Grating lobe reduction in transducer arrays through structural filtering of supercritical plates

The effect of placing a structural acoustic filter between water and the transducer elements of an array is investigated to help reduce undesirable grating lobes. A plate is mounted to transducer elements with a thin decoupling polyurethane layer between the transducers and the plate. The plate acts as a radiation/incidence angle filter to pass energy at angles near normal incidence, but suppress energy at large incidence angles. Grating lobe reduction is achieved at the expense of limiting the available steering of the main lobe. Within this steer angle limitation, the main lobe can be steered as normal while the grating lobe level is reduced by the plates angular filtering. The insertion of a plate structural filter provides an inexpensive and easily implemented approach to extend usable frequency bandwidth with reduced level grating lobes, without increasing the number of array elements. Even though some data matches theory well, a practical material has yet to be found that possesses optimal material properties. To the authors’ knowledge, this work represents the first attempt to advantageously utilize a plate to provide angular dependent sound transmission filtering above the plates critical frequency (the supercritical frequency region). [Work sponsored by ONR Code 333, Dr. David Drumheller.]