Enhancing the Fractional Bandwidth of an Ultrasonic Air Transducer Using an Acoustic Block

Abstract

Airborne ultrasonic transducers have been used in many applications such as range finders and flowmeters. However, they suffer from low fractional bandwidth (FBW) because of the acoustic impedance mismatch between the transducer and air. Improving the spatial resolution of measurements from the transducers requires a wide FBW. In this paper, a method presents to improve the FBW of a flexural-mode airborne ultrasonic transducer by using the additional structure of an acoustic block. Ultrasonic transducer was fabricated using a micro-electro-mechanical system. Electrostatic force actuates a circular plate with a radius of 800 μm and a thickness of 17 μm. In air, the plate has a low FBW, less than 1%. To improve the FBW, we designed a 350- μm-thick silicon piece with many circular holes. When the distance between the plate and the silicon piece is small enough, the air gap affects the mechanical behavior of the plate through squeeze film effects and the Helmholtz resonance effect. This combination of effects changes the FBW and the resonant frequency of the transducer. The effect of the silicon piece, i.e., the acoustic block, was investigated with variations of the following three key parameters: the thickness of the air gap, the radius of the circular holes, and the area ratio of the holes. According to our finite element analysis, area ratios of the holes have the largest effect on the FBW and can improve it by as much as 25%. In a simulation with elevated pressure, the effect of the Helmholtz resonance starts to dominate. The acoustic block has several potential uses, such as tuning the frequency of existing transducers and designing airborne ultrasound transducers for elevated pressure applications.