Development of a high target strength passive acoustic reflector for low-frequency sonar applications

The objective of this study was development of broadband, high target strength, passive acoustic reflectors. These reflectors would provide convenient, low-cost targets for low-frequency sonar trials and fleet exercises. The primary development goals included controlled, stable monostatic and bistatic reflectivity, and adjustable deployment depth down to 90 m (300 ft). The development process used both computer models and scaled physical models to find the most effective configuration capable of meeting the project goals. Review of acoustic scattering theory showed that acoustically soft reflectors (bubbly liquids or air-filled spheres and cylinders) provide higher target strength values than similarly sized hard reflectors (metal spheres or corner reflectors). Air-filled cylinders were found to provide the highest target strength values and widest useful bandwidth for a given reflector volume. This is a result of the tube resonance that occurs at ka=0.02 at the depth range of interest where k is the acoustic wavenumber and a is the tube radius. Air-filled cylinder target strength at resonance is about equal to its target strength at ka=1.0 with a deviation less than ±4 dB between these values. Guided by these study results, a prototype reflector was assembled using a 16-m length of air-filled gum rubber tubing, 5.7 cm in diameter (3.2-cm ID). A SCUBA regulator attached to a small air tank maintained tubing inflation. Test results at a depth of 90 m showed an effective target strength of 12 dB at 250 Hz with a deviation less than ±2 dB from 200 to 400 Hz. This is equivalent to the target strength of a 16-m-diameter perfectly reflecting sphere-an object with a volume about 52000 times that of the prototype cylinder reflector