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This paper describes the evaluation of the radiation field characteristics of PVDF membrane transducers, whereby the active area is defined by an appropriate electrode pattern when operating into air. Measurement, including conventional scanning of the fields and laser vibrometry, is supplemented by finite element modeling over the frequency range 500 kHz to 1.1 MHz. Contrary to expectation, the surface displacements are nonuniform, giving rise to premature axial focusing of the field. This is shown to be caused by generation of the zero order symmetrical Lamb wave, arising from the electric field discontinuity at the electrode boundary of the transducer.