An annular array design proposal with multiple geometric pre-foci.

An annular array design method is described and used for definition of an array proposal. The geometric pre-focus, realized by curving or use of an acoustic lens, varies among the annuli. Element sizes and geometric pre-focus depths are determined by a maximum allowed phase-shift within the active depth region of each element on receive, resulting in fewer elements or larger apertures compared with the standard equal area design. The method allows combination of large aperture and high frequency with a large receive depth of field. The developed design rules are used to define an array for imaging within the frequency interval [7.5, 15] MHz. Its total aperture diameter is 22 mm and the thinnest element is 0.23 mm wide. Receive beams resulting from this array are simulated. The beams and their sidelobe-to-mainlobe energy ratios are compared with an ideally focused reference where there are no phase shifts over the elements. Within nearly the entire depth of field, the sidelobe-to-mainlobe energy ratio is less than 5 dB higher than for the ideal reference.