Air-coupled ultrasound transduction improvement using vertical piezoelements’ array

Novel transducer construction is proposed: array of PVDF film strips placed parallel to each other with air gaps between. Films are oriented in such way that strip extension (31 mode) is aligned with emission direction. Transduction is improved by providing better acoustic impedance match to air and increased displacement of emitting surface. Two distinct operation modes are presented: i) emission from the gaps ii) emission from membrane attached to the edges at array top with lower edges backed. In gap emission mode, the transduction efficiency is increased because of two mechanisms. The pressure produced by the expansion of the individual films is concentrated into narrow gap. Also, emission of edges due to height extension. Height extension (31 mode) is much higher than thickness expansion (33 mode) if film height is larger than film thickness. Low, 500 Rayl, equivalent acoustic impedance of gap emission is achieved at film thickness 10 mu m and gap with 50 mu m. In membrane emission mode, only extension (31 mode) is used, pressure emitted is increased due to large membrane displacement and better match to air. Equivalent acoustic impedance is 500 kRayl at PVDF film thickness 40 mu m and 200 mu m air gap. Displacement is maximized if PVDF film with large transverse piezoelectric coefficient d31 is used. Experimental measurements are presented. Transmission sensitivity peak for gap emission was 155 mPa/V, for membrane emission mode it was 320 mPa/V. Impressive, more than 270 % fractional bandwidth was confirmed experimentally.