Pseudo-random selection of elements in a multi-element array

Simple random number generators using integrated circuit shift registers are being employed to make random selections of points on multi-element (12 000 array points) ultrasonic planar transducers. The transducer is a 2-in (5 1 cm) diameter piezoelectric plate used to detect the scattered acoustic field caused by an object placed in an ultrasonic field. The transducer is digitally scanned to enable information from discrete points to be encoded for image reconstruction by computer processing. For unambiguous reconstruction, point spacing of one halfwavelength is required. This would require 12 000 sample points, and processing this information would be unwieldy. Removing the points randomly according to a specific density distribution enables up to 95% of the points to be removed with little deterioration in transducer performance. Computer simulations of the thinned array have been completed and these confirm that the transducer directivity patterns are satisfactory in the far-field and focused near-field.