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This paper reviews capacitor micromachined ultrasonic transducers (cMUTs). Transducers for air-borne and immersion applications are made from parallel-plate capacitors whose dimensions are controlled through traditional integrated circuit manufacturing methods. Transducers for airborne ultrasound applications have been operated in the frequency range of 0.1–11 MHz, while immersion transducers have been operated in the frequency range of 1–20 MHz. The Mason model is used to represent the cMUT and highlight the important parameters in the design of both airborne and immersion transducers. Theory is used to compare the dynamic range and the bandwidth of the cMUTs to piezoelectric transducers. It is seen that cMUTs perform at least as well if not better than piezoelectric transducers. Examples of single-element transducers, linear-array transducers, and two-dimensional arrays of transducers will be presented.

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Nondestructive evaluation techniques Amlrurn , which use conventional piezoelectric transducers tvDicallv reauire liquid coupling fluids to improve " _ . -the impedance mismatch between piezoelectric materials and air. Air-coupled ultrasonic systems can eliminate this requirement if the dynamic range of the system is large enough such that the losses at the air-solid interfaces are tolerable. Capacitive miFig, Schematic cross-section of a single cMUT mem.cromachined ultrasonic transducers (cMUTs) have brane been shown t o have more than 100 dB dynamic range when used in bistatic transmission mode. This dynamic range, along with the ability to transmit ultrasound efficiently into air, makes cMUTs ideally suited for air-coupled nondestructive evaluation applications. These transducers can be used either in through transmission experiments at normal incidence to the sample or to excite and detect guided waves in aluminum and composite plates.transmission system using cMUTs that achieves aIn this paper, we present results of a pitch-catch dynamic range in excess of 100 dB. The pair of transducers is modeled with an equivalent electrical circuit which predicts the transmission system's insertion loss and dynamic range. We also demonstrate the feasibility of Lamb wave defect detection for one-sided nondestructive evaluation applications. A pair of cMUTs excites and detects the so mode in a 1 . 2 mm-thick aluminum plate with a received signal-to-noise ratio of 28 dB without signal averaging.