Influence of the Interelement Coupling on Ultrasound Array Radiation Patterns

It is well known that the performances of the acoustic imaging arrays are degraded by the interelement coupling suitained via the backing, the matching layers, and the kerf filler. The filling material inserted between the elements gives mechanical robustness to the array, but acts as a path of interaction, transmitting, between the elements, shear and lateral compressional forces. In this work, the effect on the array radiation pattern of the cross-coupling due to the filling material is investigated. A hybrid experimental-numerical technique is used. Two groups of five elements of a commercial array transducer were isolated and in one group the kerf filling material was removed. The cross-coupling waveforms, captured with a mechanical probe of small dimensions in contact with the emitting surfaces of the elements, 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 refers to the worst coupling situation caused by the shear forces exerted between the elements. Fourier transforming the temporal signals, the cross-coupling transfer function of each element was computed and the radiation pattern was simulated by a numerical model based on the Rayleigh-Sommerfeld integral. Comparing the radiation patterns for filled and unfilled groups of elements, for the three cases mentioned above, a good estimation of the influence of the filling material is obtained.