A planar acoustic lens is described in which Rayleigh waves generated by a circular interdigital transducer on LiNbO3 are converted to a focused hollow conical beam in the contiguous liquid. The focal spot is comparable to that of a standard diffraction-limited lens of similar aperture; the calculated sidelobes are higher but the measured ones are comparable.
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Farnell et al. (1980) studied this question.