Defect imaging by micromachined ultrasonic air transducers

Capacitive micromachined ultrasonic transducers (cMUTs) are shown to have over 100 dB dynamic range in air. This enables fast imaging of internal defects of solid structures with high signal-to-noise ratio. The high dynamic range is the result of a resonant structure with a fractional bandwidth limited to about 10%. Better temporal resolution is required to differentiate the defects in the depth dimension, which demands higher bandwidth devices. In this paper we present an optimized pulse-echo electronics system for cMUTs in air, simulations suggest that dynamic ranges in excess of 100 dB are attainable in pulse-echo operation using commercially available discrete components. Transmission experiments through aluminum and composite plates verify more than 100 dB dynamic range and demonstrate the ability of cMUTs to image defects in air at 2.3 MHz. We also present a variation on cMUT design which improves the useful bandwidth of the device, permitting greater depth resolution in pulse-echo imaging.