Ultrasound imaging in air

One of the main factors which has limited the use of air-borne ultrasound in man-made applications is lack of suitable transducers for air-borne propagation. Piezo-electric transducers, which are the main sources of ultrasonic energy in solid and liquids, couple far less effectively in air than capacitive transducers which are significantly more efficient, but are not available commercially except in one size and at one frequency. Work by Wykes et al. (1991, 1992) has enabled capacitive transducers to be produced in a range of frequencies and sizes and this has led to the production of transducer arrays which can be used to produce two and three dimensional images. This work discusses (i) capacitive transducers (ii) US imaging and (iii) four imaging systems, operating at 50 and 100 kHz