have been overcome. This report summarizes past and recent effort and presents a digest of the more recent results. These results were obtained as part of a program to determine how hydrophones could be moved through the sea with minimal self noise, i.e., without machinery or flow-caused-noise radiated from own ship structure. The most successful units to date are called "Towflex." These consist of special hydrophones in flexible oil-filled hoses. When towed from a surface ship by cables up to 8000 feet long, at depths to 1200 feet, and at speeds up to 13 knots, these units show lower or equivalent self-noise levels than reported on quiet submarines for the same speed. For frequencies of 20 to 500 cps, Towflex units are generally quieter - in the order of 10 to 15 decibels at 6 knots - and for the frequencies of 500 to 10000 cps, the levels are similar. There are indications that even with these units, flow noise will be a major problem starting at about 10 knots.