Increasing the active surface in random sparse 2D arrays

The design process of a two-dimensional transducer arrays prototype for NDT air-coupling inspection is described. The manufacture process for the presented prototype, based on multiuser micro-electro- mechanical process (MUMPS), impose technical restrictions (like the size of membranes or the die size) that should be considered with the usual 2D apertures design drawbacks. The development of full 2D squared matrix arrays suitable to generate high quality volumetric images is limited by the large number of elements needed. To avoid this inconvenience, several random sparse array design techniques have been proposed to reduce the number of active elements, maintaining good enough image quality. But unfortunately due to the high element impedance, as a consequence of its small size, and to the area reduction resulting from the thinning process, the image contrast is highly reduced. In this paper we propose to increase the array active surface in random sparse arrays by enlarging the elements size up to λxλ, although we hold a λ/2xλ/2 element distribution grid. This strategy allows to increase the radiated energy and to reduce the element impedance, avoiding at the same time grating lobes formation. In the paper, we study theoretically the field properties of these arrays and, moreover, we make a comparison of the proposed solution with λxλ elements and the conventional arrays whose elements are kept under λ/2xλ/2.