An air-coupled line-focused capacitive ultrasonic transducer

A new type of air-coupled, line-focusing transducer is described. This capacitive broadband device, with an active area measuring 19.5 mm/spl times/45 mm, consists of a metallised 3.5 mm thick polymer film mounted over a brass backplate cylindrically concave along its length, with a radius of 28 mm. Device bandwidth (-6 dB) was found to be typically 200 kHz to 900 kHz, with a focal line width of /spl sim/0.68 mm. A miniature air-coupled receiver was used to scan the radiated field of these devices, excited in both tone-burst and broadband mode, and the results compared to theoretical sound pressure plots using an impulse response method. A pair of devices has been used in a novel crossed arrangement to image the surface profile of solids.