An Analytical Model for Bandwidth Enhancement of Air-Coupled Unsealed Helmholtz Structural CMUTs

Xin Liu,Mang I Vai,Jiujiang Wang,Peng Un Mak,Sio Hang Pun,Yuanyu Yu

doi:10.1155/2019/3896965

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) were reported to own high potential in air-coupled ultrasonic applications such as noncontact nondestructive examination and gas flow measurement. The unsealed CMUTs which utilized the squeeze film effect were reported to overcome the narrow output pressure bandwidth of the conventional sealed CMUTs in air operation. This kind of unsealed CMUTs can also be regarded as Helmholtz resonators. In this work, we present the air-coupled unsealed Helmholtz structural CMUTs which utilize both the squeeze film effect and the Helmholtz resonant effect to enhance the output pressure bandwidth. Based on the mechanism of vibration coupling between membrane and air pistons in membrane holes, we propose an analytical model to aid the design process of this kind of CMUTs. We also use finite element method (FEM) to investigate this kind of CMUTs for our analytical model validation. The FEM results show that the significant bandwidth enhancement can be achieved when the Helmholtz resonant frequency is designed close to the fundamental resonant frequency of the CMUT membrane. Compared with the conventional sealed CMUT cell, the 4-hole unsealed Helmholtz structural CMUT cell improves both the 3-dB fractional bandwidth and SPL-bandwidth product around 35 times. Furthermore, it is found that, with more holes under the same hole area ratio or with a smaller ratio of the cavity height to the viscous boundary layer thickness, the Helmholtz resonant effect becomes weaker and thus the output pressure bandwidth decreases.

Highlights

Capacitive micromachined ultrasonic transducers (CMUTs) were invented as an alternative to conventional piezoelectric ultrasonic transducers in early 1990s
The vibration coupling characteristic of two resonant peaks is more obvious with such large dimension cells, because the proportion of thermal viscous loss energy to the total energy gets lower when the cell dimension increases from Half case to Double case, as we shall show in discussion section
We present the air-coupled unsealed Helmholtz structural CMUTs which utilize both Helmholtz resonant effect and squeeze film effect to enhance the device’s output pressure bandwidth performance in transmit mode

Summary

Introduction

Capacitive micromachined ultrasonic transducers (CMUTs) were invented as an alternative to conventional piezoelectric ultrasonic transducers in early 1990s. Compared with the piezoelectric ultrasonic transducers, CMUTs have many advantages such as they are suitable for batch production and can be integrated with modern electronics. Conventional piezoelectric ultrasonic transducers typically require matching layers to compensate the acoustic impedance mismatch between piezoelectric materials and surrounding gaseous or liquid media. CMUTs feature a better impedance matching with gaseous and liquid media, and they can transmit/receive ultrasound more efficiently into gas and liquid without the matching layer [1]. In some applications, some objects cannot contact with liquid couplants, such as paper products and electronic products [4]. CMUTs ideally are suited for air-coupled applications because of their good acoustic impedance matching with air

Results

Discussion

Conclusion