Angular dependence of target strength for simple geometric shapes

Abstract

Understanding the active sonar equation is a fundamental skill for a course in underwater acoustics and sonar such as that taught at the United States Naval Academy. Students must develop an intuitive understanding of each term in both the noise-limited and reverberation-limited cases. In this demonstration, the angular dependence of the target strength term is explored using commercially available 25- and 40-kHz airborne piezoelectric transducer/receiver pairs. By rotating standard target shapes (e.g., rectangular plates, circular plates, finite cylinders, ellipsoids), reflected intensity is measured as a function of angle of rotation. The measured data are compared with those predicted by standard theoretical predictions. While the same measurements could be made in a tank using submerged transducers, the airborne transducers offer the advantage of lower cost and simplicity of apparatus. Additionally, unwanted modal interference from sound reflection off tank surfaces is reduced in the airborne measurement. By direct observation, future naval officers are able to study target strength so as to understand how to position their sensors to optimize their own active sonar performance and position their warship to minimize detection by enemy active sonar.