Airborne Capacitive Micromachined Ultrasonic Transducers in Pitch–Catch Mode: Modeling and Characterization of the Measurement Chain

Abstract

Capacitive Micromachined Ultrasonic Transducers (CMUTs) feature broadband operation as well as high sensitivity, showing promise for applications such as pressure sensing, ultrasonic imaging, and wireless power transfer. Although widely used in medical and underwater settings, CMUTs are also well-suited to air-coupled operation. As an illustrative example, CMUT-based gas detection can be achieved by monitoring ultrasonic properties of the medium (acoustic velocity and attenuation). This paper focuses on a modeling approach for a complete CMUT-based airborne pitch-catch measurement, across a wide frequency range from 0.5 MHz to 5 MHz. A lumped-element equivalent circuit of the CMUT array is used for transducer modeling, with parameters extracted experimentally from electrical impedance measurements and interferometric displacement analysis. Acoustic pressure propagation in air is simulated using Field II in MATLAB, incorporating frequency-dependent air attenuation ( $\approx ~100$ Np/m at 2.5 MHz) obtained from dedicated experiments. The performance of three preamplifier types (voltage, charge, and transimpedance) is evaluated at a 6 mm separation distance between the transmitter and receiver. For a 2.5 MHz-centered excitation signal with an amplitude of 10 V and biased at 45 V, the measured and simulated output amplitudes reach approximately 30 dBmV with a voltage amplifier, 35 dBmV with a transimpedance amplifier, and 40 dBmV with a charge amplifier. The simulations agree well with the experimental results, confirming reliable and predictable ultrasonic sensing in air using CMUTs and paving the way for future work, such as gas-sensing modeling.