"Pipe Organ" Inspired Air-Coupled Ultrasonic Transducers With Broader Bandwidth

Piezoelectric Micromachined Ultrasonic Transducer-Integrated Helmholtz Resonator with Microliter-Sized Volume-Tunable Cavity

In this study, a piezoelectric micromachined ultrasonic transducer (PMUT) is integrated with a microliter-sized volume-tunable Helmholtz resonator. The passive Helmholtz resonator is constructed using an SU8 photolithography-defined square opening plate as the neck portion, a 3D-printed hollow structure with a threaded insert nut, and a precision set screw to form the volume-controllable cavity of the Helmholtz resonator. The fabricated piezoelectric films acted as ultrasonic actuators attached to the surface of the neck SU8 plate. Experimental results show that the sound pressure level (SPL) and operation bandwidth could be effectively tuned, and a 200% SPL increase and twofold bandwidth enhancement are achieved when setting the cavity length to 0.75 mm compared with the open-cavity case. A modified Helmholtz resonator model is proposed to explain the experimental results. The adjusting factors of the effective mass and viscous damper are created to modify the existing parameters in the conventional Helmholtz resonator model. The relationship between the adjusting factors and cavity length can be described well using a two-term power series curve. This modified Helmholtz resonator model not only provides insight into this active-type Helmholtz resonator operation but also provides a useful estimation for its optimal design and fabrication.

Broadband Piezoelectric Micromachined Ultrasonic Transducer With a Resonant Cavity

This article presents a broadband piezoelectric micromachined ultrasonic transducer (PMUT) surrounded by a resonant cavity called C-PMUT. The C-PMUT shows two resonance peaks derived from the resonances of the active PMUT cell and the passive resonant cavity. Both of the two resonances vibrate at the first-order resonant mode. An equivalent circuit model is established considering the vibration of the resonant cavity and the crosstalk between the PMUT cell and the resonant cavity. Finite element analysis (FEA) has been used to verify the theoretical model. Bandwidth optimization has been operated and the -6 dB bandwidth is extended to more than 100% in liquid. Furthermore, the theoretical model of the C-PMUT array is established based on the C-PMUT cell. The FEA models of the C-PMUT arrays are proposed, and the -6 dB bandwidth of a 4 ×4 C-PMUT array is increased to 2× compared to the traditional array. Therefore, the C-PMUT provides a novel broadband strategy for future real-time ultrasound imaging.

Equivalent Circuit Models of Cell and Array for Resonant Cavity-Based Piezoelectric Micromachined Ultrasonic Transducer

This article presents a design of resonant cavity-based piezoelectric micromachined ultrasonic transducers (PMUTs), including impedance matching tube-integrated (T) and Helmholtz resonant (HR) cavity-integrated PMUTs. In addition, equivalent circuit models for single PMUT cell and PMUT array are developed for structural optimization and complex array design. The model-derived results agree well with the FEM results. On the basis of the proposed models, an optimized design is established to achieve high output pressure and a good array working performance. The working performance of arrays that consist of HR-PMUTs and traditional circular diaphragm PMUTs (C-PMUTs) is compared. Results indicate that the HR-PMUT array has a lower crosstalk effect than the traditional C-PMUT array. Furthermore, the highest ultrasonic output pressure of HR-PMUT array at the resonant frequency can be achieved with an increase of up to 163% compared with that of the C-PMUT array because of the liquid amplification effect. Also, the cavity-based design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.