Active Control of the Reflection Coefficient of an Underwater Surface

From a strategic point of view, it is essential to protect underwater vehicles from being detected by opponents. Modern mono- or bistatic sonar systems are capable of precisely determining the position of a watercraft. In order to triangulate the positions of watercrafts, the sonar sends out acoustic signals that are reflected by the vehicles' surfaces. These deflected sound waves are subsequently detected and evaluated. How well an object can be detected using a sonar can be measured according to the target strength. Through their shape, construction and choice of materials, modern underwater vehicles are optimized in relation to minimizing their radiated and reflected sound waves; in this way, their target strength is minimized. These passive measures are particularly effective in the medium- and high-frequency range down to 1500 Hz. To effectively reduce reflections at lower frequencies, an active system is developed in this study and evaluated in a laboratory test with a water-filled impedance tube. The incident sound waves were measured in front of an active surface and then processed using an adaptive control system based on an FPGA platform. The system operates with a very thin piezoceramic applied to the surface in order to minimize the reflections of the sound waves. The laboratory tests showed the high effectiveness of the system under the influence of sonar-like signals.