Acoustic transduction in air from two bulk-micromachined silicon structures is investigated. Both contain silicon diaphragms of the order of 2 mm2 in close proximity to a metallized substrate. One diaphragm is mass-loaded; the other is not. Their resonant frequencies (70 and 360 kHz) are dominated by squeeze film trapping of ambient air, and the Q of each device is about 8. The lower frequency (LF) device is characterized by electrical and acoustic measurements using a calibrated microphone. Novel diagnostic methods that exploit the non-linear nature of the transducer are described. The adequacy of calibration by reciprocity is confirmed at 70 kHz and applied to the high frequency device. An insertion loss of 19 dB is measured, which compares well with reports of other silicon-based transducers. Observed losses are accounted for by squeeze-film damping applied to the diaphragm-substrate gap. The ability to control the bandwidth by the squeeze film effect, the good efficiency, and the relatively standard method of construction could make such ultrasonic transducers useful in specialist applications.
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