A broad frequency anechoic lining system

Abstract

Testing in indoor underwater acoustic test tanks can be limited by ambient noise and by reflections from the tank’s surfaces. The severity of reflection problems is related to a tank’s dimensions and the acoustic reflectivity of the tank’s sufaces. To reduce reflection problems, a tank may be enlarged, but in most cases, this solution is not acceptable. An alternative solution is to modify the reflectivity of the tank’s surfaces by adding an anechoic lining to the tank. A broad frequency (1 to 100 kHz) anechoic lining system was designed and developed for the 700 000 gallon Acoustic Test Facility at the Naval Undersea Warfare Center in Newport, RI. To accomplish broad frequency absorption, two parallel gradual transition absorbers are used: low-frequency, triangular wedges arranged in rows 6 in. apart and high-frequency molded pyramids spaced approximately 1 in. apart. Both the wedges and pyramids are fabricated from specially formulated, microvoided-polyurethane composites. The system is designed to provide 8 to 33 dB of echo reduction between 1 and 100 kHz. Measured prototype performance exceeded 10 dB at frequencies above 1.8 kHz. System acoustic design and materials development are discussed. The results of acoustic testing are presented and compared to design predictions.