Radiation Pattern Distortion Caused by the Interelement Coupling via the Backing and the Matching Layers in Linear Array Transducers

It is well known that the performances of the acoustic imaging arrays are degraded by the inter-element coupling sustained via the backing, the matching layers, and the kerf filler. In this work the effect of the cross-coupling due to the backing, and the matching layers is investigated. A hybrid experimental-numerical technique is used. We built several arrays with and without matching layers, and using a classical backing material; on every piezoelectric element we built-in a probe of small dimensions, in order to capture the waveform on its emitting surfaces. In order to evidence the backing contribution to the coupling, every array is divided in two groups of elements and in one group the kerf-filling material was not inserted. The cross-coupling waveforms were recorded for all the groups of elements when: only the central element of the group was driven; all the elements were driven with the same pulse applied at the same time; the elements were phased excited. Fourier transforming the temporal signals, the cross-coupling transfer function of each element was computed and the radiation pattern simulated by means of a numerical model based on the Rayleigh-Sommerfeld integral. A comparison among the radiation patterns for all the groups of elements, for the above mentioned cases, is presented.