Smart acoustically active surfaces

The use of such materials as PVDF and piezorubber in making smart acoustically active surfaces is discussed and demonstrated. By using one layer as a sensor and applying the signal detected in this layer through an amplifier and filter to a second driving layer it is possible to control either the reflection or transmission of plane waves striking the surface normally. By using two driving layers, both transmission and reflection can be simultaneously controlled. When transmissions are eliminated the control of reflection is independent of changes in backing impedance. By adding a second sensing layer along with two driving layers, reflections and radiation from a vibrating surface can be eliminated simultaneously. Stability problems involved in using pressure and/or velocity sensors in various control algorithms are discussed.