Influence of the inter-element coupling on ultrasound array radiation patterns

It is well known that the performances of the acoustic imaging arrays axe d egraded by the interelement coupling sustained via the backing, the matchin g layers, and the kerf filler. The filling material inserted between the el ements gives mechanical robustness to the array, but acts as a path of inte raction, transmitting, between the elements, shear and lateral compressiona l forces. In this work, the effect on the array radiation pattern of the cr oss-coupling due to the filling material is investigated. A hybrid experime ntal-numerical technique is used. Two groups of five elements of a commerci al array transducer were isolated and in one group the kerf filling materia l was removed. The cross-coupling waveforms, captured with a mechanical pro be of small dimensions in contact with the emitting surfaces of the element s, were recorded for filled and unfilled 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; all the elements were driven with the same pulse, but inverting the polarity alternately. This latter case re fers to the worst coupling situation caused by the shear forces exerted bet ween the elements. Fourier transforming the temporal signals, the cross-cou pling transfer function of each element was computed and the radiation patt ern was simulated by a numerical model based on the Rayleigh-Sommerfeld int egral. Comparing the radiation patterns for filled and unfilled groups of e lements, for the three cases mentioned above, a good estimation of the infl uence of the filling material is obtained.