David W. Schindel, Andy G. Bashford, and D. A. Hutchins
ABSTRACT A micromachined Fresnel zone-plate has been used to focus ultrasonic waves in air over a range of frequencies (450 to 900 kHz). The zone-plate was mounted upon a planar micromachined air-coupled capacitance transducer, which was capable of generating toneburst ultrasonic waves in air over a wide frequency bandwidth (<100 kHz to 2 MHz). A second air-coupled capacitance detector (apertured to 200 µm) was scanned in the field of the zone-plate source in order to image the generated ultrasonic field at various frequencies of operation. It was found that the ~680 µm spot size of the experimental zone-plate did not vary appreciably with changing frequency, whereas the focal length increased markedly with increasing frequency (from ~5 mm at 450 kHz up to ~15 mm at 900 kHz). These findings are shown to be in excellent agreement with previously reported theoretical predictions by the authors.
1999 IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control Vol. 46, pp. 242-246, 1999
© 1999, by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.